Hyperthyroidism is the most common endocrine disorder in middle-aged and older cats, affecting approximately 10% of cats over 10 years old. The condition significantly impacts quality of life and welfare if untreated, but responds well to several available treatments when managed appropriately.
Hyperthyroidism in cats most commonly results from benign hyperplasia or adenoma of the thyroid gland, causing excessive production of thyroxine (T4). The cause of thyroid proliferation is not fully understood but may involve dietary factors (iodine levels, goitrogens in food), environmental chemicals (flame retardants, BPAs from tin can liners), and genetic predisposition. Malignant thyroid carcinoma is rare (<2% of cases).
Classic signs include weight loss despite polyphagia (increased appetite), hyperactivity and restlessness, vomiting, diarrhoea, polyuria/polydipsia, and unkempt coat. Cardiac effects (hypertrophic cardiomyopathy, hypertension, tachycardia) can cause respiratory distress and collapse. Hypertension may damage kidneys, eyes (retinal detachment), and brain. The anxious, uncomfortable hyperthyroid cat represents a significant welfare concern requiring prompt diagnosis and treatment.
Diagnosis is typically straightforward with measurement of total serum T4 concentration. Cats with early or mild hyperthyroidism may have T4 in the upper-normal range—repeat testing, free T4, or T3 suppression test can confirm equivocal cases. Assessment for concurrent kidney disease is critical before treatment, as correcting hyperthyroidism may unmask underlying renal insufficiency.
Four treatment options are available: Medical management (methimazole/carbimazole tablets or transdermal gels) controls T4 production and is reversible but requires lifelong daily medication. Radioactive iodine (I-131) is curative in approximately 95% of cats with a single treatment but requires hospitalisation for radiation safety. Surgical thyroidectomy is curative but carries anaesthetic risks in older cats. Dietary management (iodine-restricted diets) can control hyperthyroidism but requires strict dietary compliance. Choice depends on individual cat factors, concurrent disease, and owner circumstances.
Regular monitoring of T4 levels, kidney function, blood pressure, and body weight is essential regardless of treatment method. The goal is to normalise thyroid function while avoiding hypothyroidism, which can worsen kidney function. With appropriate management, hyperthyroid cats can return to normal body condition and excellent quality of life.
Owner compliance is crucial for medical management. Understanding how to administer medication, recognising signs of over- or under-treatment, and attending regular monitoring appointments significantly affects welfare outcomes. Transdermal gels (applied to the inner ear pinna) offer an alternative to tablets for difficult-to-medicate cats, improving compliance and welfare.