CONDITION
Hyperthyroidism in Cats
A condition where the thyroid gland produces excess hormone, increasing metabolic rate and affecting weight, appetite, and organ function.
Why this matters now
Hyperthyroidism is the most common endocrine disorder in cats, predominantly affecting middle-aged to older cats with the vast majority of diagnoses occurring after 10 years of age. The condition has become increasingly recognised since it was first described in the late 1970s, and its apparent prevalence has risen substantially over subsequent decades, a trend attributed to a combination of improved diagnostic awareness, longer feline lifespans, and potentially genuine increases in incidence related to environmental or dietary factors. There is no clear breed or sex predisposition, though Siamese and Himalayan breeds may be affected at lower rates. The condition arises from excessive production of thyroid hormones (primarily T4) by one or both thyroid glands, most commonly due to benign adenomatous hyperplasia rather than malignant neoplasia. Thyroid carcinoma accounts for only 1-3% of hyperthyroid cases in cats.
Without management, hyperthyroidism follows a progressive course as the autonomous thyroid tissue continues to grow and produce increasing amounts of thyroid hormone. The metabolic effects intensify over time, with weight loss becoming more pronounced despite maintained or increased appetite, and cardiac changes progressing from compensatory tachycardia and hypertrophy to potential heart failure. The hypermetabolic state places increasing demands on multiple organ systems, and secondary complications, particularly cardiac disease and systemic hypertension, can develop or worsen. Importantly, hyperthyroidism can mask underlying chronic kidney disease by increasing renal blood flow and glomerular filtration rate; when thyroid hormone levels are normalised through treatment, previously hidden renal insufficiency may become apparent. The interplay between thyroid function and kidney health is a significant consideration in managing the condition in older cats.
Signals & patterns
Early signals
Weight loss despite good appetite
The hallmark early sign of hyperthyroidism is progressive weight loss occurring alongside a maintained or even increased appetite. The accelerated metabolic rate means the body burns calories faster than they can be replaced through eating, leading to gradual loss of body condition. Owners may initially interpret the increased appetite positively, not connecting it with the simultaneous weight loss until the change becomes more pronounced.
Increased appetite and food-seeking behaviour
Polyphagia, a noticeable increase in hunger and food-seeking behaviour, often develops early as the body's accelerated metabolism drives greater caloric demand. Cats may become more vocal around feeding times, steal food from other pets, or show interest in foods they previously ignored. Some cats may develop a ravenous quality to their eating, consuming meals rapidly and then looking for more.
Increased water consumption
Polydipsia, or increased thirst and water intake, may be noticed as the hypermetabolic state increases fluid requirements. Owners might observe the cat drinking from unusual sources, spending more time at water bowls, or emptying bowls more quickly than previously. This increased water intake often accompanies increased urination, though both changes can also be seen in other common feline conditions such as diabetes and kidney disease.
Increased activity or restlessness
The stimulatory effects of excess thyroid hormone on the nervous system may produce a noticeable increase in activity, restlessness, or apparent anxiety in some cats. Older cats that had become sedentary may seem unusually active or agitated, pacing, vocalising more, or displaying a degree of hyperactivity that seems inconsistent with their age. Some owners initially welcome this apparent 'rejuvenation' before recognising it as a sign of illness.
Changes in coat quality
The coat may become unkempt, dull, or matted as the metabolic demands of hyperthyroidism affect the skin and hair cycle. Some cats show patchy hair loss or an oily, greasy coat quality. Reduced grooming behaviour may contribute, though the coat changes can also reflect the direct effects of thyroid hormone excess on skin and hair follicle metabolism.
Later signals
Pronounced muscle wasting
As the condition progresses, muscle loss becomes increasingly apparent, with the cat developing a bony, angular appearance as muscle mass is catabolised to meet metabolic demands. The spine, pelvis, and shoulder blades may become prominently palpable beneath thinning skin and diminished subcutaneous fat. This cachexia can progress significantly even while appetite remains good, reflecting the severe metabolic imbalance.
