A cat presents with weight loss and a palpable neck mass; what is the most common feline endocrine disorder causing weight loss and what test confirms it?

• A. Hyperthyroidism; measure total T4 or free T4
• B. Hypothyroidism; measure T4
• C. Diabetes mellitus; measure blood glucose
• D. Chronic kidney disease; measure BUN

Answer: (A)

Hyperthyroidism is the most common feline endocrine disorder causing weight loss, especially in older cats, and an enlarged thyroid gland can present as a palpable neck mass from nodular hyperplasia. To confirm the diagnosis, measure circulating thyroid hormone levels. Total T4 is the standard screening test, and most hyperthyroid cats have an elevated T4. If results are inconclusive, measuring free T4 by equilibrium dialysis is more sensitive and helps confirm cases where total T4 might be normal due to other illnesses or variability. The neck mass points to thyroid involvement, making thyroid hormone testing the best confirmatory step.