by Jennifer Chaitman, VMD, ACVIM (Internal Medicine)
Hypothyroidism occurs when there is decreased production of thyroid hormones, thyroxine, and triiodothyronine. These hormones are made in the thyroid gland, at the base of the neck. Thyroid hormones work all over the body to increase the body's metabolism. They can speed up the heart rate, accelerate the response to stress, cause breakdown of muscle and fat tissue, stimulate red blood cell production, and regulate cholesterol levels. For this reason, when there is a lack of thyroid hormone, your dog may be sluggish, overweight, mildly anemic and have an elevated cholesterol level. Skin changes are also prominent, including hair loss, a scruffy coat, failure of hair to regrow after clipping, scaly skin, oily skin, chronic ear infections, and skin infections, among other things.
Hypothyroidism is a common disease in the dog. However, it is frequently overdiagnosed. Since treatment is lifelong and requires monitoring, it is very important that hypothyroidism is diagnosed properly. Sometimes it is a difficult diagnosis to make since it cannot be diagnosed by a single blood test, and signs of hypothyroidism are shared by several other conditions. It is also difficult to diagnose if your dog is sick or on medication. Illness and drugs can lower blood thyroid hormone levels and create the appearance of hypothyroidism, even if the thyroid gland is normal. Therefore, your doctor may order a panel of blood tests (Total T4, free T4, T3, TSH, autoantibodies) to evaluate the different thyroid hormones, their protein-bound and unbound forms, as well as anti-thyroid antibodies. Most of the time, this is sufficient for a diagnosis. Under very rare conditions, response testing may be suggested.
There are a few different types of hypothyroidism. The most common one is primary hypothyroidism, where for some reason, the thyroid gland atrophies or is destroyed by an inappropriate immune response (lymphocytic thyroiditis). Rarely, cancer cells may invade the thyroid gland and cause primary hypothyroidism. Secondary and Tertiary hypothyroidism are extremely rare and caused by brain abnormalities. Usually there will be other problems aside from the signs of hypothyroidism.
Sometimes the abnormal immune response causing hypothyroidism will also attack other glands, causing hypoadrenocorticism and diabetes mellitus. Your doctor will look for signs of these other conditions while examining your dog.
Treatment of hypothyroidism is simple and usually very well tolerated. You will give your dog a pill twice a day. There is no set dose. Therefore, your dog will be started on a suggested amount and then monitored. After a few weeks of hormone therapy, blood will be tested four to six hours after the morning pill to make sure your dog is receiving the proper dose of hormone. You will see changes in your dog's activity level within a few weeks. Improvement in skin and haircoat may take up to a few months.
We recommend testing a thyroxine level four times a year to ensure adequate dosing.