Vomiting and diarrhoea
Gastrointestinal signs including intermittent vomiting and diarrhoea or soft stools may develop as the increased gastrointestinal motility caused by thyroid hormone excess leads to malabsorption and reduced transit time. Some cats develop a pattern of eating voraciously followed by vomiting, which can further compromise nutritional status. Chronic gastrointestinal signs contribute to ongoing weight loss and can reduce quality of life.
Cardiac changes
Tachycardia (elevated heart rate), heart murmurs, and gallop rhythms may become detectable on clinical examination as the cardiovascular system responds to prolonged hyperthyroid demand. Echocardiographic changes, particularly left ventricular hypertrophy, develop as the heart muscle thickens in response to chronic volume overload and the direct effects of thyroid hormone on myocardial cells. In advanced cases, congestive heart failure with respiratory signs may develop.
Behavioural deterioration
Some hyperthyroid cats develop significant behavioural changes as the condition progresses, including aggression, marked irritability, reduced tolerance for handling, or apparent confusion. Loud, persistent vocalisation at night is commonly reported and can be distressing for owners. These behavioural signs reflect the neurological effects of sustained thyroid hormone excess and may overlap with signs of concurrent cognitive dysfunction in geriatric cats.
Sudden loss of vision with widely dilated pupils
Systemic hypertension associated with hyperthyroidism can cause sudden retinal detachment, which owners may notice as the cat appearing to lose its sight abruptly, bumping into furniture, or showing pupils that stay widely dilated even in bright light. Blood pressure measurement and examination of the retina can identify this complication, and some vision may be recovered if blood pressure is brought under control soon after it develops.
Fast, laboured, or open-mouthed breathing
As thyroid-related heart changes progress, some cats develop fluid accumulation around or within the lungs. This shows as a faster breathing rate at rest, more visible effort with each breath, or breathing with the mouth open, a pattern that differs from the cat's normal resting breathing. Of everything described on this page, this is the change that moves fastest: cats breathing this way can deteriorate within hours, and in veterinary practice breathing of this kind is assessed the same day it is seen.
Lethargy, fever, bruising or bleeding during anti-thyroid medication
Thiamazole and carbimazole can, in a small proportion of cats, affect the bone marrow's production of blood cells. Owners may notice unusual tiredness, reduced appetite, a raised temperature, unexplained bruising, nosebleeds, or bleeding from the gums, particularly within the first few months of starting or increasing the medication. These signs prompt a blood count to check for changes in blood cell numbers. A marked fall in white cells or platelets can be serious, and the UK datasheet for carbimazole notes that serious reactions can be fatal if the medication is continued. In veterinary practice this blood test is therefore carried out when the signs appear and is not left to the next routine check.
Click to read about the biological mechanisms
How this is usually investigated
Investigation of suspected hyperthyroidism typically begins with measurement of thyroid hormone levels alongside assessment of the condition's effects on other organ systems. The diagnostic approach aims to both confirm the thyroid diagnosis and evaluate for concurrent conditions, particularly renal and cardiac disease, that may influence management decisions.
Total T4 measurement
Free T4 measurement
Comprehensive biochemistry and haematology
Blood pressure measurement
Thyroid scintigraphy
Options & trade-offs
Hyperthyroidism is notable among feline conditions for having multiple distinct management approaches available, each with different mechanisms, practicalities, and long-term implications. The choice of approach often reflects individual factors including the cat's overall health status, concurrent conditions (particularly renal function), owner circumstances, and the availability of specific treatments.
Radioactive iodine therapy (I-131)
Radioactive iodine treatment involves a single dose of iodine-131, which is selectively taken up by hyperactive thyroid tissue and destroys it through localised radiation while sparing normal thyroid tissue and surrounding structures. This approach is curative in approximately 95% of cases with a single treatment, does not require anaesthesia, and has minimal side effects. Whether it suits a particular cat depends on the individual, including whether concurrent conditions rule it out. Iodine-131 is not authorised as a veterinary medicine in the UK; it is used under the veterinary medicines cascade, at centres holding the radiation permits the treatment requires.
