Background: Causes are genetic, environmental, and possibly immunologic. Horses are commonly hypothyroid, but rarely is it a primary problem. Measuring thyroid function is generally inconclusive; treatment with thyroid hormone is less than satisfactory.
Symptoms: Obesity, lethargy, intolerance of cold, dull coat, dandruff, skin disease, slow heart rate and metabolism.
Diagnostics: If the disease has progressed to the point where thyroid function is severely compromised, it is easily diagnosed. In horses, many commonly used drugs such as phenylbutazone suppress thyroid function or interfere with the tests.
Special Notes: Most horses with apparent thyroid dysfunction probably have insulin resistance and pituitary dysfunction as the primary condition.
Principles for Supplementation: Natural treatment supports the thyroid gland and immune system; treatment may need to be multifactorial, including homeopathy, acupuncture and herbs to make permanent changes.