Hyperthyroidism in Senior Cats: Symptoms, Diagnosis & Treatment

Feline hyperthyroidism is the most common endocrine (hormonal) disorder in cats and one of the most common conditions diagnosed in cats over 10 years of age - affecting approximately 10% of senior cats. The thyroid gland (a butterfly-shaped gland in the neck) becomes enlarged and produces excess thyroid hormone (primarily T4), accelerating virtually every metabolic process in the body.

**Why does it happen?**

In over 98% of cases, feline hyperthyroidism results from benign adenomatous hyperplasia (non-cancerous overgrowth) of the thyroid tissue. The remaining small percentage have thyroid carcinoma. The exact cause of the benign hyperplasia isn't fully established, though dietary factors (iodine levels in pet food) and environmental factors have been proposed.

Additional signs: vomiting, diarrhoea, poor coat condition, vocal, and hypertension (high blood pressure) - which can cause sudden blindness from retinal detachment.

1. **Daily medication (methimazole/carbimazole)**: Blocks thyroid hormone synthesis. Effective and affordable. Requires lifelong twice-daily dosing. Side effects include facial itching and, rarely, bone marrow suppression (monitoring required). 2. **Prescription low-iodine diet (Hill's y/d)**: By strictly limiting dietary iodine, thyroid hormone production is reduced. Highly effective but requires the cat to eat exclusively this food - not suitable for multi-cat households or cats that won't eat only one food. 3. **Radioactive iodine (I-131)**: The gold standard treatment. A single subcutaneous injection of radioactive iodine concentrates in thyroid tissue and destroys the overactive cells. Cure rate >95%. Requires short hospital stay for radiation safety. Not available everywhere. 4. **Surgery (thyroidectomy)**: Removal of affected thyroid tissue. Effective but carries anaesthetic risk in senior cats and risk of hypoparathyroidism (low calcium) if parathyroid glands are inadvertently removed.