Trade-offs: The treatment requires a period of hospitalisation in a licensed facility (typically around one to three weeks, depending on the centre, the dose given and UK radiation safety requirements), during which the cat cannot be visited. Availability is limited to centres with nuclear medicine capabilities. The cost of treatment, including hospitalisation, can be significant compared to initial costs of other approaches, though it is often cost-effective when compared to the cumulative lifetime cost of ongoing medical management. A small proportion of cats may develop hypothyroidism post-treatment. Precautions at home typically last two to four weeks, varying between centres, and cover contact between the cat and children or anyone who is or may be pregnant; precautions on handling and disposing of used litter can run for several weeks longer, and each centre issues its own written conditions.
Anti-thyroid medication (thiamazole/methimazole)
Oral anti-thyroid medication works by blocking the synthesis of thyroid hormones within the thyroid gland, providing medical management of hyperthyroidism without destroying thyroid tissue. Thiamazole (the name used in the UK; known as methimazole in some other countries) is the agent most often used, and is available as a UK-licensed tablet under the brand name Felimazole. Carbimazole, a related prodrug that the body converts to thiamazole, is also used, and is available in the UK as the prolonged-release tablet Vidalta. A transdermal gel applied to the inner ear flap is sometimes prescribed for cats that cannot be dosed orally, though this formulation does not hold a UK marketing authorisation and is typically supplied as a veterinary 'special' rather than a licensed product. The medication can reduce thyroid hormone levels within 1-3 weeks, and dosing can be adjusted to achieve target thyroid levels.
Trade-offs: Medical management requires lifelong daily medication, which can be challenging with uncooperative cats, though transdermal formulations applied to the inner ear flap offer an alternative to oral dosing. Side effects occur in approximately 15-20% of cats and can include gastrointestinal signs, facial scratching, blood cell abnormalities, and rarely, hepatotoxicity. Regular blood monitoring is needed to assess thyroid levels, kidney function, and haematological parameters. The medication manages rather than cures the condition, meaning the underlying thyroid tissue continues to grow.
Surgical thyroidectomy
Surgical removal of the affected thyroid gland(s) can cure the condition by removing the abnormal tissue, although hyperthyroidism can return if thyroid tissue left behind, such as the other gland, later becomes overactive. Unilateral thyroidectomy is performed when only one gland is affected, while bilateral thyroidectomy is needed when both glands are involved. Pre-operative stabilisation with anti-thyroid medication is typically used to reduce anaesthetic and surgical risk. Thyroid scintigraphy may be performed pre-operatively to determine the extent of thyroid involvement.
Trade-offs: Surgery requires general anaesthesia, which carries inherent considerations, particularly in older cats with cardiac complications from hyperthyroidism. The parathyroid glands, small structures intimately associated with the thyroid, are at risk of damage or inadvertent removal during bilateral thyroidectomy, which can lead to life-threatening hypocalcaemia in the post-operative period. Recurrence is possible if ectopic thyroid tissue is present in locations not accessible to the surgeon. Surgical expertise in thyroid surgery varies between centres, which may be a relevant factor when this approach is discussed.
Iodine-restricted prescription diet
A commercially available prescription diet containing severely restricted iodine levels (Hill's y/d) aims to limit the thyroid gland's ability to produce excess thyroid hormone by depriving it of the iodine the gland needs. When fed as the sole food source, this diet can reduce thyroid hormone levels in some cats. The approach does not require medication or procedures and may appeal to owners who prefer a dietary management strategy.
Trade-offs: The diet only works as the cat's sole food source, and any access to other food, treats, or hunting prey provides iodine and can undermine the approach. This can be particularly challenging in multi-cat households or for cats with outdoor access. Not all cats find the diet palatable, and dietary monotony over months or years can lead to food fatigue. The degree of thyroid hormone reduction may be less predictable or complete than with other approaches, and the diet does not address the underlying thyroid pathology, which continues to progress. Concerns have been raised about whether severe iodine restriction may have other metabolic effects over long-term feeding.
Combination and staged approaches
In practice, management of hyperthyroidism often involves a staged or combined approach. Anti-thyroid medication is frequently used as an initial step to stabilise the cat, assess the impact of normalising thyroid levels on kidney function, and provide time to evaluate which curative treatment may be most appropriate. This 'medical trial' can reveal previously masked renal insufficiency, which may then influence whether a curative approach (radioactive iodine, surgery) or ongoing medical management is pursued.
