Who gets it and how it presents
The AAFP guidelines say the disease is now diagnosed in 1.5-11.4% of older cats worldwide. Most affected cats have toxic nodular goiter, a benign condition in which growth and function are autonomous [1]. Most cases are bilateral, and the guidelines put the share with malignant carcinoma at diagnosis at 2% [1]. Merck gives a figure of under 3% [3].
The cause is unknown. Epidemiologic studies have linked canned food, deodorized litter, bisphenol A, phthalates, soy isoflavones and fire-retardant chemicals to the disease. However, the AAFP panel advises telling owners that "all associations are conjecture, and not proven fact" [1].
The Vaske review puts the mean age at 12 to 13 years, with fewer than 5% of cases in cats younger than 10 [2]. Typical signs listed in the sources include [1][2]:
- weight loss despite a good or increased appetite
- increased thirst and urination
- restlessness and vocalization
- vomiting and diarrhea
- an unkempt coat
- a fast heart rate
The same review notes that about 10% of cats show "apathetic hyperthyroidism," with decreased appetite and lethargy. It also reports that more than 90% have a palpable thyroid nodule at diagnosis [2]. Merck cautions that the thyroid can also be palpable in euthyroid cats [3].
Cardiovascular findings are common. According to the Vaske review, about half of hyperthyroid cats have a systolic murmur, while congestive heart failure is uncommon [2]. The same authors report hypertension in roughly 10% of cats at diagnosis and in a further 20% after return to euthyroidism, possibly reflecting unmasked kidney disease [2]. The AAFP panel describes monitoring blood pressure at every visit as optimal. It notes that a complete fundic exam may determine whether hypertensive retinopathy is present [1].
Diagnosis and testing pitfalls
The AAFP guidelines define a definitive diagnosis as persistently elevated thyroid hormone (T4, or T4 plus free T4 by equilibrium dialysis) occurring with at least one typical clinical sign [1]. For any suspected case, the panel recommends a minimum database of CBC, serum chemistry, urinalysis and T4, both to confirm the disease and to find comorbidities [1]. Differential diagnoses listed in the guidelines include [1]:
- diabetes mellitus
- gastrointestinal malabsorption or maldigestion
- neoplasia (especially gastrointestinal lymphosarcoma)
- chronic kidney disease
- parasitism
Common laboratory changes, per the Vaske review, include mild increases in packed cell volume in 40%-50% of cats and elevated ALT or ALP in more than 90% [2]. The authors add that these liver changes usually do not affect liver function tests and typically resolve with treatment [2].
The main pitfall is a normal T4 in a cat that has the disease. Merck estimates that 5%-10% of affected cats have T4 within the reference interval [3]. The Vaske review attributes this to early disease in most such cats and to concurrent illness in about 20%-30% [2]. For a suspicious cat with a normal result, those authors describe repeating total T4 several weeks later while ruling out nonthyroidal illness, then measuring free T4 if T4 is still normal [2]. Merck adds that T4 falls with age, so a senior cat with signs and a high-normal T4 "may actually be hyperthyroid" [3].
Free T4 detects more cases but is less specific. The Vaske review states that more than 95% of hyperthyroid cats with normal total T4 have an elevated free T4, and that free T4 is unnecessary when total T4 is already high [2]. Merck describes a high-normal T4 with a high free T4 as diagnostic [3].
For persistently equivocal cases, the sources describe these further tests:
| Test | What the sources report |
|---|
| T3 suppression | Euthyroid cats suppress total T4 by 50% or more; hyperthyroid cats show little or no drop. Results depend on owner compliance, which can be checked by a rise in T3 [2] |
| TRH stimulation | Considered about as reliable as T3 suppression, but generally not recommended because of product availability, side effects and difficult interpretation [2] |
| TSH response | Not currently recommended for diagnosing feline hyperthyroidism [2] |
| Random TSH | Low TSH may precede T4 elevation, but only about half of cats with undetectable TSH became hyperthyroid within 14 months in a 104-cat study [2] |
| Thyroid scintigraphy | Confirms disease, shows bilateral, ectopic or metastatic tissue and informs treatment choice [2] |
The Vaske review states that ultrasound "is not useful for the diagnosis of hyperthyroidism," though it shows gland structure [2].
Classification and concurrent kidney disease
The AAFP guidelines sort suspected cases into six clinical categories, each with a management strategy [1]. The cited source text identifies Group 5 as cats with hyperthyroidism and concurrent renal azotemia. It does not list the definitions of all six groups.
For kidney status, the AAFP panel recommends IRIS staging, including blood pressure and urine protein measurement, before treatment begins [1]. The panel recommends managing IRIS stage 1 and 2 cats as though non-azotemic. For stage 3 and 4 cats it recommends "a more prudent approach," such as lower methimazole doses and more aggressive kidney management [1].
According to a review in the Journal of Feline Medicine and Surgery, hyperthyroidism raises renal blood flow and glomerular filtration rate and is accompanied by reduced muscle mass. Together these can make creatinine look normal and hide kidney disease [4]. That review reports pretreatment azotemia in 10%-23% of hyperthyroid cats and post-treatment azotemia in about 15%-49% [4]. In one study of 268 non-azotemic cats treated with antithyroid drugs alone or with thyroidectomy, 15% were azotemic by 8 months [4]. The authors also state that no single serum or urine marker reliably predicts which cats will become azotemic after treatment [4].
