Dog: Hyperthyroidism

General information

Hyperthyroidism occurs when the thyroid glands produce too much thyroid hormone. This condition is much less common in dogs than hypothyroidism. The thyroid glands in the dog are located on either side of the trachea (windpipe) and exist as two separate halves. The thyroid gland produces two thyroid hormones, thyroxine (T4) and triiodithyronine (T3).

Thyroid hormones affect multiple body systems. They play an essential role in regulating growth, metabolism, immune function and heart function.

Production of thyroid hormone is controlled by the pituitary gland located at the base of the brain. The pituitary gland produces thyroid stimulating hormone (TSH) which stimulates the thyroid gland to produce thyroid hormones.


Hyperthyroidism in dogs is usually caused by a malignant thyroid tumor.

Cardinal symptom

Increased appetite


Dogs with hyperthyroidism have an increased metabolism. Because of this, weight loss is seen despite their increased appetite. The thyroid gland is enlarged in hyperthyroidism. Other clinical signs include increased thirst, increased urination, increased heart rates, vomiting and diarrhea.


Hyperthyroidism is diagnosed by physical examination and blood tests to measure thyroid hormone levels. Thyroid hormone results are not straightforward. Without evidence of thyroid cancer, the increased levels of thyroid hormones should be repeated to confirm. Ultrasonography and radiography are used to diagnose thyroid cancer. Radiographs should be taken of the chest to check for spread of the cancer.


Treatment of hyperthyroidism involves treatment of the thyroid cancer. If the tumor is freely moveable without being adhered to deeper structures, surgical removal of the thyroid (thyroidectomy) is the treatment of choice. Chemotherapy and/or radiation can be used to shrink the tumor for palliative treatment of tumors too large for surgery, too tightly adhered or that have evidence of spread to other organs. These treatments can also help reduce the symptoms of hyperthyroidism. They are usually performed at universities or veterinary specialty referral clinics.


If the tumor is small, easily moveable and can be completely removed with surgery, the prognosis is good for resolution of clinical signs of hyperthyroidism. However, in dogs, most cases of thyroid cancer are malignant and have already spread to other areas of the body, so the prognosis is poor. In these cases, any treatment aimed at reducing pain and clinical symptoms is usually of short duration.

Update version: 4/24/2014, © Copyright by
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