Trade-offs: A staged approach requires multiple veterinary visits, ongoing monitoring, and decision-making at several points. The initial medical phase adds cost before the curative treatment is undertaken. However, the information gained during the medical trial period, particularly regarding renal function, can inform long-term management decisions. Some cats do well on long-term medical management and may never proceed to a curative approach if the medication is well tolerated and kidney function is stable.
Common misconceptions
"A cat that is eating well and active cannot be seriously unwell"
The combination of increased appetite and apparent increased energy that characterises hyperthyroidism can create a misleading impression of good health, particularly in the early stages. Some owners describe their older cat as seeming 'rejuvenated' or 'kitten-like', interpreting the hypermetabolic signs positively. In reality, the increased appetite reflects the body's attempt to keep up with unsustainable metabolic demands, and the apparent energy often represents thyroid-driven agitation rather than genuine vitality. The concurrent weight loss, if noticed, may be attributed to the increased activity rather than recognised as a sign of disease.
"Treating hyperthyroidism will always make the cat better overall"
While controlling excess thyroid hormone production is important, the relationship between hyperthyroidism and kidney function adds a significant layer of complexity. Hyperthyroidism increases renal blood flow and glomerular filtration rate, which can mask significant underlying chronic kidney disease. When thyroid hormone levels are normalised through any treatment approach, renal function may decline as the 'protective' hyperthyroid effect on kidney perfusion is removed. This does not mean treatment is harmful, but it means that kidney function is usually monitored during and after treatment, and the degree of unmasked renal insufficiency may influence long-term management strategies.
"Radioactive iodine treatment is dangerous or harmful to the cat"
Despite the concerning terminology, radioactive iodine therapy is a well-established treatment for feline hyperthyroidism, with a cure rate of approximately 95% from a single dose. The radioactive iodine is selectively concentrated in hyperactive thyroid tissue, delivering localised radiation that destroys the abnormal cells while sparing normal tissue. The radiation the cat gives off diminishes over time, but cats are typically hospitalised for around one to three weeks after treatment under UK radiation safety regulations, and are not radiation-free at the point of discharge. Precautions at home then typically run for two to four weeks, with those on used litter running longer, as set out in each centre's written conditions. Side effects are uncommon and typically mild, and the procedure does not require general anaesthesia or surgery.
Related conditions
Chronic Kidney Disease in Cats
Hyperthyroidism can mask declining kidney function because the increased metabolic rate maintains higher glomerular filtration, meaning chronic kidney disease may become apparent only after thyroid treatment begins. Both conditions are common in older cats and often occur together.
Hypertrophic Cardiomyopathy in Cats
Excess thyroid hormone increases heart rate and contractility, and prolonged hyperthyroidism commonly causes the heart muscle to thicken in a pattern that can look similar to hypertrophic cardiomyopathy on a scan. This thyroid-driven thickening is largely reversible once thyroid hormone levels are controlled, unlike primary hypertrophic cardiomyopathy, a heart muscle disease that is not caused by thyroid status. Both conditions may produce a heart murmur, and telling them apart can need cardiac reassessment once thyroid hormone levels have been brought into the normal range.
Diabetes Mellitus in Cats
Both conditions affect metabolic regulation and commonly produce an overlapping picture of weight loss despite a maintained or increased appetite, together with increased thirst and urination, so these signs alone cannot reliably distinguish one from the other. Blood and urine testing, including thyroid hormone and glucose measurement, is used to tell them apart, and in some older cats both conditions are present together.
Understanding hyperthyroidism involves appreciating its systemic nature: while the primary problem originates in the thyroid gland, its effects extend across virtually every organ system, and management decisions often need to consider the whole patient rather than the thyroid alone. The relationship between thyroid function and kidney health is a particularly important dimension, as the interplay between these two systems significantly influences management strategies and monitoring. The availability of multiple management approaches means that decisions can be tailored to individual circumstances, and what works well for one cat may not suit another. Long-term monitoring tends to be an integral part of living with this condition, regardless of the management approach chosen.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS