Thyroid carcinoma in cats: when to suspect it
14-year-old cat treated with thiamazole for 4 years. The dose has had to be increased several times to control T4. Large, irregular, poorly mobile neck masses are now palpable.
Which statement is correct?
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A. This is the expected course of an adenoma and increasing thiamazole further is enough
Although adenomas grow over time, an increasing need for thiamazole and large, irregular, fixed masses are warning signs of malignancy that must be investigated.
B. Thyroid carcinoma is the most common cause of feline hyperthyroidism
More than 95% of cases are due to adenomatous hyperplasia or benign adenomas. Carcinoma accounts for only a small percentage.
C. Thiamazole controls both hormone production and carcinoma growth
Thiamazole has no antitumour effect: it may control T4 for a while, but the tumour keeps growing and metastasises.
D. Carcinoma should be suspected; it is confirmed by histopathology and the treatment of choice is high-dose iodine-131, sometimes with surgery
Correct. Feline thyroid carcinoma is rare (around 1–3%), but is suspected with large or invasive masses, multifocal or intrathoracic uptake on scintigraphy, and an increasing need for thiamazole. It responds to iodine-131 doses much higher than those used for adenoma.
lightbulb Key takeaway
Suspect feline thyroid carcinoma with large, fixed or multiple masses, an increasing need for thiamazole, or recurrence after iodine-131. Treatment is high-dose iodine-131, with or without surgery.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.