Hyperthyroidism is the most common endocrine disorder of older cats, caused by excessive production of thyroid hormones (T3 and T4) from enlarged thyroid glands. First described in cats in 1979, it now affects approximately 1 in 10 cats over the age of 10. Understanding welfare impacts and treatment options is essential for owners and veterinary teams.
Pathophysiology
The majority (98%) of cases involve benign hyperplasia or adenoma of thyroid tissue — thyroid carcinoma is rare (<2%). The cause is multifactorial: dietary factors (iodine levels in commercial cat food, canned food consumption), endocrine disruptors (polybrominated diphenyl ethers in household dust and furnishings), and possibly genetic factors have been implicated. Iodine content in cat food was historically implicated; reformulation has reduced but not eliminated incidence.
Clinical Signs
- Weight loss despite ravenous appetite (polyphagia)
- Hyperactivity, restlessness, vocalisations
- Vomiting and diarrhoea
- Polyuria/polydipsia (PU/PD)
- Poor coat condition; unkempt appearance
- Palpable thyroid nodule(s) in the ventral neck
- Tachycardia, hypertrophic cardiomyopathy in advanced cases
- Hypertension with ocular, neurological, or renal complications
Diagnosis
Total serum T4 is elevated in >90% of cases. Equivocal T4 values may require repeat testing, free T4 by equilibrium dialysis, or T3 suppression testing. Nuclear scintigraphy allows imaging of thyroid tissue distribution and is the gold standard for confirming ectopic thyroid tissue.
Treatment Options
Methimazole/Carbimazole: Medical management using anti-thyroid drugs is first-line for most cats. Daily oral dosing or twice-daily methimazole; transdermal ear flap gels available for difficult-to-medicate cats. Side effects (facial pruritis, vomiting, bone marrow suppression — rare) require monitoring.
Radioactive Iodine (¹³¹I): Gold standard curative treatment. A single injection destroys abnormal thyroid tissue while sparing normal tissue. Requires specialist facility and brief hospitalisation (typically 2 weeks in UK due to radiation regulations). High cure rate (95%+).
Surgical thyroidectomy: Effective but carries anaesthetic risk in older cats with concurrent disease. Bilateral disease or ectopic tissue reduce cure rates.
Dietary management: Iodine-restricted prescription diets (Hill's y/d) can control hyperthyroidism if fed exclusively but compliance is challenging in multi-cat households.
Complicating Concurrent Disease
Hyperthyroidism can mask concurrent chronic kidney disease (CKD) by maintaining glomerular filtration rate through elevated blood pressure. Treatment may unmask CKD — monitoring renal parameters after treatment initiation is essential. Managing both conditions simultaneously requires careful dosing and monitoring.
Welfare Considerations
Untreated hyperthyroidism causes chronic distress — weight loss, agitation, GI upset, and cardiovascular strain all reduce quality of life. Prompt diagnosis and effective management dramatically improves wellbeing. Regular monitoring (T4 and kidney parameters every 3–6 months) ensures sustained control and early detection of complications.