For many years it was believed that hypothyroidism was a fairly common syndrome in horses, as it is in dogs and humans. Many of the so-called easy keeper horses were tested for hypothyroidism by checking a single serum T4 level and if it was low, those horses were diagnosed with hypothyroidism. These horses were given thyroid supplementation. Recent research has now shown that these easy keeper horses are not hypothyroid and that a single low T4 level is not diagnostic of this condition. It has been proven that most of these easy keeper horses actually have a condition called metabolic syndrome.

Although thyroid hormone supplementation may have some benefit for 2 to 3 months in these horses to help them lose weight and prevent laminitis or founder, it is not indicated long term as the thyroid glands in these horses are normal. Thyroid supplement has also been used in brood mares as it was also believed that hypothyroidism was the reason some mares would not get pregnant. To determine if thyroid hormone concentration was related to pregnancy in mares, a study was performed in Kentucky with over 300 mares. Serum T4 levels were measured and ultrasound exams were performed for pregnancy at 15 to 16 days after ovulation. Sixty of these 300 mares were receiving thyroid hormone supplementation due to low T4 thyroid levels. The study revealed that 70% of both groups of mares were pregnant, indicating that thyroid hormone concentration and pregnancy were not related. Since thyroid supplementation did not increase pregnancy rates, it is believed that thyroid supplementation is not beneficial or indicated in brood mares.