The sources differ on what post-treatment azotemia means for survival:
- The AAFP guidelines state that survival time "decreases significantly" when azotemia develops after treatment [1].
- The two Kansas State reviews report that cats developing azotemia after treatment did not have shorter survival than cats that stayed non-azotemic, while cats azotemic before treatment did [2][4].
Treatment options
The AAFP panel lists four common options. It states that the goal is to restore euthyroidism, avoid hypothyroidism and minimize side effects [1].
Radioactive iodine
The AAFP guidelines report single-treatment success above 95% in most studies, with T4 falling into the reference interval by 4-12 weeks [1]. The Vaske review cites a study of 524 cats with these results [2]:
- 94.2% had resolution of signs and normal T4 within 6-12 months
- 2.5% relapsed
- 2.1% developed clinical hypothyroidism needing supplementation
Those authors note that no randomized trial has compared all treatment options [2].
The AAFP panel reports that up to 75% of cats may become hypothyroid for some period after treatment. Isolation after treatment can range from 3 days to 4 weeks depending on regulations and dose [1]. The panel states that physiologically stable cats respond best. Cats with clinically significant cardiovascular, renal, gastrointestinal or endocrine disease may not be good candidates [1].
Antithyroid drugs
The AAFP guidelines state that methimazole "should be started at a dose of 1.25-2.5 mg per cat twice daily (q12h)." They add that twice-daily dosing carries fewer serious side effects than a higher once-daily dose [1]. The Vaske review notes that a lower start of 1.25 mg twice daily or 2.5 mg once daily has historically been recommended for cats with previously diagnosed kidney disease [2].
According to the same review, one study of 262 cats reported side effects in about 18% [2]. These included:
- vomiting and anorexia
- facial excoriation
- blood dyscrasias
- liver disease
The AAFP panel notes that methimazole does not destroy abnormal tissue, so doses may need to rise over time and some cats become resistant [1]. Merck states that carbimazole converts to methimazole and should not be given to cats that do not tolerate methimazole [3]. The AAFP panel also describes a methimazole trial before radioiodine or bilateral surgery as a way to gauge renal risk [1].
Thyroidectomy
The AAFP guidelines report that more than 90% of cats reach euthyroidism after surgery in most studies, with relapse approaching 5% within 3 years [1]. Reported complications include hypocalcemia (6-82% depending on technique), Horner's syndrome and laryngeal paralysis [1]. Ectopic thyroid tissue, which the guidelines estimate at 4-9% of cases, is likely to be missed at surgery [1]. Merck gives a less favorable long-term picture. It reports over 40% persistent or recurrent hyperthyroidism and over 15% hypothyroidism with long follow-up [3].
Iodine-restricted diet
The AAFP guidelines describe a prescription diet containing 0.2 ppm iodine on a dry matter basis [1]. They report that 75% of cats show lower T4 within 28 days and that 83% were in remission in a 1-year study [1]. Palatability problems affected up to 12% of cats studied [1]. Merck states that the cat "must eat only this diet for the rest of its life," with no other food, treats or prey [3]. The Vaske review raises concern that many cats on the diet do not reach T4 in the lower half of the reference interval [2].
Monitoring
The AAFP panel recommends this schedule [1]:
- first follow-up at 2-4 weeks
- repeat testing 2-4 weeks after any dose change
- T4, CBC, chemistry and urinalysis every 4-6 months in stable cats
For surgery and radioiodine patients, the panel generally recommends testing at 30, 60, 90 and 180 days [1].
The sources do not agree on the target T4:
- The Vaske reviews favor the lower half of the reference interval [2][4].
- Merck describes the low to mid range [3].
- The AAFP guidelines state that in cats with renal insufficiency, T4 "should be maintained in the upper half of the reference interval" [1].
The JFMS review recommends monitoring creatinine for at least 6 months after euthyroidism. It also recommends confirming iatrogenic hypothyroidism with a low total T4 plus a high TSH rather than T4 alone [4]. In one study of 80 treated cats, 57% of the hypothyroid cats became azotemic, compared with 30% of euthyroid cats [4].
Prognosis and client communication
The AAFP guidelines report median survival of up to 5.3 years in cats without concurrent kidney disease. They state that outcomes in well-managed cats depend more on comorbidities than on the thyroid disease itself [1]. In a study of 300 cats treated with antithyroid drugs alone or with thyroidectomy, median survival was 178 days in cats azotemic before treatment and 612 days in non-azotemic cats [2].
Points the sources flag for client conversations include:
- possible thyroid hormone replacement after radioiodine, particularly with kidney disease [1]
- strict lifelong compliance on the diet [3]
- lifelong daily medication with methimazole alone [2]
- the chance that treatment reveals hidden kidney disease [3][4]
On cost, the Vaske review states that the relatively larger upfront cost of thyroidectomy and radioiodine must be compared with the monthly costs of the noncurative options [2].
