07 July 2026
Sarah M A Caney BVSc, PhD, DSAM(Feline), MRCVS, RCVS, specialist in feline medicine, discusses feline hyperthyroidism, a common, manageable condition
Figure 1. Weight loss is the most common clinical sign associated with hyperthyroidism, and this diagnosis should be considered in any cat with unintended weight loss, even if body condition is normal or overweight.
Hyperthyroidism – the clinical syndrome resulting from excessive circulating levels of thyroid hormones – is the most common endocrinopathy in cats. Hyperthyroidism affects around 10% of cats aged 10 and above; the median age at diagnosis is usually 12 to 13 years. In most cats, hyperthyroidism is caused by bilateral adenomatous hyperplasia; 2% of patients have carcinomas. Diagnosis and management are often straightforward.
The most common clinical signs of hyperthyroidism are weight loss, often in spite of a normal or increased appetite (Table 1). Signs are most obvious in those cats that have been suffering with the illness for longer and in those that have concurrent illnesses. Chronic kidney disease (CKD) is the most common concurrent illness, estimated to affect up to a third of patients. Many cats with hyperthyroidism can appear “healthy” to an owner – polyphagia and hyperactivity can be interpreted as a cat with a “healthy joie de vivre” and weight loss often passes unnoticed.
Physical examination often reveals weight loss, presence of a goitre and tachycardia (Table 1, Figure 1 and Figure 2). Up to a quarter of hyperthyroid cats have systemic hypertension at the time their hyperthyroidism is diagnosed (Figure 3).
In most cases the diagnosis can be confirmed in cats with compatible clinical signs, simply by measuring resting serum total thyroxine (tT4) levels. Serum biochemistry and haematology are important to rule out other causes of similar clinical signs, identify concurrent disease and reveal clues of hyperthyroidism. Common changes include elevated liver enzymes, leucocytosis, eosinopenia and erythrocytosis.
Occasionally, a normal tT4 result is received in a cat suspected of having hyperthyroidism. This is especially common in cats that have concurrent disease, such as CKD, resulting in suppression of total T4 levels. Repeat tT4 testing at a reference laboratory is a sensible first step; adding free T4 and TSH assessment can also be helpful. Cats suffering from hyperthyroidism typically have elevated free and total T4 and low or undetectable levels of TSH. Diagnosis of hyperthyroidism is rarely an emergency as this is a slowly progressive illness. Therefore, if in doubt, repeat assessment (a “watch and wait” approach) is sensible rather than treating the cat.
Hyperthyroidism is typically a gradually progressive condition so it is easy for early and subtle clinical signs to pass unnoticed and for carers to only become aware of an issue once the disease is advanced. The earlier the diagnosis, the better the prognosis since damaging consequences of hyperthyroidism will be milder and more easily reversible.
The author’s tips for optimising early diagnosis include:
Hyperthyroidism is typically straightforward to treat and the outcome is often excellent (Figure 4).
Sarah Caney is internationally recognised as 1 of around a dozen RCVS specialists in feline medicine working in the UK. Sarah has always enjoyed seeing a mixture of first opinion and referral feline medicine patients, and has a particular interest in elderly cat medicine. Sarah founded Vet Professionals in 2009.
Baral R, Peterson ME (2012). Thyroid gland disorders. In Little SE (ed), The cat: clinical medicine and management, Elsevier Saunders, Philadelphia, PA: 571–592
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 18(5): 400-416.