Feline Hyperthyroidism

What you might notice

• Losing weight even though she is eating the same amount or more - this is the single most typical sign
• A big appetite: begging, raiding food bowls, seeming hungry all the time
• Drinking noticeably more water, and more or heavier clumps in the litter box
• Vomiting, or looser stool than usual
• Restlessness - pacing, not settling, and loud yowling that is often worst at night
• A coat that looks unkempt, greasy, clumped or thin because she has stopped grooming properly
• Breathing faster, or a heart your vet says is racing or has a new murmur
• Sometimes the opposite picture: a smaller group of cats become weak, quiet and off their food instead of hungry and hyper, which is why this is easy to miss

Standard of care

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

   Approved / guideline-backed

   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.

   Named products: 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.

   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.

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

   Approved / guideline-backed

   Record creatinine, SDMA, USG, UPC 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.

   Named products: 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' (Yu et al. 2022 review). After radioiodine, median creatinine rose significantly from 1.3 to 2.0 mg/dL in one prospective cohort of 13 cats (PMID 41170923). Hyperthyroidism may mask CKD and vice versa (PMID 35481810).

   Yu L, Lacorcia L, Johnstone T. Hyperthyroid cats and their kidneys: a literature review. Aust Vet J. 2022;100(9):415-432. PMID 35711100. Also: Geddes R, Aguiar J. Feline Comorbidities: Balancing hyperthyroidism and concurrent chronic kidney disease. J Feline Med Surg. 2022;24(7):641-650. PMID 35481810.

3. Methimazole - the approved first-line antithyroid drug in cats

   Approved / guideline-backed

   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. Its reversibility is exactly why it is often used first: if unmasking CKD makes the cat worse, the dose can be reduced or stopped. Titrate to a TT4 in the lower half of the reference interval rather than to the lowest achievable value.

   Named products: Methimazole - Felimazole Coated Tablets, NADA 141-292 (Dechra Veterinary Products), 2.5 mg and 5 mg tablets, 'For oral use in cats only'.

   APPROVED PRODUCT. Felimazole Coated Tablets (methimazole), NADA 141-292, Dechra Veterinary Products; label indication verbatim: 'Felimazole Coated Tablets (methimazole tablets) are indicated for the treatment of hyperthyroidism in cats.' Label starting dose verbatim: 'The starting dose of Felimazole Coated Tablets is 2.5 mg administered every 12 hours.' EFFICACY: 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); the maintenance dose in the 81 cats treated with methimazole alone ranged 2.5-20 mg/day (mean 11.9 mg/day) (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 (PMID 24372075).

   Felimazole Coated Tablets label, NADA 141-292 (Dechra), DailyMed setid 2d70b747-314c-4baa-95c4-1734855a1556, label updated December 2022. Efficacy: 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 practice: Daminet S, Kooistra HS, Fracassi F, et al. Best practice for the pharmacological management of hyperthyroid cats with antithyroid drugs. J Small Anim Pract. 2014;55(1):4-13. PMID 24372075.

4. Carbimazole (sustained-release) where it is licensed - a methimazole pro-drug, not a US option

   Approved / guideline-backed

   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 where owner dosing frequency is the limiting factor. It is NOT an FDA-approved product in the United States, so a US clinic should not be sold or shown this as a domestic option.

   Named products: Carbimazole sustained-release (marketed in the UK as Vidalta, MSD Animal Health). Marketing-authorisation number NOT verified - do not publish one.

   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 (PMID 23591627). Recommended starting dose in the European best-practice review: 10-15 mg once daily for the sustained-release carbimazole formulation (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. Owner preference data are reassuring for twice-daily dosing: 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 (PMID 23591627).

   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, et al. J Small Anim Pract. 2014;55(1):4-13. PMID 24372075.

5. Radioiodine (I-131) - the definitive, usually curative treatment, and the lowest effective dose is the right dose

   Controlled trial in this species

   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 matters: a lower dose achieves the same cure rate with substantially less iatrogenic hypothyroidism.

   Named products: Sodium iodide I-131. Administered only at a licensed radiopharmaceutical facility under local radiation-safety regulation.

   Prospective non-randomised cohort of 189 cats with mild-to-moderate hyperthyroidism (T4 >= 4.0 and < 13.0 ug/dL) treated with 2 mCi (n=150) or 4 mCi (n=39) I-131. No significant difference in persistent hyperthyroidism at 3 months (0% vs 5.3%, P=.34) or 6 months (0% vs 3.3%, P=.51), i.e. a cure rate greater than 95% with the low dose. At 6 months, overt hypothyroidism was significantly MORE common after the standard dose (18% vs 1%, P=.0005) and so was subclinical hypothyroidism (46% vs 21%, P=.004); standard-dose cats had higher creatinine concentrations (P<.05) and higher percentage rises in creatinine (P<.0001). Authors' conclusion: low-dose I-131 is safe and effective with reduced frequency of iatrogenic hypothyroidism and azotaemia (PMID 28158908).

   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.

6. Surgical thyroidectomy - effective where radioiodine is unavailable, but scintigraphy first

   Controlled trial in this species

   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. Post-operative parathyroid injury causing hypocalcaemia is the main early complication and requires serial calcium monitoring.

   Named products: Surgical procedure, no drug. Anaesthetic protocol should minimise adverse cardiovascular effects in a thyrotoxic patient.

   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 (recurrence rate difference with vs without EHTT, P<.001). Authors' conclusion: thyroidectomy is effective with a low complication rate when performed by an experienced surgeon, pre-operative scintigraphy is advised, and surgery is not recommended when EHTT is present (PMID 16635010).

