Feline Hyperthyroidism

The thyroid is a two-lobed gland located in the neck region with one lobe on either side of the trachea (windpipe). The thyroid hormone produced by the gland is transported to every cell in the body via the blood stream. The primary function of thyroid hormone is to enhance the rate at which cells function; too much hormone makes the cells work very fast while too little causes the cells to slow down.

Hyperthyroidism is the production of too much thyroid hormone. The disease is caused by a benign growth of cells that secrete thyroid hormone in excess of the normal levels.  It is rare in dogs but is one of the most common diseases diagnosed in cats 7 years of age and older (usually over 10 years of age).


Clinical signs for hyperthyroidism vary among affected cats. Among the most common signs seen by owners is extreme weight loss, increased appetite, muscle weakness, heart disease (increase in the size of the heart, increased heart rate, changes in heart rhythm, cardiac arrest), intolerance to stress, patchy hair loss, failure to groom, increases in water intake and urine output and restlessness or nervousness. Vomiting and diarrhea are also common. Rarely seen are signs of panting, loss of appetite, listlessness or seeking cool places to rest.

Whenever your pet is showing signs of a health issue your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from feline hyperthyroidism or another serious condition, a veterinary specialist is available at an ExpertVet certified hospital.


To diagnose hyperthyroidism blood tests of thyroid activity are necessary. Most hyperthyroid cats will have an elevated thyroid hormone (T4) in their blood stream. However a small percentage of cats do not have a "diagnostic" elevation in their blood T4 level. In these instances follow up testing or additional assessment of thyroid function is indicated.


Hyperthyroid cats that are not treated tend to become increasingly ill, whereas treatment will usually restore a patient's health. Three different treatments are available for managing hyperthyroidism in cats.

Methimazole (Tapazole®) is an oral medication that reduces the thyroid gland’s ability to produce thyroid hormone. The major disadvantage to methimazole is that it must be used for the life of the patient. Methimazole may cause vomiting, loss of appetite, liver damage, decreases in white and red blood cells and platelets idiosyncratically. Full blood work is necessary within one week of the introduction of methimazole to ensure that the liver and bone marrow are tolerant to the drug. Finally regular reassessment of thyroid function is required to confirm the methimazole dose is controlling the hyperthyroidism (usually every three months). Methimazole, if tolerated, is often recommended in patients with concurrent chronic renal (kidney) insufficiency.

Surgical removal of abnormal thyroid gland is a treatment option that is no longer recommended. Although surgery does resolve the hyperthyroidism quickly, it requires anesthesia and hospitalization. This is contraindicated as some hyperthyroid cats are at increased anesthetic risk. Rarely, parathyroid glands may be accidentally removed during thyroidectomy; a surgical consequence that results in hypocalcaemia (low blood calcium) and lifelong calcium supplementation. Finally surgical correction of thyroid hyperactivity may be detrimental to the patient's kidney health. Rapid reduction in thyroid activity alters renal (kidney) blood flow. In patients with concurrent renal disease this rapid change in blood flow may aggravate the renal disease.