Hyperthyroidism
Learn what the condition is, how it may be detected early, how it is treated or managed, and which breeds or species are linked to it.
Condition snapshot
A concise guide to the condition’s pattern, detection, management and urgency.
This snapshot is a general guide, not a diagnosis or treatment plan. New, severe or worsening signs require veterinary assessment.
About the condition
Hyperthyroidism is excessive production of thyroid hormones and is one of the most common endocrine disorders of older cats. In most cases the thyroid tissue has undergone benign overgrowth, often affecting one or both thyroid lobes. Thyroid cancer is a much less common cause.
Excess thyroid hormone accelerates metabolism. Affected cats often lose weight despite eating well or even becoming ravenously hungry. Increased thirst and urination, restlessness, vocalisation, vomiting, diarrhoea and a poorly groomed coat may occur. The heart can beat rapidly and become thickened from sustained stimulation. Some cats are unusually quiet rather than hyperactive, especially when advanced disease or other illness is present.
Untreated hyperthyroidism places ongoing strain on the cardiovascular system and can contribute to high blood pressure. It also alters blood flow through the kidneys. As a result, kidney disease may be partly masked while thyroid hormone is high and become more obvious after treatment restores a normal metabolic state.
A palpable thyroid nodule may be present.
Hyperthyroidism is treatable and, in many cases, curable. Monitoring is essential whatever option is chosen because both thyroid levels and kidney function can change after treatment.
Because the disease changes metabolism throughout the body, successful treatment often produces noticeable changes beyond the thyroid level itself. Weight may increase, appetite may normalise and restlessness may settle. These changes are expected, but excessive weight gain or marked lethargy can signal over-treatment and should prompt reassessment.
A downward weight trend is particularly important even when appetite remains excellent. A consistently rapid heart rate, new heart murmur or high blood pressure can also be associated with thyroid disease. After treatment begins, thyroid hormone and kidney values are rechecked to ensure the cat is neither persistently hyperthyroid nor pushed into hypothyroidism.
Screening & Early Detection
Older cats should have their weight, heart rate and thyroid region assessed during routine health checks. Regular senior blood testing that includes total T4 can identify hyperthyroidism before severe weight loss or heart complications develop.
At home, increased hunger combined with weight loss, restlessness, increased thirst, vomiting, diarrhoea or a change in coat quality should prompt assessment.
Total T4 is the usual first-line blood test. When results are borderline but clinical suspicion remains high, the veterinarian may repeat T4 after an interval or use additional tests such as free T4 or thyroid imaging. Blood chemistry, urinalysis and blood pressure are assessed at the same time because kidney and cardiovascular health influence treatment planning.
Monitoring intervals depend on the chosen treatment and stability. Early detection allows treatment before severe muscle loss, hypertension or cardiac effects become established.
Diagnosis is usually made with a total T4 blood test interpreted alongside history and examination. Cats with strongly suggestive signs but a normal or borderline T4 may need repeat testing or additional thyroid tests because early disease can fluctuate.
Treatment & Management
Radioactive iodine therapy is often considered the preferred definitive treatment because it selectively destroys overactive thyroid tissue and can cure the disease without daily medication. Availability and cost vary, and cats require a period of controlled hospitalisation because of radiation-safety regulations.
Antithyroid medication such as methimazole or carbimazole can control hormone production and is useful both long term and as a reversible trial before definitive therapy. Blood tests are needed to monitor thyroid levels, kidney function and potential drug side effects. Surgical thyroidectomy can also be curative but requires appropriate case selection and anaesthetic planning.
An iodine-restricted prescription diet can control thyroid hormone in some cats if it is fed exclusively. Even small amounts of other food can reduce its effectiveness, so it is not practical for every household.
Whatever treatment is chosen, kidney function and blood pressure should be reassessed as thyroid levels normalise. Some cats reveal previously masked CKD. The treatment goal is a stable, comfortable cat with appropriate thyroid hormone rather than simply driving T4 as low as possible. Long-term follow-up remains necessary because hypothyroidism, recurrent disease or kidney complications can develop.
Treatment choice depends on the cat’s kidney function, general health, access to specialist services and the owner’s ability to give medication or manage a prescription diet.
Breeds or species affected
Abyssinian
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American Bobtail
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American Curl
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American Shorthair
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American Wirehair
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Arabian Mau
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Aegean
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Australian Mist
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Balinese
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Bengal
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Birman
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Bombay
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British Longhair
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British Shorthair
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Burmese
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Burmilla
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Chartreux
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Chausie
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Colorpoint Shorthair
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Cornish Rex
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Cymric
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Devon Rex
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Domestic Longhair
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Domestic Medium Hair
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Domestic Shorthair
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Donskoy
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Egyptian Mau
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European Shorthair
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Exotic Shorthair
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Havana Brown
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Highlander
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Himalayan
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Japanese Bobtail
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Khao Manee
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Korat
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Kurilian Bobtail
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LaPerm
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Lykoi
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Maine Coon
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Manx
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Minuet
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Minskin
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Munchkin
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Nebelung
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Norwegian Forest Cat
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Ocicat
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Oriental Longhair
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Oriental Shorthair
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Persian
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Peterbald
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Pixiebob
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RagaMuffin
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Ragdoll
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Russian Blue
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Savannah
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Scottish Fold
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Scottish Straight
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Selkirk Rex
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Siamese
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Siberian
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Singapura
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Snowshoe
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Somali
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Sphynx
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Thai
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Tonkinese
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Toybob
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Toyger
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Turkish Angora
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Turkish Van
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Bambino
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Cheetoh
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Dwelf
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Elf Cat
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Genetta
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Kinkalow
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Lambkin
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Serengeti
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Skookum
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Ukrainian Levkoy
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