The Effect of Vitamin A, Retinoids and Retinoid Receptors on the Hypothalamic-Pituitary-Thyroid Axis

Abstract

The effects of vitamin A on thyroid hormone production and action have been known for many years. In the 1940s, Simkins demonstrated that patients with hyperthyroidism, were successfully treated with high dose of vitamin A (1) . When treated with vitamin A, these hyperthyroid patients had decreased symptoms of hyperthyroidism as well as decreased metabolic rate. Shadu and Brody studied this effect in euthyroid rats which were fed 30,000 IU of vitamin A per day for 7 days (2). The vitamin A fed rats had a 10% decrease in oxygen consumption and interestingly were found to have a 35% decrease in thyroid weight. The authors hypothesized that vitamin A suppresses pituitary thyrotropic hormone secretion thereby decreasing thyroid hormone production and oxygen consumption. This was the first suggestion that large doses of vitamin A may be affecting thyroid hormone production and function through a central mechanism. Morley and colleagues performed complementary experiments by feeding rats diets that were deficient in vitamin A and pair feeding animals with the same diet plus a supplement of 1,000 IU of vitamin A to normalize the vitamin A level (3). The animals were fed the vitamin A deficient diet until they had early signs of vitamin A deficiency, which included eye lesions and slight weight decrease.