Feline Hyperthyroidism

Hyperthyroidism is a common endocrine disorder of cats. It is a potentially serious disease that causes weight loss, hair coat changes, gastrointestinal disturbances, hypertension, and cardiac problems iff untreated. Hyperthyroidism increases renal blood flow (RBF) and glomerular filtration rate (GFR) that may mask concurrent chronic renal disease. Treatment of hyperthyroidism may decrease GFR by as much as 50% and blood urea nitrogen (BUN) and serum creatinine (SCr) concentrations may increase substantially. The following guidelines have been developed at The Ohio State University Veterinary Medical Center to screen hyperthyroid cats to determine if they are candidates for radioactive iodine (131I) therapy.


  • Urinalysis, biochemical profile and serum thyroxine (T4) concentration are performed.
  • "Methimazole challenge" is recommended to "unmask" concurrent chronic renal disease. The cat is treated with methimazole (Tapazole) orally for 2 weeks and serum chemistry tests are repeated. If renal function remains stable but serum thyroxine concentration has not sufficiently decreased, the dosage of methimazole is increased for 2 more weeks at which time renal function is re-evaluated. This process is continued until the cat is receiving a therapeutic dosage of methimazole and serum thyroxine concentration is normal or until adverse clinical signs (e.g. anorexia, lethargy, vomiting). If adverse clinical signs do not develop after a therapeutic dosage of methimazole has been achieved, (131I) therapy is considered safe and is recommended to the owner.
  • If lowering serum thyroxine concentration with methimazole therapy results in adverse clinical signs or worsening of azotemia, (131I) therapy may not be recommended.

For appointments, call the OSU Veterinary Medical Center: (614) 292-3551.