Hypothyroidism is a condition in which there are low levels of thyroid hormones in the blood. Low thyroid levels are a result of a dysfunctional thyroid gland or pituitary gland. Dogs are most prone to hypothyroidism and it is rare in cats. In many cases, hypothyroidism is hereditary. Hypothyroidism is simply treated by oral supplementation of thyroid hormone in the form of a pill.

Regular checks of blood thyroid levels are necessary to determine adequate supplementation. Normal thyroid gland function is a negative feedback system. If the level of thyroid hormones in the blood reaches a low level, the hypothalamus in the brain produces thyroid-releasing hormone (TRH). TRH stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH).

TSH stimulates the thyroid gland to produce thyroid hormones (T3 and T4). T3 regulates the body's overall energy production and metabolism. T4 is converted into T3 by body tissues based upon their particular need. T4 also ends the feedback system by cutting off further production of TRH and TSH by the hypothalamus and pituitary gland.

If either the pituitary gland or thyroid gland becomes dysfunctional, then the lack of thyroid hormones in the body causes low energy and low metabolism. Signs of hypothyroidism include lethargy, recurrent infections, weight gain with normal appetite, heat-seeking behavior, low heart rate, hair loss, and dry skin. Intact females go into heat less often or stop ovulating. It will also decrease growth rate in young dogs. However, the signs of hypothyroidism are usually not apparent until an advanced stage of the disease.

Hypothyroidism is usually detected early when a thyroid test is performed along with a general blood profile to assess a pet's health. The thyroid test by radioimmunoassay (RIA) measures the total T4 levels in the blood. If the results of the RIA test appear to be inconclusive and the pet seems to be in between the high and low ranges, there is also a test called the "free T4 by equilibrium dialysis test" that measures the active levels of T4 available to the body tissues. Free levels are a much more sensitive indicator of hypothyroidism than are total levels.

The drug of choice used to treat hypothyroidism is synthetic levothyroxine (T4). The treatment regimen starts with a twice daily schedule of a calculated dose of the thyroid supplement. After about 4-6 weeks of therapy, the patient's response is evaluated and thyroid levels are checked again. If needed, the doctor will adjust the dosage and frequency of the medication. Thyroid tests are most accurate 6 hours after the morning dose at which time the thyroid levels are at their highest. To maintain the correct supplementation, thyroid levels should be checked every 6 months and the dosage altered as required.

l. Ackerman, Lowell, Pet Skin and Haircoat Problems, Veterinary Learning Systems Co., Inc., Trenton, NJ, p.137-141, 1993
2. Bill, Robert (Pete), Pharmacology for Veterinary Technicians, American Veterinary Publications, Inc., Goleta, CA, p.222-225, 1993