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 definitive 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 immediately seeking 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. Nighttime vocalisation — loud, persistent crying, particularly 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.

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

Purpose: Measurement of total thyroxine (T4) in the blood is the primary screening test for feline hyperthyroidism. In most hyperthyroid cats, total T4 is elevated above the normal reference range, providing a straightforward diagnosis. The test is widely available, relatively inexpensive, and provides results quickly, making it the standard initial investigation.
Considerations: A proportion of hyperthyroid cats — estimated at 2-10% — may show total T4 values within the normal range, particularly in early disease or when concurrent non-thyroidal illness suppresses thyroid hormone levels. Fluctuation of T4 levels over time means a single normal result does not definitively exclude the diagnosis in a cat with suggestive clinical signs. Serial testing over weeks to months may be needed in equivocal cases.

Free T4 measurement

Purpose: Free T4 (the unbound, biologically active fraction of T4) is more sensitive than total T4 for detecting hyperthyroidism and can identify cases where total T4 remains within the normal range. This test is particularly useful in cats with high clinical suspicion but normal total T4 results, capturing cases of early or mild hyperthyroidism that might otherwise be missed.
Considerations: Free T4 is more sensitive but less specific than total T4, meaning that some non-hyperthyroid cats (particularly those with concurrent illness) may show mildly elevated free T4 values. For this reason, free T4 is generally interpreted alongside total T4 and clinical findings rather than in isolation. The test uses equilibrium dialysis methodology for accurate results, which may not be available at all laboratories.

Comprehensive biochemistry and haematology

Purpose: Blood work helps assess the systemic effects of hyperthyroidism and identify concurrent conditions. Common findings include elevated liver enzymes (particularly ALT and ALP), mild erythrocytosis, and stress leucogram patterns. Critically, renal parameters (urea, creatinine, SDMA) establish baseline kidney function before treatment, as hyperthyroidism can mask underlying kidney disease through increased renal blood flow.
Considerations: The interpretation of renal parameters requires careful context — apparently normal kidney values in a hyperthyroid cat do not guarantee normal renal function, as the thyroid-driven increase in GFR may be maintaining creatinine and urea within reference ranges despite significant underlying nephron loss. Post-treatment reassessment of kidney function is typically recommended to unmask any concealed renal insufficiency. Liver enzyme elevation in hyperthyroidism usually reflects the metabolic effects of thyroid hormone excess rather than primary liver disease.

Blood pressure measurement

Purpose: Systemic hypertension is a common complication of feline hyperthyroidism, occurring in approximately 10-20% of cases at diagnosis, and can cause target organ damage including retinal detachment, cardiac changes, and renal injury. Blood pressure assessment at diagnosis establishes whether hypertension is present and requires concurrent management, and helps guide monitoring during treatment.
Considerations: Blood pressure measurement in cats is subject to situational variability, and the stress of the veterinary environment can produce artificially elevated readings. Multiple measurements in a calm environment improve reliability. Hypertension may resolve with successful management of the thyroid condition, or it may persist independently, requiring ongoing monitoring and potentially separate management.

Thyroid scintigraphy

Purpose: Nuclear medicine imaging using technetium-99m pertechnetate provides functional imaging of the thyroid gland, showing the location, size, and activity of thyroid tissue. This investigation is particularly valuable for identifying bilateral versus unilateral thyroid involvement, detecting ectopic thyroid tissue (particularly in the thorax), and distinguishing between benign hyperplasia and thyroid carcinoma based on uptake patterns.
Considerations: Thyroid scintigraphy requires specialised nuclear medicine facilities, which limits its availability. The procedure involves handling of radioactive material and requires a period of isolation for the cat. While it provides the most comprehensive anatomical and functional assessment of the thyroid glands, it is typically reserved for cases where surgical thyroidectomy is planned (to assess bilateral involvement) or where the diagnosis is complex.

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 injection 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. It is widely considered the definitive treatment for feline hyperthyroidism when concurrent conditions do not preclude it.

Trade-offs: The treatment requires a period of hospitalisation in a licensed facility (typically 1-3 weeks depending on jurisdiction and radiation safety regulations), 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.

Anti-thyroid medication (methimazole/thiamazole)

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. Methimazole (or its prodrug carbimazole) is the most commonly used agent, available in oral tablet, liquid, or transdermal formulations. The medication reduces 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) provides a definitive cure by physically removing the abnormal tissue. 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 recommended 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, and outcomes are best when performed by experienced surgeons.

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 essential substrate. 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 must be fed exclusively — 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 definitive 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 definitive treatment is undertaken. However, the information gained during the medical trial period — particularly regarding renal function — can be invaluable for making informed 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

Misconception:

"A cat that is eating well and active cannot be seriously unwell"

Reality:

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.

Misconception:

"Treating hyperthyroidism will always make the cat better overall"

Reality:

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 monitoring during and after treatment is essential, and the degree of unmasked renal insufficiency may influence long-term management strategies.

Misconception:

"Radioactive iodine treatment is dangerous or harmful to the cat"

Reality:

Despite the concerning terminology, radioactive iodine therapy is one of the safest and most effective treatments for feline hyperthyroidism, with a cure rate of approximately 95% from a single injection. The radioactive iodine is selectively concentrated in hyperactive thyroid tissue, delivering localised radiation that destroys the abnormal cells while sparing normal tissue. The cat's radiation exposure diminishes rapidly, and by the time of discharge, radiation levels are within safe limits for normal household interaction. Side effects are uncommon and typically mild, and the procedure does not require general anaesthesia or surgery.

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 be the optimal choice for another. Long-term monitoring tends to be an integral part of living with this condition, regardless of the management approach chosen.

Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS