Hipertiroidismo felino (tiroides hiperactiva en gatos)

Qué podría notar

• Baja de peso aunque coma lo mismo o incluso más: esta es la señal más típica
• Mucho apetito: pide comida todo el tiempo, se mete a otros platos, parece que nunca queda satisfecho
• Toma bastante más agua, y usted encuentra más orina o piedritas más pesadas en la caja
• Vómito, o heces más blandas de lo normal
• Inquietud: camina sin parar, no se acomoda, y maúlla fuerte, sobre todo de noche
• El pelo se ve descuidado, grasoso, con nudos o más ralo, porque dejó de acicalarse bien
• Respira más rápido, o su veterinario le escucha el corazón acelerado o un soplo nuevo
• A veces su veterinario le palpa un bulto en la parte baja y delantera del cuello, en uno o en los dos lados
• A veces ocurre lo contrario: algunos gatos se ponen débiles, callados y sin apetito en lugar de acelerados, y por eso se pasa por alto
• Un gato de esta edad es también la edad clásica de la enfermedad renal, la presión alta y la enfermedad del músculo del corazón, y por fuera se ven iguales: por eso se revisan la tiroides, los riñones, la presión y el corazón al mismo tiempo, no uno por uno

Estándar de atención

1. Treat every hyperthyroid cat - comorbidity is a reason to plan carefully, not a reason to withhold treatment

   Aprobado / respaldado por guías

   Do not adopt a watch-and-wait approach in a confirmed hyperthyroid cat because it also has kidney disease or heart disease. Choose and sequence the treatment around the comorbidity instead, and monitor the comorbidity through the transition to euthyroidism. Where the comorbidity is one the approved drug's label excludes, that is a reason to choose a different modality or to make and document an explicit extralabel decision - not a reason to treat nothing.

   Productos nombrados: No drug at this step - this is the decision framework.

   The 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism explicitly 'emphasize the importance of treating all hyperthyroid cats, regardless of comorbidities' and separate the diagnosis into six clinical categories with matched management strategies, and explicitly set out to dispel myths about the disease and replace them with an evidence-based narrative. The comorbidity review adds that 'Hyperthyroidism treatment options should be carefully considered when CKD has also been diagnosed' (PMID 35481810).

   Carney HC, Ward CR, Bailey SJ, Bruyette D, Dennis S, Ferguson D, Hinc A, Rucinsky AR. 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. J Feline Med Surg. 2016;18(5):400-16. PMID 27143042. Comorbidity framing: Geddes R, Aguiar J. Feline Comorbidities: Balancing hyperthyroidism and concurrent chronic kidney disease. J Feline Med Surg. 2022;24(7):641-650. PMID 35481810.

2. Stage the kidneys and measure blood pressure BEFORE the first dose of anything

   Aprobado / respaldado por guías

   Record creatinine, SDMA, USG, UPC, a CBC and systolic blood pressure while the cat is still hyperthyroid. This is the only chance to get a pre-treatment baseline, and it is the number against which every later decision is made. Expect creatinine to rise as euthyroidism is restored; a rise from a known baseline is interpretable, a rise from nothing is not. Never promise an owner that kidney values will stay where they are.

   Productos nombrados: No drug at this step.

   Hyperthyroidism increases renal blood flow and GFR and causes protein catabolism, which 'render traditional renal markers insensitive for the detection of CKD in cats with uncontrolled hyperthyroidism'; the same review notes that SDMA's behaviour specifically in hyperthyroid cats 'remains in its infancy' (PMID 35711100). Hyperthyroidism may mask CKD and vice versa (PMID 35481810). The magnitude of the post-treatment creatinine rise is best supported by the radioiodine cohort work: cats given standard-dose I-131 had significantly higher creatinine concentrations (P<.05) and higher percentage rises in creatinine (P<.0001) than low-dose cats at 6 months in a 189-cat prospective cohort (PMID 28158908). A SECONDARY, INCIDENTAL observation pointing the same way comes from a small case-control study whose actual subject was insulin-like growth factor 1, not the kidney: in 13 hyperthyroid cats (with 14 healthy controls) serum creatinine rose from a median 1.3 mg/dL (IQR 1.2-1.6) to 2.0 mg/dL (IQR 1.7-2.3) six months after radioiodine, P=0.006 (PMID 41170923). Cite that figure only as a small incidental finding, not as a renal cohort study.

   Yu L, Lacorcia L, Johnstone T. Hyperthyroid cats and their kidneys: a literature review. Aust Vet J. 2022;100(9):415-432. PMID 35711100. Geddes R, Aguiar J. Feline Comorbidities: Balancing hyperthyroidism and concurrent chronic kidney disease. J Feline Med Surg. 2022;24(7):641-650. PMID 35481810. Creatinine change after radioiodine: Lucy JM, Peterson ME, Randolph JF, et al. J Vet Intern Med. 2017;31(2):326-334. PMID 28158908. Incidental secondary finding in a 13-cat case-control IGF-1 study: Shin D, Chae Y, Yun T, Kang BT, Kim H. Change in insulin-like growth factor type 1 concentration after radioactive iodine treatment in cats with hyperthyroidism. J Feline Med Surg. 2025;27(12):1098612X251395870. PMID 41170923. DOI https://doi.org/10.1177/1098612X251395870.

3. Methimazole - the only FDA-approved antithyroid drug for cats. Quote its contraindications, its maximum dose and its human-safety warnings, not just its starting dose

   Aprobado / respaldado por guías

   Reversible medical control. It does not shrink or cure the nodule; it suppresses hormone production for as long as it is given, so it is lifelong unless a definitive treatment follows. Titrate to a TT4 in the lower half of the reference interval rather than to the lowest achievable value, and do not escalate past the label maximum in a cat that is not responding - escalate the diagnostic work-up instead (see the carcinoma reassessment triggers under diagnosis). Reversibility is why it is often used first, but the label does NOT support unrestricted use in the CKD-comorbid population: it CONTRAINDICATES the drug in cats with renal failure, and separately requires that use 'in cats with renal dysfunction should be carefully evaluated.' Widespread real-world use of methimazole in mildly azotaemic cats is defensible clinical practice and is what the European best-practice review and the CKD comorbidity review describe (PMID 24372075; PMID 35481810) - but it is a departure from the approved label and must be presented as an explicit extralabel clinical judgement, not as label-supported first-line therapy. Owner instructions that go with every prescription: do not break or crush the tablets; wear gloves; if a dose is missed give the next at the normal time and never double up; stop the tablet and call at once if the cat becomes ill.

   Productos nombrados: Methimazole - Felimazole Coated Tablets, NADA 141-292 (Dechra Veterinary Products), FDA-approved in the UNITED STATES, 2.5 mg and 5 mg tablets, 'For oral use in cats only'. HUMAN SAFETY, which must appear wherever this drug is named: 'Methimazole is a human teratogen and crosses the placenta.' / 'Pregnant women or women who may become pregnant, and nursing mothers should wear gloves when handling tablets, litter or bodily fluids of treated cats.' / 'Do not break or crush tablets.' / 'Wear protective gloves to prevent direct contact with litter, feces, urine, or vomit of treated cats.' / 'Wash hands with soap and water after administration.' / 'Not for use in humans. Keep out of reach of children.' The tablet coating exists to limit human exposure, which is a further reason not to break or crush it - and a reason a compounded transdermal gel, which has no such coating, carries a different human-handling risk.

   APPROVED PRODUCT (US FDA). Felimazole Coated Tablets (methimazole), NADA 141-292, Dechra Veterinary Products. INDICATION verbatim: 'Felimazole Coated Tablets (methimazole tablets) are indicated for the treatment of hyperthyroidism in cats.' STARTING DOSE verbatim: 'The starting dose of Felimazole Coated Tablets is 2.5 mg administered every 12 hours.' MAXIMUM DOSE verbatim: 'The maximum total dosage is 20 mg per day divided, not to exceed 10 mg as a single administration.' Plus: 'Cats receiving doses greater than 10 mg per day should be monitored more frequently.' CONTRAINDICATIONS verbatim: 'Do not use in cats with hypersensitivity to methimazole, carbimazole or the excipient, polyethylene glycol. Do not use in cats with primary liver disease or renal failure. Do not use in cats with autoimmune disease. Do not use in cats with hematological disorders (such as anemia, neutropenia, lymphopenia, or thrombocytopenia) or coagulopathies. Do not use in pregnant or lactating queens.' RENAL PRECAUTION verbatim: 'Use of Felimazole Coated Tablets in cats with renal dysfunction should be carefully evaluated. Reversal of hyperthyroidism may be associated with decreased glomerular filtration rate and a decline in renal function, unmasking the presence of underlying renal disease.' MONITORING SCHEDULE verbatim: 'Hematology, biochemistry, and TT4 should be evaluated prior to initiating treatment and monitored after 3 weeks and 6 weeks of treatment. Thereafter, bloodwork should be monitored every 3 months and the dose adjusted as necessary.' SERIOUS-REACTION SURVEILLANCE verbatim: 'Due to potentially serious adverse reactions such as hepatopathy, immune-mediated anemia, thrombocytopenia, and agranulocytosis, cats on methimazole therapy should be monitored closely for any sign of illness including anorexia, vomiting, head/facial pruritus or edema, depression/lethargy, weight loss, anemia, skin lesions, diarrhea, fever, or lymphadenopathy.' STOP RULE verbatim: 'If a cat becomes ill while on Felimazole Coated Tablets, the drug should be stopped and appropriate hematological and biochemical testing should be done.' BLEEDING verbatim: 'Methimazole has anti-vitamin K activity and may induce bleeding diathesis without evidence of thrombocytopenia.' INTERACTIONS verbatim: 'Anticoagulants may be potentiated by the anti-vitamin K activity of Felimazole Coated Tablets. Concurrent use of phenobarbital may reduce the clinical effectiveness of Felimazole Coated Tablets.' / 'A reduction in dose of certain drugs (beta-adrenergic blocking agents, digitalis glycosides, and theophylline) may be needed when the patient becomes euthyroid.' / 'Methimazole is known to reduce the hepatic oxidation of benzimidazole anthelmintics (e.g. fenbendazole), leading to increased plasma concentration of these anthelmintics when administered concurrently.' BREEDING MALES verbatim: 'Felimazole Coated Tablets caused delayed maturation of the testes in young male cats in the 12-week safety study. The safety of Felimazole Coated Tablets has not been evaluated in male cats intended for breeding.' LABEL ADVERSE REACTIONS: in a US field study of 113 cats the most common were change in food consumption, lethargy, vomiting, diarrhoea/loose stool, skin lesions and abnormal vocalisation; three cats were withdrawn early, one for 'unmasking of latent renal disease' and two for skin lesions; and in the extended-use study 'Eighteen cats died or euthanized during the extended use study, four of which may have been related to Felimazole Coated Tablets due to the unmasking/acceleration of chronic renal failure.' NO MINIMUM AGE OR WEIGHT is stated on this label - do not publish one. EFFICACY AND HARM FROM THE SAME STUDY, reported together: in 262 hyperthyroid cats, 2-3 weeks of methimazole at 10-15 mg/day reduced mean serum T4 from 12.1 to 2.1 ug/dL (P<0.001), and the maintenance dose in the 81 cats treated with methimazole alone ranged 2.5-20 mg/day (mean 11.9 mg/day). That mean reflects 1988 dosing practice and MUST NOT be read as a target: modern practice starts at 2.5 mg q12h and titrates (PMID 24372075), and the label ceiling is 20 mg/day divided with no single dose above 10 mg. The SAME paper established this drug's toxicity profile: clinical side effects in 48 of 262 cats (18.3%), usually within the first month - anorexia, vomiting, lethargy, self-induced excoriation of the face and neck, bleeding diathesis, and icterus from hepatopathy; mild haematological abnormalities in 43 of 262 (16.4%), usually within the first two months - eosinophilia, lymphocytosis, slight leukopenia; and SERIOUS haematological reactions in 10 of 262 (3.8%) - agranulocytosis and thrombocytopenia with bleeding, which resolved within a week of stopping the drug. Antinuclear antibodies developed in 52 of 238 tested (21.8%) and red cell autoantibodies in 3 of 160 tested (1.9%) (PMID 3265728). A European expert best-practice review recommends a starting dose of 2.5 mg methimazole twice daily, titrated to a TT4 in the lower half of the reference interval, with monitoring for side effects 'especially during the first months of treatment', notes that 'Some side-effects may require discontinuation of treatment', and once euthyroidism is achieved recommends monitoring visits at 1 month, 3 months and biannually thereafter with attention to azotaemia, hypertension and iatrogenic hypothyroidism (PMID 24372075).

   Felimazole Coated Tablets label, NADA 141-292 (Dechra Veterinary Products), DailyMed setid 2d70b747-314c-4baa-95c4-1734855a1556, read at source 2026-09-12. Efficacy AND safety: Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. J Vet Intern Med. 1988;2(3):150-7. PMID 3265728. Dosing and monitoring practice: Daminet S, Kooistra HS, Fracassi F, Graham PA, Hibbert A, Lloret A, Mooney CT, Neiger R, Rosenberg D, Syme HM, Villard I, Williams G. Best practice for the pharmacological management of hyperthyroid cats with antithyroid drugs. J Small Anim Pract. 2014;55(1):4-13. PMID 24372075.

4. Radioiodine (I-131) - the definitive, usually curative treatment, and the household gets written radiation-safety instructions

   Aprobado / respaldado por guías

   A single subcutaneous or oral dose of I-131 is selectively taken up by hyperfunctioning thyroid tissue and ablates it. It requires a licensed radiation facility and a period of hospitalisation with regulated handling of litter and waste. It treats ectopic thyroid tissue that surgery cannot reach. Dose is individualised. In mild-to-moderate disease a lower dose produced substantially less iatrogenic hypothyroidism, and the trial did not detect a difference in cure rate - but absence of a detected difference in a non-randomised, unbalanced comparison is not proof of equivalence, and severe hyperthyroidism, large thyroid volume and suspected carcinoma were not studied and are not addressed by this evidence. The cat also goes home radioactive: the household, not only the facility, has obligations.

   Productos nombrados: Sodium iodide I-131. Administered only at a licensed radiopharmaceutical facility under local radiation-safety regulation. Not a product a pharmacy or online retailer can supply, and not something an owner ever handles. We did NOT verify a US NADA or ANADA for a veterinary sodium iodide I-131 product in this research pass - do not publish an approval number for it.

   Prospective NON-RANDOMISED, unbalanced cohort of 189 cats with MILD-TO-MODERATE hyperthyroidism (T4 >= 4.0 and < 13.0 ug/dL) treated with low-dose 2 mCi (n=150) or standard-dose 4 mCi (n=39) I-131. Persistent hyperthyroidism at 3 months: 0% on the 4 mCi STANDARD dose versus 5.3% on the 2 mCi LOW dose, P=.34; at 6 months 0% on 4 mCi versus 3.3% on 2 mCi, P=.51. Read those group labels carefully - the source lists standard dose first. The correct statement is that the study DID NOT DETECT a difference in cure rate, not that the cure rate was the same; with 39 cats in the comparator arm and p-values of .34 and .51 this is an under-powered non-inferiority question that the design cannot settle. The authors' own headline efficacy statement is a cure rate of >95% with the low dose. THE HARM SIGNAL IS THE POINT OF THE PAPER: at 6 months, overt hypothyroidism was significantly MORE common after the 4 mCi standard dose (18% versus 1%, P=.0005) and so was subclinical hypothyroidism (46% versus 21%, P=.004); standard-dose cats had higher creatinine concentrations (P<.05) and higher percentage rises in creatinine (P<.0001), with no difference in the incidence of azotaemia between groups. Authors' conclusion verbatim in substance: low-dose I-131 is safe and effective for cats with mild-to-moderate hyperthyroidism, with reduced frequency of iatrogenic hypothyroidism and azotaemia (PMID 28158908). Nothing in this paper licenses a universal 'lowest dose always' rule outside the severity band enrolled. HOUSEHOLD RADIATION SAFETY: post-discharge handling is the one genuine human radiation exposure in this protocol. The treated cat goes home with WRITTEN instructions from the treating facility covering a defined period of restricted close contact, separate handling and retention of litter, and stricter limits for pregnant women and children. These requirements are set by the local radiation-safety authority and VARY BY JURISDICTION, so the owner must be given the facility's own written instructions and must be told the isolation and litter requirements BEFORE committing. We do not publish a specific number of days, because we have not verified one for any given jurisdiction.

   Lucy JM, Peterson ME, Randolph JF, Scrivani PV, Rishniw M, Davignon DL, Thompson MS, Scarlett JM. Efficacy of Low-dose (2 millicurie) versus Standard-dose (4 millicurie) Radioiodine Treatment for Cats with Mild-to-Moderate Hyperthyroidism. J Vet Intern Med. 2017;31(2):326-334. PMID 28158908. Curative-pathway framing and the benign-disease restriction: Yu L, Lacorcia L, Johnstone T. Aust Vet J. 2022;100(9):415-432. PMID 35711100.

5. Surgical thyroidectomy - effective where radioiodine is unavailable, but render the cat euthyroid first and do scintigraphy first

   Ensayo controlado en esta especie

   A curative option in experienced hands, and the route that yields histopathology (which is how thyroid carcinoma gets identified). It cannot address ectopic thyroid tissue, and operating on a cat that has ectopic tissue predicts recurrence. Two pre-operative obligations that the older version of this protocol under-specified. First, MEDICAL STABILISATION: the standard approach is to render the thyrotoxic cat euthyroid with an antithyroid drug before anaesthesia rather than operating on an uncontrolled thyrotoxic patient - and note that the label itself warns that doses of beta-adrenergic blocking agents, digitalis glycosides and theophylline may need reducing as the patient becomes euthyroid. Second, ELECTROLYTES AND CARDIAC ASSESSMENT: hypokalaemia is common pre-operatively and should be identified and corrected, and a thyrotoxic cat of this age warrants cardiac assessment and an anaesthetic plan built around minimising adverse cardiovascular effects. Post-operative parathyroid injury causing hypocalcaemia is the main early complication; it requires serial calcium monitoring in hospital AND an owner briefed on the home signs of hypocalcaemic tetany for the first 72 hours after discharge (see red flags).

   Productos nombrados: Surgical procedure, no drug for the procedure itself. Pre-operative antithyroid drug: methimazole (Felimazole, NADA 141-292) - see that step for its full contraindications and human-safety warnings. Anaesthetic protocol should minimise adverse cardiovascular effects in a thyrotoxic patient; we name no specific anaesthetic agent or protocol because we did not verify one.

   Retrospective series of 101 hyperthyroid cats, modified intracapsular dissection. Pre-operative scintigraphy found ectopic hyperplastic thyroid tissue (EHTT) in 9 cats. Two cats died within 3 days of surgery; 5 of 86 developed transient post-operative hypocalcaemia; 29 of 91 were HYPOKALAEMIC pre-operatively. Thyroid carcinoma was found on histology in 3 of 88 cats. Hyperthyroidism recurred in 5 cats between 3 and 59 months, and 4 of those 5 had EHTT pre-operatively (P<.001). Authors' conclusion: 'Complications were uncommon after thyroidectomy performed by an experienced surgeon when combined with an anesthetic regimen associated with minimal adverse cardiovascular effects'; pre-operative thyroid scintigraphy is advised; and 'Surgery is not recommended when EHTT is present, because of a higher chance of developing recurrent disease' (PMID 16635010). The pre-operative medical-stabilisation practice is directly evidenced in the largest methimazole series, in which 181 of 262 cats received methimazole 'as a preoperative preparation for thyroidectomy' for 7 to 130 days (mean 27.7 days) (PMID 3265728). WE DID NOT LOCATE a controlled feline trial of beta-blockade for thyrotoxic tachycardia and therefore publish NO beta-blocker efficacy claim, dose, or heart-rate threshold for proceeding to anaesthesia.

   Naan EC, Kirpensteijn J, Kooistra HS, Peeters ME. Results of thyroidectomy in 101 cats with hyperthyroidism. Vet Surg. 2006;35(3):287-93. PMID 16635010. Pre-operative medical preparation: Peterson ME, Kintzer PP, Hurvitz AI. J Vet Intern Med. 1988;2(3):150-7. PMID 3265728.

6. Measure, treat and re-measure the blood pressure - the protocol that diagnoses hypertension must also treat it

   Ensayo controlado en esta especie

   Check systolic blood pressure at diagnosis and again after euthyroidism is restored, with a FUNDIC EXAMINATION if it is high, and treat hypertension that persists on repeated properly taken measurements. Roughly half of hypertensive hyperthyroid cats normalise after successful treatment and the other half do not - and a small number become hypertensive only afterwards, so this cannot be a one-off measurement at diagnosis. Two separate things need distinguishing: chronic hypertension, which is managed with a daily oral antihypertensive and repeated BP checks; and a hypertensive crisis with acute target-organ damage - sudden blindness, retinal haemorrhage or detachment, neurological signs - which is an emergency managed differently and urgently, and which is the reason sudden blindness sits in the red flags. Situational ('white-coat') hypertension is real and common in this population, so a single high reading in a nervous cat is not a diagnosis and is not a reason to start a drug.

   Productos nombrados: JURISDICTION MATTERS HERE AND MUST BE STATED. (1) TELMISARTAN - Semintra (telmisartan oral solution), NADA 141-501, Boehringer Ingelheim Animal Health USA Inc. This is FDA-APPROVED IN THE UNITED STATES; label indication verbatim: 'SEMINTRA is indicated for the control of systemic hypertension in cats.' Label dose verbatim: 'The initial dose of SEMINTRA is 1.5 mg/kg (0.68 mg/lb) orally twice daily for 14 days, followed by 2 mg/kg (0.91 mg/lb) orally once daily.' AGE LIMIT verbatim: 'the safe use of SEMINTRA has not been evaluated in cats less than 9 months of age.' CONTRAINDICATION verbatim: 'Do not use in cats with a hypersensitivity to telmisartan.' HUMAN SAFETY verbatim: 'Not for human use. Keep out of reach of children. Pregnant women should avoid contact with SEMINTRA because substances that act on the renin-angiotensin-aldosterone system (RAAS) such as angiotensin receptor blockers (ARBs) can cause fetal and neonatal morbidity and death during pregnancy in humans.' (2) AMLODIPINE BESYLATE is identified as the treatment of choice for feline systemic hypertension in the ISFM consensus guidelines and has the strongest feline RCT, but we did NOT locate a US FDA feline approval for amlodipine in this research pass - so it must be described as evidence-supported and guideline-preferred, never as 'approved', and its use in a US cat is extralabel from a human product. Dose individualisation is required for either drug; the ISFM guidelines note the effective amlodipine dose varies between cats. Do not publish an amlodipine NADA number.

   Prospective study of 401 hyperthyroid NON-AZOTAEMIC cats: 108 (27%) were hypertensive (systolic BP >= 160 mmHg) before treatment, against a literature-reported prevalence range of 7% to 48%. Of 87 initially hypertensive cats re-examined after successful radioiodine, 43 (49%) normalised and 44 (51%) remained hypertensive; 16 of 168 initially normotensive cats (9.5%) first developed hypertension after treatment. Persistent or new hypertension was NOT related to azotaemia or iatrogenic hypothyroidism, and a significantly higher proportion of hypertensive cats were nervous or excited than normotensive cats (47% vs 12% before treatment, P<.001; 50% vs 17% among those remaining hypertensive, P<.001) - the authors conclude that many of these cats 'might have "situational" hypertension, as hyperthyroid-induced hypertension should resolve after treatment' (PMID 38440934). The ISFM consensus guidelines state that BP must be measured 'as accurately as possible with a reproducible technique', that the clinical consequences of hypertension relate to target-organ damage to 'eye, heart and vasculature, brain and kidneys', that routine BP monitoring 'is generally performed infrequently, probably leading to underdiagnosis', that 'amlodipine besylate is the treatment of choice to manage feline hypertension and is effective in the majority of cats, but the dose needed to successfully manage hypertension varies between individuals', that 'Some cats require long-term adjuvant therapy' and that 'occasionally, additional therapy is necessary for emergency management of hypertensive crises' (PMID 28245741). The best feline antihypertensive efficacy data are for amlodipine: randomised, double-blinded, placebo-controlled trial in 77 client-owned hypertensive cats (median age 14 years), 0.125-0.50 mg/kg once daily for 28 days, 61 completing. Responder rate 63% on amlodipine versus 18% on placebo, odds of response 7.9x (95% CI 2.6-24.1, P<.001); mean systolic BP fell from 181 (+/-12) mmHg to 154 (+/-17) mmHg on amlodipine versus 170 (+/-21) mmHg on placebo (P<.001); voluntary acceptance of the chewable formulation 73%; the authors note it 'can be used concomitantly with angiotensin-converting enzyme inhibitors and in cats with chronic kidney disease' (PMID 25857394). Note the 18% PLACEBO response rate - untreated hypertensive cats 'respond' nearly one time in five on a single-timepoint definition, which is why uncontrolled testimonials are worthless in this disease.

   Stammeleer L, Xifra P, Serrano SI, Rishniw M, Daminet S, Peterson ME. Blood pressure in hyperthyroid cats before and after radioiodine treatment. J Vet Intern Med. 2024;38(3):1359-1369. PMID 38440934. Taylor SS, Sparkes AH, Briscoe K, Carter J, Sala SC, Jepson RE, Reynolds BS, Scansen BA. ISFM Consensus Guidelines on the Diagnosis and Management of Hypertension in Cats. J Feline Med Surg. 2017;19(3):288-303. PMID 28245741. Huhtinen M, Derre G, Renoldi HJ, Rinkinen M, Adler K, Aspegren J, Zemirline C, Elliott J. Randomized placebo-controlled clinical trial of a chewable formulation of amlodipine for the treatment of hypertension in client-owned cats. J Vet Intern Med. 2015;29(3):786-93. PMID 25857394. Semintra label: NADA 141-501 (Boehringer Ingelheim Animal Health USA Inc.), DailyMed setid f436a826-9f5b-4269-b4ec-97d5522eaca6, read at source 2026-09-12.

7. Detect and correct iatrogenic hypothyroidism after treatment - measure TSH and creatinine, not just T4

   Ensayo controlado en esta especie

   This is the most consequential and most frequently skipped step in feline hyperthyroid care. Overshooting into hypothyroidism is a recognised complication of all four modalities, is associated with azotaemia, and is largely invisible if only TT4 is measured. Where the cat is medically managed, reduce the antithyroid dose. Where the gland has been ablated, consider levothyroxine supplementation - but understand what the evidence does and does not show, because the two best sources do not agree, and say so rather than smoothing it over.

   Productos nombrados: Levothyroxine sodium for DOCUMENTED iatrogenic hypothyroidism. JURISDICTION AND SPECIES MATTER: the FDA-approved levothyroxine sodium products we located are labelled for DOGS (canine hypothyroidism). We did NOT verify any FDA-approved FELINE levothyroxine product in this research pass, so use in a cat is extralabel from a canine or human product under AMDUCA and must be described that way - do NOT publish a NADA number for feline levothyroxine, and do not write 'approved' unqualified. Document the hypothyroidism before supplementing (paired TT4 and TSH, or a TSH stimulation test), and monitor TT4, TSH and creatinine afterwards. Where the cat is on methimazole or carbimazole, the first move is to REDUCE the antithyroid dose, not to add levothyroxine on top.

   (1) In 75 hyperthyroid cats documented euthyroid 6 months after treatment, azotaemia was significantly more common in the hypothyroid group (16/28) than the euthyroid group (14/47), P=.028; 28 of 41 cats (68%) with TT4 below reference range had raised TSH; and - this is the comparison the older draft of this protocol misstated - hypothyroid cats that DEVELOPED AZOTAEMIA had significantly shorter survival (median 456 days, range 231-1589) than hypothyroid cats that REMAINED NON-AZOTAEMIC (median 905 days, range 316-1869), P=.018. That is a within-hypothyroid comparison in which azotaemia is the survival discriminator; it is not a hypothyroid-versus-euthyroid survival result, and n=80 across two retrospective arms (PMID 20695989). (2) In 19 methimazole- or carbimazole-treated cats with documented iatrogenic hypothyroidism (subnormal TT4 plus raised TSH), reducing the dose to restore euthyroidism (TT4 10-40 nmol/L) significantly decreased plasma creatinine (median 2.61 to 2.07 mg/dL, P<.001) - though body weight also fell significantly (P=.019), which the authors flag as a confounder, and heart rate and alkaline phosphatase rose (PMID 24773059). (3) Prospective cohort of 117 radioiodine-treated cats in which hypothyroidism was diagnosed by TSH STIMULATION TEST, not by a single TSH: euthyroid azotaemic cats (median survival 934 days) and non-supplemented hypothyroid cats (azotaemic and non-azotaemic combined, 1232 days) had shorter survival than euthyroid non-azotaemic cats (1616 days), P=.003 and P=.002. Levothyroxine-supplemented hypothyroid NON-AZOTAEMIC cats survived longer than non-supplemented ones (1037 vs 768 days, P=.027), but supplementation was NOT associated with prolonged survival in hypothyroid AZOTAEMIC cats (771 vs 152 days, P=.991). The authors state plainly that randomised controlled trials are still needed (PMID 39831449). THE HONEST TENSION, WHICH MUST BE PUBLISHED RATHER THAN RESOLVED BY US: the CKD comorbidity review states that 'Therapy with levothyroxine is recommended for cats that are hypothyroid and azotaemic' (PMID 35481810), while the 117-cat prospective cohort found a survival benefit only in hypothyroid NON-azotaemic cats and none in hypothyroid azotaemic cats (PMID 39831449). Present both; do not pick one and present it as settled. (5) THE REASON TT4 ALONE IS NOT ENOUGH, QUANTIFIED: in an evaluation of 312 cats (169 untreated hyperthyroid, 53 treated with radioiodine, 12 with CKD, 78 clinically healthy), test sensitivity for identifying I-131-induced hypothyroidism was only 45.5% for T4 versus 100.0% for TSH by either the canine chemiluminescent assay or a more sensitive bulk-acoustic-wave assay (P=.03); positive predictive value was 71.9% for T4, 81.2% for TSH-CLIA and 100% for TSH-BAW (PMID 38382201). A cat monitored on T4 alone after radioiodine will have iatrogenic hypothyroidism missed more often than it is found. Note the instrument: feline TSH is measured with a CANINE TSH immunoassay (PMID 17641276), and the chemiluminescent versions in routine use cannot resolve below 0.03 ng/mL - so read TSH as suppressed versus detectable, not as a feline number. (4) Confirming hypothyroidism in a cat is not straightforward: repeat TT4, free T4 and TSH, a TSH stimulation test, or scintigraphy may be needed, and false positives and negatives occur with all thyroid function tests, especially with concurrent non-thyroidal illness (PMID 23975586).

   Williams TL, Elliott J, Syme HM. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. J Vet Intern Med. 2010;24(5):1086-92. PMID 20695989. Williams TL, Elliott J, Syme HM. Effect on renal function of restoration of euthyroidism in hyperthyroid cats with iatrogenic hypothyroidism. J Vet Intern Med. 2014;28(4):1251-5. PMID 24773059. Cox SE, Wakeling J, Hall T, Williams TL. Survival of radioiodine treated hyperthyroid cats that are euthyroid and hypothyroid after treatment, and effect of levothyroxine supplementation on survival time of cats with iatrogenic hypothyroidism. J Vet Intern Med. 2025;39(1):e17295. PMID 39831449. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: hypothyroidism. Compend Contin Educ Vet. 2013;35(8):E4. PMID 23975586. Geddes R, Aguiar J. J Feline Med Surg. 2022;24(7):641-650. PMID 35481810. TSH assay performance and the failure of T4-alone monitoring: Peterson ME, Dougherty E, Rishniw M. Evaluation of a novel, sensitive thyroid-stimulating hormone assay as a diagnostic test for thyroid disease in cats. Am J Vet Res. 2024;85(5). PMID 38382201. Peterson ME, Guterl JN, Nichols R, Rishniw M. Evaluation of Serum Thyroid-Stimulating Hormone Concentration as a Diagnostic Test for Hyperthyroidism in Cats. J Vet Intern Med. 2015;29(5):1327-34. PMID 26192742. Heterologous canine TSH assay applied to feline serum: Ferguson DC, Caffall Z, Hoenig M. J Endocrinol. 2007;194(2):267-73. PMID 17641276.

