Hyperthyroidism – A Common Elderly Feline Condition

What is Hyperthyroidism?

Hyperthyroidism is an endocrine disorder resulting from an “overactive” thyroid gland, a “butterfly” shaped gland located in the neck. The hypothalamus, a small region of brain located above the pituitary gland releases thyrotropin releasing hormone (TRH), which binds to the receptors on the thyroid, initiating the release of the thyroid stimulating hormone (TSH). The thyroid stimulating hormone regulates the amount of the iodine-containing hormones triiodothyronine (T3) and thyroxine (T4) secreted from the thyroid gland. The thyroid gland mainly secretes T4 which is converted to T3, a more potent form in the peripheral tissues. The majority of T4 and T3 is bound to plasma proteins. Free T4 and T3 are active and immediately available for uptake into cells. Hyperthyroidism occurs when levels of T4 and T3 are elevated.

In order to maintain homeostasis, T4 exerts negative feedback on the hormones TRH and TSH which stops their release when they are not needed. These thyroid hormones play an important role in metabolism, regulating body temperature and heart rate.

Hyperthyroidism occurs most frequently in elderly cats and rarely in dogs. The most common cause of an overactive thyroid in cats is from a benign thyroid tumor called an adenoma. Occasionally, a malignant tumor may develop called a thyroid adenocarcinoma.

Clinical Symptoms Include...

  • Weight loss
  • Excessive hunger and eating (polyphagia)
  • Vomiting
  • Restlessness
  • Increased urination and thirst (Polyuria/polydipsia)
  • Diarrhea
  • Unkempt coat, hair loss 
  • Palpable thyroid mass (thyroid slip)
  • Increase heart rate (heart murmur/tachycardia)
  • Dehydration
  • Aggression
  • High blood pressure (hypertension)

A thyroid panel is test used to evaluate thyroid functioning. It includes the concentration of T4, free T4 (active form) and the thyroid stimulating hormone (TSH). An elevated level of T4 and T3 combined with a decrease in TSH hormone are indicative of hyperthyroidism.

How is Hyperthyroidism Treated?

There are a couple different treatment options for hyperthyroidism, however radioactive iodine therapy is the gold standard.

Radioactive Iodine

Radioactive iodine is the treatment of choice for hyperthyroidism. Radioactive iodine is injected under the skin (subcutaneous) and concentrates in thyroid tissue where it kills the overactive cells. After this procedure is performed, the patient becomes “radioactive” and needs to remain separated from other pets and people for about two weeks. This treatment option is about 95% effective at curing this disease.

Oral Medication

Methimazole (Tapazole) is an oral life-long medication that needs to be given daily to control thyroid hormones. It may not be suitable for cats with chronic liver or kidney disease. The disadvantage to this treatment option is that the thyroid tumor is still present and may continue to grow.


 A thyroidectomy is the surgical removal (-ectomy) of the thyroid gland. A disadvantage to this method of treatment is if both thyroid glands are affected (~70% of cats) and need to be removed, the cat will develop hypothyroidism and need to take daily oral medications. The parathyroid glands are two tiny glands located directly above the thyroid. During surgery these glands may accidentally be removed leading to electrolyte imbalances, most commonly a decrease in calcium (hypocalcemia). This method of treatment is more invasive compared to other options and is typically not used except in the rare case of that malignant thyroid tumor is present.  

Prescription Diet

Hill’s y/d thyroid prescription diet is an option if other treatment methods are declined. It’s a nutritionally balanced diet that can help restore normal thyroid levels. It is more effective for mild to moderate hyperthyroidism and alone cannot effectively treat severe hyperthyroidism. It may be used in conjunction with other treatments. 


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