Feline Hyperthyroidism

Hyperthyroidism in Cats | Rochester MN

Hyperthyroidism is a common disease of middle-aged and older cats. Enlarged thyroid glands in the neck region secrete excessive levels of thyroid hormones. Thyroid gland enlargement is usually due to benign changes within the thyroid gland, and is rarely associated with malignant tumors.

Signs on Hyperthyroidism

Hyperthyroidism can cause a myriad of clinical signs including weight loss, vomiting, diarrhea, vocalizing, hyperactivity, poor hair coat, and increased appetite, thirst, and urination.


After considering history and physical exam findings (such as weight loss, thyroid gland enlargement, increased heart rate, heart arrhythmias), blood is drawn to perform multiple tests. Hyperthyroidism is diagnosed with a blood test checking for elevated thyroid hormone levels. A chemistry panel screens for other diseases (such as diabetes) and ensures that the kidneys are healthy enough to tolerate the medication/treatment.


  1. Medication – methimazole
    • Methimazole is a daily medication given lifelong to manage hyperthyroidism.
    • This medication can be administered in pill, liquid, treat, or transdermal gel forms.
    • While methimazole is usually well-tolerated, side effects may include vomiting, diarrhea, lethargy, and facial itching.
  2. Food – y/d
    • Hill's Science Diet recently introduced this product as another treatment option for hyperthyroidism.
    • This diet severely limits the iodine level in the food to reduce circulating levels of thyroid hormone in the blood stream.
    • Within a multi-cat household, there are complications in feeding this diet. The hyperthyroid patient can only eat y/d and absolutely nothing else. Normal thyroid cats can eat y/d for part of their diet but they must be supplemented with regular food to stay healthy.
  3. Surgery – thyroidectomy
    • This surgery is not done as commonly as it was in the past, due to the potential for complications.
    • The most dangerous surgical complication is the inadvertent removal of the parathyroid glands located next to the thyroid glands. If this happens, then lifelong daily calcium supplementation is needed to avoid life-threatening hypocalcemia.
    • Another potential complication of surgery is the failure to remove all of the abnormal thyroid tissue. If this happens, then the patient is still hyperthyroid right away or soon after surgery.
  4. Radioactive iodine
    • Only available at referral specialty clinics, this is a good option for those who cannot give pills to their cats.
    • Radioactive iodine is given as an injection. This iodine gets taken up by and destroys only the abnormal thyroid tissue, leaving the surrounding tissues undamaged. The cat is hospitalized for up to two weeks until the radioactivity is eliminated.
    • This costs about $1,300 and can only be done on otherwise healthy patients.

Follow Up

If methimazole is used, give meds as directed for four weeks. Then schedule a recheck exam, where weight, general body condition and repeat bloodwork will be assessed. The thyroid hormone level needs to be rechecked to determine if the initial dosing strength needs to be adjusted, and other bloodwork ensures that the kidneys are tolerating the use of this medication.

Heart murmurs should be reassessed to ensure they resolve with use of the medication. Weight gain should be seen. Gastrointestinal signs (vomiting, diarrhea), vocalization, and hyperactivity should also improve.

Quarry Hill Park Animal Hospital
2554 Clare Ln NE
Rochester, MN 55906

Phone: 507-285-1059

Hours of Operation
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Tue: 7:30 am - 5:30 pm
Thu: 7:30 am - 6:30 pm
Sat - Sun: Closed

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