8. Iodine-restricted therapeutic diet - a genuine option with honest, specific limits, and real household constraints

   Evidencia preliminar

   Exclusive feeding of a commercially prepared iodine-restricted food lowers TT4 in most cats without a tablet, injection or anaesthetic. It is a CONTROL strategy, not a cure: the nodule continues to grow. Three hard constraints must be stated to the owner. (1) EXCLUSIVITY: it only works if the diet is the ONLY thing the cat eats - no treats, no other cats' food, no hunting, no table scraps - and it fails silently, with no signal to the owner until TT4 is rechecked. (2) IT MAY NOT FULLY CONTROL THE THYROTOXIC STATE: normalising TT4 on this diet does not clearly prove the disease is controlled. (3) HOUSEHOLD AND DIET CONFLICTS: the food works by restricting iodine, which is an ESSENTIAL nutrient, so it is not an appropriate maintenance food for a cat that does not need iodine restriction - feed it only to the diagnosed cat, and follow the manufacturer's own feeding instructions, which are the authority on which animals in the household must not be fed it. In a multi-cat household where bowls cannot be separated, and in a cat with outdoor access that hunts, this option is a poor fit. And a cat that needs BOTH iodine restriction and a phosphorus-restricted renal diet cannot be on both foods at once; that trade-off is a veterinary decision, not something to solve by mixing foods.

   Productos nombrados: Therapeutic iodine-restricted diet (the studied product in PMID 24232246 and PMID 26081922 is a commercially available iodine-restricted therapeutic food; the 2014 study was Hill's-affiliated). REGULATORY FRAMING, CORRECTED - the absence of a NADA here is NOT reassurance. No NADA applies because the product is marketed as a FOOD rather than as a drug; that is precisely why disease-treatment claims for it require care, because describing a food as treating a diagnosed disease with efficacy data is what can render it an unapproved new animal drug. Any PetSmartMeds page naming this option must describe it as a veterinary therapeutic food used under veterinary direction, and must not carry a drug-style efficacy claim. WE DID NOT READ a specific manufacturer's feeding-restriction label in this research pass; the household restriction above is stated on the mechanistic ground that iodine is an essential nutrient, and the manufacturer's own feeding instructions remain the authority.

   TIERED 3, NOT 2, DELIBERATELY: MULTIPLE STUDIES, NONE BLINDED OR PLACEBO-CONTROLLED. This option is not of equal evidentiary standing to radioiodine or surgery and must not be presented as if it were. (1) Prospective, multicentre, NON-CONTROLLED, OPEN-LABEL, manufacturer-affiliated study of 225 client-owned cats (median age 15 y): TT4 fell by week 4 (P<0.0001) and was within reference range in 56 of 88 cats at week 4 and 51 of 68 at week 8; vomiting, PU/PD, hyperactivity, polyphagia, weight loss, coat quality and quality of life all improved by week 4 (P<0.0001); creatinine decreased from week 0 to week 4 (P=0.001) (PMID 24232246; two of five authors were Hill's Pet Nutrition employees - disclose that). (2) Retrospective series of 49 cats: TT4 normalised in 20 of 48 (42%) at 21-60 days and 39 of 47 (83%) at 61-180 days, but body weight did NOT significantly increase (P=.34) and heart rate did NOT significantly decrease (P=.64) - 'not all clinical signs of hyperthyroidism improved' (PMID 26081922). (3) Twelve-month study in 8 cats with TT4 > 6.0 ug/dL: 6 of 8 normalised TT4 by 4 weeks; the two that did not had the highest baseline TT4; 3 of 8 were WITHDRAWN because CKD emerged (PMID 30859084). (4) Controlled but NON-RANDOMISED 3-arm trial in 34 cats - iodine-restricted food (group A, n=14) versus transdermal methimazole in PLO (group B, n=11) versus oral methimazole (group C, n=9). TT4 fell significantly in all three groups. Creatinine rose significantly only in the two methimazole groups. THE LIVER-ENZYME FINDING MUST BE REPORTED ACCURATELY AND NOT ATTRIBUTED TO THE DIET ALONE: verbatim, 'Liver enzyme activities decreased significantly only in group B, while no significant decreases were detected in groups A and C at any time' - i.e. enzymes failed to improve in BOTH the diet arm AND the ORAL methimazole arm, and improved only in the transdermal arm. The authors' interpretation, which is worth giving clients as the honest limit of the dietary option, is that the diet 'failed to improve liver enzymes abnormalities' and that 'These findings could indicate a persistent hyperthyroid state in cats treated with iodine-restricted food despite normalization of serum total thyroxine concentrations.' But arms of 9 to 14 cats cannot settle this: a non-significant result in an arm of 9 is weak evidence of anything, and the earlier draft of this protocol wrongly presented the finding as specific to the diet (PMID 31998612).

   van der Kooij M, Becvarova I, Meyer HP, Teske E, Kooistra HS. Effects of an iodine-restricted food on client-owned cats with hyperthyroidism. J Feline Med Surg. 2014;16(6):491-8. PMID 24232246. Hui TY, Bruyette DS, Moore GE, Scott-Moncrieff JC. Effect of Feeding an Iodine-Restricted Diet in Cats with Spontaneous Hyperthyroidism. J Vet Intern Med. 2015;29(4):1063-8. PMID 26081922. Loftus JP, DeRosa S, Struble AM, Randolph JF, Wakshlag JJ. One-year study evaluating efficacy of an iodine-restricted diet for the treatment of moderate-to-severe hyperthyroidism in cats. Vet Med (Auckl). 2019;10:9-16. PMID 30859084. Grossi G, Zoia A, Palagiano P, Leoni N, Bubini-Regini F, Malerba E, Peli A, Biagi G, Fracassi F. Iodine-restricted food versus pharmacological therapy in the management of feline hyperthyroidism: A controlled trial in 34 cats. Open Vet J. 2019;9(3):196-204. PMID 31998612.

9. Transdermal methimazole for the cat that genuinely cannot be pilled - compounded, slower, legally an unapproved new animal drug

   Evidencia preliminar

   Methimazole in pluronic lecithin organogel applied to the inner pinna is an option when oral dosing is genuinely impossible. It achieves control more slowly than the oral route and is a COMPOUNDED preparation, not the FDA-approved product. Owners must wear gloves and rotate ears - and note that this formulation has none of the tablet coating that exists on the approved product specifically to limit human exposure, so the human-handling precautions are if anything more important here, not less. All of the methimazole contraindications, monitoring requirements and human teratogen warnings from the approved-product step apply unchanged.

   Productos nombrados: Compounded methimazole in pluronic lecithin organogel (PLO). This is a compounded formulation, NOT Felimazole and NOT covered by NADA 141-292, and in the US it is an unapproved new animal drug permitted only under the enforcement discretion described in FDA CVM GFI #256. Same human-safety warnings as the tablet and then some: methimazole is a human teratogen that crosses the placenta, gloves must be worn to apply the gel and to handle the treated cat's litter, faeces, urine and vomit, and pregnant women, women who may become pregnant and nursing mothers should not handle the gel or those materials without gloves. There is no protective coating on a gel.

