Primary Hypothyroidism and Thyroid Goiter in an Adult Cat

A 5-year-old, neutered male domestic shorthair cat was referred for evaluation of suspected hypothyroidism. In the previous 2 months, the owner reported mild lethargy, weight gain with decreased appetite, unkempt hair coat, and an episode of bilateral external otitis that temporarily improved after a 10-day treatment with ear medications containing gentamicin, betamethasone, and clotrimazole; treatment was completed 50 days before presentation. The cat had been slightly overweight for several months before the referring veterinarian was consulted, but more precise information was not available from the owner. The cat was started on a commercial diet to control obesity. One month before admission, the referring veterinarian performed a CBC, serum biochemical profile, and urinalysis, and the results were unremarkable. In addition, serum total thyroxine concentration (TT4) was within normal limits (1.1 lg/dL; reference range, 0.8– 4.7) and free thyroxine concentration (fT4), measured by chemiluminescence immunoassay (CLIA), was low (<3.9 lg/dL; reference range, 9.0–33.5). On admission, the cat was obese (body condition score, 8/9) with a body weight of 7.6 kg, and had an unkempt hair coat with diffuse scaling and hypotrichosis ventrally (Fig 1). On palpation of the thyroid region, bilateral symmetric nodules with a diameter of approximately 2–3 cm each were detected. Based on a dermatologic examination, widespread exfoliative dermatosis and bilateral ceruminous otitis externa were diagnosed. Fungal culture of plucked hairs and scraped scales identified Microsporum canis infection. The thyroid tests were repeated at the same laboratory and results included a low TT4 (<0.7 lg/dL; reference range, 0.8–4.7), low fT4 measured by equilibrium dialysis (<0.4 ng/dL; reference range, 0.7–2.3), and high canine thyroid-stimulating hormone (TSH) concentration (5.6 ng/mL; reference range, <0.5), leading to a diagnosis of primary hypothyroidism. The cat was started on levothyroxine at the dosage of 0.1 mg PO q24h. In addition, topical econazole, q3d and itraconazole, 5 mg/kg PO q24h, were prescribed for the dermatophytosis. To investigate the thyroid gland and identify any ectopic thyroid tissue and to characterize the pituitary gland, computed tomography (CT) of the head, neck, and chest was scheduled 5 days later under general anesthesia, along with surgical excision of a thyroid nodule. Before induction of anesthesia, an electrocardiogram and echocardiography were performed that identified no abnormality. The CT was obtained with helical acquisition using a 4-slice scanner with 1.25 mm slice thickness (acquisition parameters: 120 kV, 160 mAs, 1 pitch). After IV administration of 2 mL/kg iohexol, a dynamic study of the pituitary gland was obtained continuously scanning in sequence from the rostral to the caudal margin of the sella turcica until the contrast