EXPRESS: Nasal non-hypersecretory ectopic follicular thyroid carcinoma in a cat

Case summary: An 8-year-old neutered male domestic shorthair cat was evaluated for stertor, sneezing and right-sided nasal discharge. Rhinoscopy identified a mass occupying the choanae and nasopharynx. Histopathological examination of samples from two endoscopic debulking procedures revealed well-differentiated thyroid tissue composed of colloid-containing follicles lined by uniform cuboidal cells. Computed tomography (CT) demonstrated a right nasal and nasopharyngeal mass; both cervical thyroid lobes appeared normal. Serum total thyroxine concentration was within the reference interval (2.21 µg/dl; reference interval 1.0–4.0) with no evidence of hyperthyroidism. Technetium-99m pertechnetate scintigraphy revealed faint, poorly delineated activity over the right nasal cavity. As radiotherapy was unavailable and invasive surgery was declined, 370 MBq (10 mCi) iodine-131 was administered following progression. Four weeks later, worsening respiratory signs and epistaxis prompted repeat CT, which documented destruction of the ethmoidal turbinates, medial orbital wall and cribriform plate, with retrobulbar and intracranial extension. Given the extensive disease and poor prognosis, the owner elected euthanasia. The progressive, destructive behaviour supported a diagnosis of well-differentiated, non-hypersecretory ectopic follicular thyroid carcinoma despite its bland histological appearance. Relevance and novel information: To the authors’ knowledge, this is the first reported ectopic follicular thyroid carcinoma arising in the nasal cavity and choanae of a cat. Well-differentiated thyroid tissue in a feline nasal mass should raise suspicion of ectopic thyroid neoplasia despite normal cervical thyroid glands and absent hyperthyroidism. Rapid recurrence and progressive local invasion demonstrate that bland histological features do not exclude malignant behaviour, supporting cross-sectional staging and early consideration of definitive local treatment.