   Randomised trial in 47 newly diagnosed hyperthyroid cats (44 completed: 17 oral, 27 transdermal), 2.5 mg q12h by either route, evaluated at weeks 0, 2 and 4 with physical exam, weight, CBC, biochemistry, urinalysis, TT4 and indirect Doppler blood pressure. At 2 weeks, significantly MORE oral-treated cats had TT4 within reference range (14/16 vs 14/25, P=.027); the difference was no longer significant at 4 weeks (9/11 vs 14/21), which the authors attribute to possibly inadequate numbers evaluated by then. Gastrointestinal adverse effects were significantly MORE frequent with oral methimazole (4/17 vs 1/27, P=.04), with no difference in neutropenia, hepatotoxicity or facial excoriation. Authors' conclusion: overall efficacy of the transdermal route is not as high as oral at 2 weeks, but it causes fewer GI adverse effects (PMID 15515580). REGULATORY POSITION, STATED COMPLETELY: in the United States a drug compounded from bulk drug substances is legally an UNAPPROVED NEW ANIMAL DRUG, tolerated only under FDA enforcement discretion as set out in CVM GFI #256, 'Compounding Animal Drugs from Bulk Drug Substances' (final, August 2022). The conditions of that enforcement discretion narrow sharply when an FDA-APPROVED product already exists for the same indication in the same species - which it does here: Felimazole, NADA 141-292, is approved for the treatment of hyperthyroidism in cats. Compounded transdermal methimazole is therefore appropriate only on an individual-patient clinical need that the approved product cannot meet (a cat that genuinely cannot be dosed orally), documented as such - and it must never be offered as a general convenience alternative or promoted as equivalent to the approved tablet.

   Sartor LL, Trepanier LA, Kroll MM, Rodan I, Challoner L. Efficacy and safety of transdermal methimazole in the treatment of cats with hyperthyroidism. J Vet Intern Med. 2004;18(5):651-5. PMID 15515580. Regulatory: FDA CVM Guidance for Industry #256, Compounding Animal Drugs from Bulk Drug Substances (final, August 2022). Approved comparator: Felimazole Coated Tablets label, NADA 141-292 (Dechra Veterinary Products), DailyMed setid 2d70b747-314c-4baa-95c4-1734855a1556, read at source 2026-09-12.

10. NON-US ANNEX: carbimazole (sustained-release) where it is licensed - a methimazole pro-drug, NOT available in the United States

    Solo evidencia de laboratorio

    THIS STEP IS NOT A US OPTION AND MUST NOT BE SHOWN TO A US CLINIC OR OWNER AS ONE. Carbimazole is converted to methimazole in vivo. A once-daily sustained-release formulation is veterinary-licensed in the UK and parts of Europe and is a reasonable alternative there where owner dosing frequency is the limiting factor. There is NO FDA-approved carbimazole product for cats in the United States. Note also that the owner-preference data do not support once-daily dosing as a priority: in a 111-owner survey, NO owner ranked once-daily treatment as most important, 79% said they were or would be happy to dose twice daily, and for 62% twice-daily pilling was not a problem - so the convenience argument for this formulation is weaker than it looks.

    Productos nombrados: Carbimazole sustained-release (marketed in the UK as Vidalta, MSD Animal Health). NOT FDA-approved in the United States for any species or indication we verified. Marketing-authorisation number NOT verified in any jurisdiction - do not publish one. Never write 'approved' for this product without naming the regulator and the country. Human-safety handling should be treated as for methimazole, since carbimazole is converted to methimazole in vivo.

    Named as a UK veterinary-licensed oral antithyroid medication (Vidalta, MSD Animal Health) alongside Felimazole (Dechra) in a survey of 111 owners of hyperthyroid cats, with 72-75% owner-assessed clinical success; 27% of the cats in that survey had concurrent CKD (PMID 23591627). Recommended starting dose in the European best-practice review: 10-15 mg once daily for the sustained-release carbimazole formulation, titrated to a TT4 in the lower half of the reference interval (PMID 24372075). WE DID NOT RETRIEVE A MARKETING-AUTHORISATION NUMBER for carbimazole in any jurisdiction in this research pass, and none should be published until it is obtained. Because we cannot cite its authorisation and it has no US approval at all, it is tiered 4 and confined to this annex - it must not outrank radioiodine, methimazole or surgery on a US-facing page. The methimazole label's contraindication list applies here by pharmacology and by label: Felimazole is contraindicated in cats 'with hypersensitivity to methimazole, carbimazole or the excipient, polyethylene glycol' - so a carbimazole reaction is a methimazole contraindication and vice versa.

    Caney SMA. An online survey to determine owner experiences and opinions on the management of their hyperthyroid cats using oral anti-thyroid medications. J Feline Med Surg. 2013;15(6):494-502. PMID 23591627. Dosing: Daminet S, Kooistra HS, Fracassi F, et al. J Small Anim Pract. 2014;55(1):4-13. PMID 24372075. Cross-hypersensitivity: Felimazole Coated Tablets label, NADA 141-292 (Dechra Veterinary Products), DailyMed setid 2d70b747-314c-4baa-95c4-1734855a1556, read at source 2026-09-12.

11. What we are NOT recommending, and why

    No se localizó evidencia

    Beta-blockade for thyrotoxic tachycardia is used adjunctively in clinical practice, but we did not locate a controlled feline trial of it in this research pass and will not publish an efficacy claim, a dose, or a heart-rate threshold. No stem cell, exosome, extracellular-vesicle or peptide product has any role here (see the biologic assessment). No supplement, iodine drop, herbal product or 'thyroid support' formula has published controlled evidence in hyperthyroid cats, and in a cat whose treatment depends on either iodine restriction or radioiodine uptake, an unmeasured iodine supplement is not merely useless - it can work against the treatment. Nothing on this list is an alternative to the established modalities, and nothing on this list should delay them.

    Productos nombrados: None. No product recommended at this step.

    NO EVIDENCE LOCATED for any of these in cats with hyperthyroidism. The intersection of (mesenchymal stem cells OR exosomes OR extracellular vesicles) AND cats AND (hyperthyroidism OR thyroid) returned exactly ONE PubMed record (searched 2026-09-12), and it is a technetium-99m biodistribution study of equine peripheral-blood MSCs in healthy cats and cats with chronic gingivostomatitis in which 'thyroid' appears only as a site of FREE technetium accumulation - not a thyroid treatment (PMID 42655747). No record combined BPC-157, TB-500 or thymosin beta-4 with feline thyroid disease. No controlled feline trial of beta-blockade for thyrotoxicosis was located. No controlled trial of any supplement, herbal product or iodine preparation in hyperthyroid cats was located.

