The Most Common Endocrine Disease in Older Cats
Hyperthyroidism occurs when the thyroid gland — a small, two-lobed structure in the neck — produces excess thyroid hormone (thyroxine, or T4). This causes a systemic acceleration of metabolism that, if left untreated, places enormous strain on the heart, kidneys, and other organs. In the UK and across Europe, hyperthyroidism is diagnosed in cats over the age of eight more than any other hormonal condition, with mean age at diagnosis around 12 to 13 years.
In the vast majority of cases (approximately 98%), the cause is a benign adenomatous hyperplasia (non-cancerous overgrowth) of thyroid tissue — one or both lobes may be affected. Thyroid carcinoma is an uncommon but recognised cause in a small percentage of affected cats and carries a different management pathway.
Clinical Signs of Hyperthyroidism
The signs of hyperthyroidism stem from the effects of elevated thyroid hormone on multiple organ systems. Because the condition develops gradually, owners often normalise the changes — attributing weight loss to ageing, for example — which can delay diagnosis. Key signs include:
- Weight loss despite a normal or increased appetite: One of the most consistent and striking features. Affected cats may eat voraciously yet lose condition steadily.
- Hyperactivity and restlessness: Hyperthyroid cats are often described as more vocal, anxious, or hyperactive than usual. Some become aggressive.
- Vomiting and diarrhoea: Gastrointestinal signs are common due to increased gut motility and the metabolic effects of excess thyroid hormone.
- Tachycardia and heart murmurs: Elevated T4 drives increased heart rate and, in time, secondary hypertrophic cardiomyopathy. Cardiac complications are a major concern in untreated cats.
- Unkempt, matted, or greasy coat: Cats often reduce grooming effort despite being seemingly active.
- Polyuria and polydipsia (PU/PD): Increased thirst and urination, which can overlap with other conditions including diabetes mellitus and CKD.
- Hypertension: Elevated blood pressure secondary to hyperthyroidism can cause retinal detachment and sudden blindness.
In some older cats, a paradoxical "apathetic" form occurs, with lethargy and reduced appetite rather than the classic hyperactive presentation — making it even easier to miss.
Diagnosis
Diagnosis is confirmed by measuring total serum T4 (total thyroxine). In most hyperthyroid cats, total T4 is markedly elevated above the reference range. However, in cats with early or mild hyperthyroidism, total T4 can fluctuate into the normal range — in these cases, a free T4 by equilibrium dialysis (fT4 by ED) provides greater sensitivity.
A complete blood screen and urinalysis are essential at baseline to assess concurrent conditions, particularly chronic Kidney Disease Early Signs">Kidney Disease in Cats: Diet, Symptoms & Prognosis">Kidney Disease Diet">Kidney Disease in Dogs: Diet, Supplements & Quality of Life">kidney disease (CKD), which is common in the same age group and has important implications for treatment choice. Thoracic radiographs and echocardiography may be recommended if cardiac signs are present. In specialist centres across Europe, thyroid scintigraphy (nuclear imaging) is available to confirm the site and extent of abnormal thyroid tissue and to detect ectopic thyroid tissue — useful when surgery or radioactive iodine therapy is planned. ECVIM-CA guidelines recommend a structured diagnostic and monitoring approach to hyperthyroid cats, particularly given the complexity of concurrent kidney disease.
Treatment Options in the EU
There are four main treatment options, each with distinct advantages and considerations:
Antithyroid Medication
Methimazole and its pro-drug carbimazole (marketed as Felimazole in the EU) are the most widely used treatments. They block thyroid hormone synthesis without destroying thyroid tissue. Felimazole is licensed for use in cats in Europe and is typically given once or twice daily. Medication is effective within two to four weeks and allows titration to the individual cat's needs. Side effects — including vomiting, facial pruritus, and, less commonly, blood dyscrasias — require monitoring via periodic blood tests. Medication does not cure the disease; it must be continued for life or until another treatment is undertaken.
Radioactive Iodine (I-131) Therapy
A single injection of radioactive iodine destroys abnormal thyroid tissue while sparing the parathyroid glands. It is curative in around 95% of cases and is considered the gold standard treatment by many specialists. It requires hospitalisation (usually five to seven days in a specialist facility) until radiation levels fall to safe levels, and is available at referral centres across the EU. It is particularly valuable for cats where medication compliance is poor or side effects are problematic.
Surgical Thyroidectomy
Surgical removal of the affected thyroid lobe(s) can be curative. It requires general anaesthesia and is generally reserved for cats that are medically stabilised first and are otherwise good anaesthetic candidates. Risk of hypoparathyroidism (low calcium) is a recognised post-operative complication if parathyroid tissue is inadvertently damaged.
Iodine-Restricted Diet
Hill's Prescription Diet y/d is a diet that controls hyperthyroidism by severely restricting dietary iodine, which is required for thyroid hormone synthesis. It is an option for cats where medication is not tolerated and referral for radioiodine is not possible. The cat must eat only this diet exclusively — even small amounts of other food will undermine control. Zooplus stocks Hill's y/d and other veterinary prescription diets, which can be ordered conveniently for ongoing management.
The Concurrent Kidney Disease Problem
One of the most clinically challenging aspects of feline hyperthyroidism is its relationship with chronic kidney disease. Elevated T4 artificially inflates glomerular filtration rate (GFR), meaning that renal function may appear better than it actually is while the cat is hyperthyroid. When thyroid hormone is normalised, underlying CKD may be unmasked. ECVIM-CA recommends a monitored treatment trial — typically four to six weeks of medical therapy — with renal function assessment before committing to permanent treatment, to ensure that treating the hyperthyroidism will not precipitate clinically significant renal failure.
Monitoring
Cats on methimazole or carbimazole should have blood tests at two to four weeks after starting therapy, then every three to six months once stable. Blood pressure should be checked regularly given the risk of hypertension-related complications.
When to Seek Urgent Care
Contact your vet urgently if your cat:
- Suddenly loses vision (possible retinal detachment from hypertension)
- Develops laboured breathing or an open-mouth breathing episode
- Collapses or has a seizure
- Stops eating for more than 24 hours
- Develops facial swelling or severe skin irritation after starting medication
This article was written by Sarah Bennett. Always consult a registered veterinary surgeon for advice specific to your pet.