Spontaneous Hyperthyroidism in the Cat.

Peterson M.E., Johnson J.G. and Andrews L.K.

Conference Proceedings, (1979). American College of Veterinary Internal Medicine, Seattle: p.108


Spontaneous hyperthyroidism due to functional thyroid adenomas was diagnosed in 5 cats ranging in age from 11 to 15 years. Clinical signs included weight loss, polyphagia, hyperdefecation, hyperactivity, and tachycardia. One cat had congestive heart failure. Screening tests (complete blood count, serum biochemistries, urinalysis) were noncontributory; 24-hour fecal fat analyses were normal. Electrocardiograms revealed sinus tachycardia in all 5 cats and increased voltage in all leads in 3 cats. Thyroxine (T4) levels by radioimmunoassay (RIA) ranged from 8.0 to 24.0 mg per 100 mL (normal = 1.4 to 4.0 mg per 100 mL). Triiodothyronine (T3) levels ranged from 120 to 260 ng per 100 mL (normal = 75 to 200 ng per 100 mL). Scintiscans performed 24 hours post-IV injection of 10 mCi 131I revealed a nodule of increased radionuclide uptake in one thyroid lobe in all cats. Uptake in the contralateral lobe was suppressed. Radioactive iodine uptake (RIU) calculated at 24 hours ranged from 32 to 58%. Hemithyroidectomy was performed on all cats. Histopathologic analysis revealed thyroid adenomas. Postoperatively, T4 values returned to normal with remission of clinical signs. It was concluded that: (1) Functional thyroid tumors causing signs of hyperthyroidism occur in the cat, (2) Results of 131I scanning procedures and RIU were consistent with a functional adenoma, (3) T4 levels were consistently elevated in these cats prior to therapy, while T3 levels were usually normal, (4) These thyroid adenomas were easily removed surgically, and signs of hyperthyroidism resolved rapidly following removal.