    Verification search run against PubMed 2026-09-12. Sole intersecting record: Beerts C, Debosschere Y, Tack L, Pauwelyn G, Carlier S, Depuydt E, Saunders JH, Peremans K, Spaas JH. The Biodistribution of Tc Labelled Equine Peripheral Blood Mesenchymal Stem Cells in Healthy and Chronic Gingivostomatitis Cats. Vet Sci. 2026;13(8):727. PMID 42655747. (Most authors are employees of Boehringer Ingelheim Veterinary Medicine Belgium NV - disclose that.)

Cómo se diagnostica

• Full physical examination with an accurate weight and body condition score, heart rate and auscultation for murmur or gallop, palpation of the ventral neck for a thyroid nodule (recording its size, mobility and whether it is uni- or bilateral), and an indirect (Doppler or oscillometric) systolic blood pressure. Blood pressure must be interpreted with the situational-hypertension caveat: 108 of 401 (27%) untreated hyperthyroid non-azotaemic cats had systolic BP >= 160 mmHg, reported prevalence in the literature ranges 7-48%, and 47% of the hypertensive cats versus 12% of normotensive cats were nervous or excited (P<.001) - so one high reading is not a diagnosis and repeated measurement with a consistent technique is required (PMID 38440934; PMID 28245741).
• Serum total T4 (TT4) as the first-line test. A clearly elevated TT4 in a cat with consistent clinical signs confirms the diagnosis; the 2016 AAFP guidelines note that including TT4 in senior wellness panels now detects the disease long before the classic emaciated, agitated presentation (PMID 27143042).
• When TT4 is high-normal but the clinical picture fits: repeat the TT4, then add free T4 by equilibrium dialysis TOGETHER WITH TSH. Free T4 must not be used as a sole criterion - in 98 cats with non-thyroidal illness and 50 healthy controls, a few sick euthyroid cats had free T4 above the reference range and the calculated free T4 fraction was significantly higher in sick cats (0.24 +/- 0.30% vs 0.10 +/- 0.06%, P<0.001) (PMID 8707673).
• DISCLOSE THE LIMITS OF THE TSH RESULT. 'The potential for false-negative and false-positive results exists with all thyroid function tests, especially in older cats that may have concurrent nonthyroidal illness,' and confirming a thyroid state may require repeating TT4, measuring free T4 and TSH, a TSH STIMULATION test, or thyroid scintigraphy (PMID 23975586). Both of the best studies of post-treatment thyroid status relied on paired TT4 plus TSH (PMID 20695989) or on a TSH stimulation test (PMID 39831449) rather than a single TSH value - so use TSH as part of a pattern, principally to show whether it is suppressed, and do not present a feline TSH figure as a calibrated feline number. DISCLOSE THE INSTRUMENT AS WELL AS THE RESULT: feline TSH is measured with a CANINE TSH immunoassay used in cats, validated against recombinant feline TSH rather than sold as a feline-specific assay (PMID 17641276). Its behaviour is asymmetric, and that asymmetry is the practical point - measured against scintigraphy in 917 untreated hyperthyroid cats and 163 euthyroid cats, TSH alone was 98.0% sensitive but only 69.9% specific for hyperthyroidism, with specificity rising to 98.8% when paired with T4 or free T4, so a DETECTABLE TSH argues against hyperthyroidism much more strongly than any given value confirms a thyroid state (PMID 26192742). The canine chemiluminescent assays in routine use cannot measure below 0.03 ng/mL and therefore cannot distinguish low-normal from truly suppressed (PMID 38382201). Report feline TSH as suppressed versus detectable, not as a calibrated feline number.
• Interpret TT4 against concurrent illness. In the same study, mean serum TT4 was significantly LOWER in sick non-thyroidal-illness cats (17.18 +/- 8.14 nmol/L) than in healthy cats (26.00 +/- 7.62 nmol/L), P<0.001 - so a mid-range TT4 in a sick older cat does not exclude hyperthyroidism (PMID 8707673).
• Baseline renal and systemic work-up BEFORE treatment: CBC, serum biochemistry, creatinine and SDMA, urinalysis with specific gravity and UPC, and IRIS staging where possible. Hyperthyroidism raises GFR and masks CKD, which makes pre-treatment renal markers insensitive (PMID 35711100; PMID 35481810). Note that SDMA's performance specifically in hyperthyroid cats 'remains in its infancy' (PMID 35711100) - do not present it as a solved problem. Retrovirus (FeLV/FIV) status per practice protocol. A CBC is not optional: the drug that will most likely be started is contraindicated in cats with haematological disorders.
• Thyroid scintigraphy where available - essential before surgery. In 101 surgical cases, scintigraphy found ectopic hyperplastic thyroid tissue in 9 cats, and 4 of the 5 cats whose hyperthyroidism recurred had ectopic tissue pre-operatively (recurrence difference P<0.001) (PMID 16635010).
• KEEP THYROID CARCINOMA ON THE LIST, AND DO NOT ESCALATE THROUGH IT. Benignity is not established without histopathology, and only the surgical route yields it: 3 of 88 thyroidectomy specimens were carcinoma (PMID 16635010). Reassess - rather than simply raising the antithyroid dose - a cat with a rapidly enlarging, large, fixed, multilobular or ventrally displaced cervical mass, or one that fails to respond to escalating antithyroid medication. Reassessment means scintigraphy, imaging and referral for histopathology. Note that the low-dose radioiodine evidence base explicitly enrolled only mild-to-moderate disease (T4 >= 4.0 and < 13.0 ug/dL) and a review of the curative pathway frames fixed-dose radioiodine as the treatment for 'benign hyperthyroidism (i.e. thyroid hyperplasia or adenoma)' - suspected carcinoma, severe disease and large thyroid volume are outside that evidence (PMID 28158908; PMID 35711100). WE DID NOT LOCATE a validated clinical prediction rule for feline thyroid carcinoma; the criteria above are reassessment triggers, not a diagnostic test.
• Consider and screen for the common comorbidities of this age group rather than attributing everything to the thyroid: CKD, systemic hypertension with a FUNDIC EXAMINATION where blood pressure is high, and cardiac disease. Systemic hypertension in cats is frequently secondary to CKD or hyperthyroidism, its consequences are target-organ damage to eye, heart and vasculature, brain and kidneys, and it is probably underdiagnosed because blood pressure is measured infrequently (PMID 28245741). Hypertension and hyperthyroidism must both be excluded before attributing ventricular wall thickening to primary cardiomyopathy.