Hyperthyroidism

Seen most commonly in the cat. Elderly cats are affected with the average age of onset being 12-13 years.

Clinical signs:

• Weight loss despite polyphagia
• Increased activity, nervousness (approximately 10% will show apathy)
• Polyuria and polydipsia
• Heat intolerance, panting
• Tachycardia >240 bpm
• Poor coat, matted and unkempt
• Palpable thyroid mass often present

Often see a hypertrophic cardiomyopathy due to chronically increased heart rate and activity.

Thyroid adenoma. Courtesy of T. Scase

Aetiopathogenesis:

>98% are a functional adenoma of the thyroid gland and many will be palpable.

Treatment:

Surgical removal of the affected thyroid gland(s). If bilateral it is important to preserve at least one parathyroid gland to maintain calcium homeostasis.

Medical treatment:

• Carbamizole: Interferes with thyroid hormone synthesis.
• Radioactive iodine therapy. Useful for intrathoracic thyroid nodules.

• In young animals causes accelerated maturation of growth plate
• In adults causes osteoporosis

Presentation/Clinical Signs

1. Weight loss
2. Cardiovascular effects
3. Palpable goitre
4. Hyperactive
5. Polyuria
6. Polydipsia

Diagnosis

1. T4 Assay
2. T4 and TSH assay

Treatment

1. Surgical - Thyroidectomy
2. Medical - Thiamazole blocks synthesis of T3 and T4.
3. Radioactive Iodine