Feline Hyperthyroidism

Feline Hyperthyroidism

Evaluation for I-131 Therapy

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 if untreated. Hyperthyroidism causes abnormally increased renal blood flow (RBF) and glomerular filtration rate (GFR) that can “mask” concurrent chronic kidney disease by making the kidney values (Serum creatinine and BYN) appear better than they actually are.  Therefore when hyperthyroidism is treated, GFR returns to normal and serum creatinine may increase substantially.  As radioactive iodine (I-131) is a curative (i.e. irreversible) treatment, the following guidelines have been developed at The Ohio State University Veterinary Medical Center to evaluate hyperthyroid cats to determine if they are good candidates for radioactive iodine therapy.


  • In order to fully evaluate the hyperthyroid patient, biochemistry profile, urinalysis, CBC, and blood pressure are recommended in addition to serum total thyroxine (T4) measurement. This data is needed as a baseline to monitor for changes associated with therapy and also to check on the status of other concurrent diseases.
  • Cats requiring multiple medications, insulin, SQ fluids for other concurrent diseases are not good candidates for I-131 due to the isolation period and radiation safety requirements for limited handling.
  • Cats that are extremely stressed or have concurrent illnesses that may be exacerbated by a hospital stay (idiopathic cystitis, herpes virus) may also not be considered good candidates for I-131 therapy.
  • A "Methimazole challenge" is recommended to "unmask" concurrent chronic kidney disease.  However, methimazole challenge may not be possible in patients with adverse reactions to this medication.

Recommended protocol for methimazole challenge:

  • The cat is treated with methimazole (Tapazole) orally for 2 weeks.  
  • Biochemistry profile, urinalysis, CBC and blood pressure are repeated while on methimazole. This is recommended in any cat receiving methimazole not only to monitor for underlying kidney disease but also for side effects such as bone marrow suppression and increase liver enzymes.
  • If renal function remains stable but serum thyroxine concentration has not sufficiently decreased (middle of normal range), 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, I-131 therapy is considered safe and is recommended to the owner.
  • If lowering serum thyroxine concentration with methimazole therapy results in significant worsening of azotemia or adverse clinical signs associated with kidney disease, I-131 therapy may not be recommended.

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