Feline Hyperthyriodism

February 26, 2015 / Cat's Meow / Leave a comment

The thyroid gland is responsible for regulating metabolism through the production of “Thyroid Hormone”, or “T4”. The level of T4 secretion controls heart rate, weight loss and gain, kidney function, and numerous other bodily processes.  Older cats are prone to developing benign growths of the thyroid gland known as “adenomas”. These growths are not usually cancerous, but secrete an abnormally large amount of thyroid hormone. Chronically high levels of T4 are damaging to the body and will eventually lead to death.

Signs and Symptoms

Cats with hyperthyroidism often initially experience signs the owners may perceive as positive. These include weight loss, increased activity, and “kitten like” behaviour.  Over time, this progresses to increased blood pressure, severe weight loss, heart disease, blindness, and kidney damage. The slow onset of signs means that cat owners will often miss these changes until the late stages of disease. Regular examinations with your veterinarian can help identify these early signs of disease before they progress to the point of being irreversible.


Diagnosis of hyperthyroidism is based on a combination of clinical signs and bloodwork.  Weight loss, muscle wasting, heart disease and a thyroid nodule are all indicators to a veterinarian that a cat may have hyperthyroidism. Definitive diagnosis is based on bloodwork, specifically an elevated level of “T4” or “thyroid hormone”. Thyroid Hormone levels above the normal range, especially in an older cat, are very specific for diagnosis of hyperthyroidism in cats, though rare cases may require additional testing.

Treatments and Prognosis

There are several treatment and management strategies available for hyperthyroidism. The most common among these include:

  • Thyroidectomy: Surgical removal of the affected thyroid gland was a common treatment option in the past, however it is rarely performed today due to high complication rates and better modern alternatives.


  • I131 Therapy: This is considered the gold standard, ideal treatment for hyperthyroidism. Administration of a single injection of radioactive iodine selectively kills the thyroid nodule with a very low rate of side effects and a high success rate. This must be performed at specialty centers and can be costly; however it is the only modern curative treatment.


  • Methimazole: Daily medication is a commonly used option for medical management of hyperthyroidism. Administration of pills or transdermal gels to suppress thyroid activity can maintain thyroid hormone levels in the normal range. Regular bloodwork and dose adjustment are required to ensure levels stay in the normal range.


  • Dietary Therapy: An iodine-depleted diet is commercially available which prevents cats from synthesizing thyroid hormone. This “y/d” diet is able to restrict thyroid hormone levels in the normal range, however care must be taken to prevent cats managed this way from eating any other food, and blood levels must be closely monitored.


Cats with hyperthyroidism can be well managed and with careful treatment may live normal lives for many years.  Possible complications of treatment include “unmasking” underlying kidney disease; or “overcompensation” leading to hypothyroidism, or low T4 levels.

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