   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.

7. Iodine-restricted therapeutic diet - a genuine option with honest, specific limits

   Controlled trial in this species

   Exclusive feeding of a commercially prepared iodine-restricted food lowers TT4 in most cats without a tablet, injection or anaesthetic. Two hard constraints must be stated to the owner: 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 normalising TT4 on this diet does not clearly prove the thyrotoxic state is fully controlled. It is a control strategy, not a cure, and the nodule continues to grow.

   Named products: 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). Not a drug; no NADA applies.

   MULTIPLE STUDIES, NONE BLINDED OR PLACEBO-CONTROLLED. (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). (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 (non-randomised) 3-arm trial in 34 cats vs transdermal and oral methimazole: TT4 fell in all groups; creatinine rose significantly only in the methimazole groups; liver enzyme activities did NOT improve in the diet group, which the authors state 'could indicate a persistent hyperthyroid state in cats treated with iodine-restricted food despite normalization of serum total thyroxine concentrations' (PMID 31998612). Report that last sentence to clients - it is the honest limit of this option.

   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, et al. 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.

8. Transdermal methimazole for the cat that genuinely cannot be pilled - compounded, slower, and with a trade-off

   Controlled trial in this species

   Methimazole in pluronic lecithin organogel applied to the inner pinna is an option when oral dosing is 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.

   Named products: Compounded methimazole in pluronic lecithin organogel (PLO). This is a compounded formulation, NOT Felimazole and NOT covered by NADA 141-292. Compounding from bulk drug substances is governed by FDA GFI #256 in the US.

   Randomised trial in 47 newly diagnosed hyperthyroid cats (44 completed: 17 oral, 27 transdermal), 2.5 mg q12h by either route. 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), possibly from inadequate numbers. 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).

   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.

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

   Controlled trial in this species

   This is the most consequential and most frequently skipped step in feline hyperthyroid care. Overshooting into hypothyroidism is a complication of all four modalities, is associated with azotaemia and shorter survival, and is largely invisible if only TT4 is measured. Where the cat is medically managed, reduce the antithyroid dose; where the gland is ablated, consider levothyroxine supplementation.

   Named products: Levothyroxine sodium for documented iatrogenic hypothyroidism. Recommended for cats that are hypothyroid and azotaemic (PMID 35481810). We did NOT verify an FDA-approved feline levothyroxine product in this research pass - do not publish a NADA number for it.

   (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 with TT4 below reference range had raised TSH; and hypothyroid cats that became azotaemic had significantly shorter survival (median 456 days, range 231-1589) than those that did not (median 905 days, range 316-1869), P=.018 (PMID 20695989). (2) In 19 methimazole- or carbimazole-treated cats with documented iatrogenic hypothyroidism, reducing the dose to restore euthyroidism significantly decreased plasma creatinine (2.61 to 2.07 mg/dL, P<.001) - though body weight also fell (P=.019), which the authors flag as a confounder (PMID 24773059). (3) Prospective cohort of 117 radioiodine-treated cats: euthyroid azotaemic cats (median survival 934 days) and non-supplemented hypothyroid cats (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). (4) Confirming hypothyroidism in a cat is not straightforward; repeat TT4, free T4 and TSH, a TSH stimulation test, or scintigraphy may be needed (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.

10. Measure and manage blood pressure through treatment, and do not assume it resolves

    Controlled trial in this species

    Check systolic blood pressure at diagnosis and again after euthyroidism is restored, with a fundic examination if it is high. Roughly half of hypertensive hyperthyroid cats normalise after successful treatment and the other half do not - and a small number become hypertensive only afterwards.

    Named products: Amlodipine besylate is identified as the treatment of choice for feline systemic hypertension in the ISFM consensus guidelines (PMID 28245741). Dose individualisation is required; the guidelines note the effective dose varies between cats.

    Prospective study of 401 non-azotaemic hyperthyroid cats: 108 (27%) were hypertensive (systolic BP >= 160 mmHg) before treatment. 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 persistently hypertensive cats were nervous or excited (50% vs 17%, P<.001), suggesting a large situational-hypertension component (PMID 38440934). The ISFM consensus guidelines identify amlodipine besylate as the treatment of choice for feline hypertension and note that routine BP monitoring is performed infrequently, probably leading to underdiagnosis (PMID 28245741).

    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, et al. ISFM Consensus Guidelines on the Diagnosis and Management of Hypertension in Cats. J Feline Med Surg. 2017;19(3):288-303. PMID 28245741.

11. What we are NOT recommending, and why

    No evidence located

    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 or a dose. 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 nothing on this list is an alternative to the four established modalities.

    Named products: 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 MSCs in cats with 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.

    Verification search run against PubMed 2026-09-12. Sole intersecting record: Beerts C, Debosschere Y, Tack L, et al. 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.

How it is diagnosed

• 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, and an indirect (Doppler or oscillometric) systolic blood pressure - 27% of untreated hyperthyroid cats are hypertensive (PMID 38440934).
• 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 (PMID 8707673).
• 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). Retrovirus (FeLV/FIV) status per practice protocol.
• 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).
• Consider and screen for the common comorbidities of this age group rather than attributing everything to the thyroid: CKD, systemic hypertension with fundic examination, and cardiac disease. Systemic hypertension in cats is frequently secondary to CKD or hyperthyroidism and is probably underdiagnosed because blood pressure is measured infrequently (PMID 28245741).