Hypothyroidism (HT) is a frequently misdiagnosed condition in adult horses that are overweight and have chronic founder. A sample measuring the two thyroid hormones, T3 and T4, is usually taken to confirm the diagnosis. The problem with this diagnosis is two fold:

• Truly hypothyroid horses are unthrifty, have poor appetites, and are mildly depressed.
• The blood levels of these hormones are quite variable in the healthy horse and do not accurately reflect thyroid function.

This article discusses the diagnosis, treatment, and prognosis of hypothyroidism in adult equines. Hypothyroidism has been well documented in newborn foals and for more information see: Hypothyroidism in Foals.

Thyroid hormone regulation is a complicated mechanism that starts in the brain. In response to numerous inputs the brain releases thyroid releasing hormone (TRH) that stimulates another part of the brain to release thyroid-stimulating hormone (TSH) into the blood where it circulates to the thyroid glands. There is a pair of them located on the back of the trachea, high in the neck close to the head.

Thyroid hormones, primarily T4, is released in response to stimulation from thyroid-stimulating hormone released by the brain. Tissues convert the T4 to the more active T3 form. These hormones affect the rate of many physiologic processes including: fat, protein and carbohydrate metabolism, increasing protein synthesis, increasing gluconeogenesis and promoting mobilization and utilization of glycogen stores.

Thyroid hormones also increase oxygen consumption, body temperature, heart rate and cardiac output, blood volume, enzyme system activity, and growth and maturity. Thyroid hormone is particularly important for adequate development of the central nervous system. While the exact mechanisms how thyroid hormones exert their effects are not well understood, it is known that thyroid hormones (primarily triiodothyronine) act at the cellular level. In humans, triiodothyronine (T3) is the primary hormone responsible for activity. Approximately 80% of T3 found in the peripheral tissues is derived from thyroxine (T4) which is the principal hormone released by the thyroid.