Hyperthyroidism in Pets

  • Hyperthyroidism is a disorder resulting from the excessive production of thyroxine (T4) and triiodothyronine (T3) by the thyroid gland.
  • 95% are caused by thyroid adenoma (benign hyperplastic thyroid gland), 5% are caused by malignant thyroid adenocarcinoma.
  • Most common endocrinopathy affecting cats older than 8 years of age.
  • There is no sex predilection and domestic shorthair and longhair cats are most commonly affected.

Clinical Signs:

  • Weight loss
  • Hyperactivity
  • Excessive appetite
  • Unkempt hair coat
  • Excessive drinking
  • Urinating large amounts
  • Vomiting or diarrhea
  • Less common signs: weakness/lethargy, decreased appetite, heat and stress intolerance, panting, hair loss


  • Physical examination may reveal a palpable thyroid mass in the neck, cardiac changes (heart murmur, arrhythmia, elevated heart rate), weight loss
  • Laboratory analysis of blood, including but not limited to CBC/ serum chemistry profile, urinalysis, and a thyroid panel.

Treatment of Hyperthyroidism in Pets:

Dietary therapy:

  • Hill’s® Prescription Diet® y/d™
  • Production of thyroid hormone requires the thyroid gland to have sufficient amounts of iodine, which is provided solely by dietary intake.
  • This diet is a limited iodine food, which can be used to control thyroid hormone production.
  • It can take an average of 4-12 weeks for the thyroid hormone to return to normal.

Medical therapy:

  • Methimazole- inhibits the synthesis of T4 and T3.
  • Side effects of the medication: anorexia, vomiting, lethargy, hematologic abnormalities.
  • Cats receiving methimazole should have a laboratory analysis of blood every 3-4 weeks for the first 3 months of treatment and then every 6 months thereafter.

Radioactive iodine therapy (I131):

  • The goal of therapy is to restore normal thyroid condition by administering a single dose of radiation without producing hypothyroidism (underactive thyroid).
  • Prior to treatment need to screen the patient for heart and renal (kidney) disease.
  • Give a trial of methimazole prior to I131 therapy to evaluate kidney function. If the patient has underlying kidney disease, the kidneys’ will usually decompensate several weeks after the start of methimazole. Those patients are not candidates for I131 therapy.
  • Requires hospitalization in a special treatment facility for 3-5 days. Visit www.radiocat.com for more information.