Effect of vitamin A on the hypothalamo-pituitary-thyroid axis

J. E. Morley, S. Melmed, A. Reed, B. G. Kasson, S. R. Levin, A. E. Pekary and J. M. Hershman This study reports the effects of the administration of pharmacologic doses of vitamin A on multiple parameters of thyroid function. Vitamin A decreased total T4 and T3 levels. With vitamin A treatment, there was a marked increase in the percentage dialyzable T3 and T4 both in vivo and in vitro. The serum-free T3 and T4 levels as measured by dialysis were on the whole normal in vitamin A-treated rats. Following thyroidectomy, the total T4 levels were still decreased, suggesting that vitamin A produced its effects by increasing peripheral clearance of thyroxine. Vitamin A did not alter basal thyroid stimulating hormone (TSH) or its response to thyroid releasing hormone, suggesting a relatively normal hypothalamic-pituitary-thyroid axis in vitamin A-treated animals. Vitamin A may decrease tissue responsiveness to thyroid hormones as evidenced by the tendency to decreased Na-K-ATPase activity in the livers from vitamin A-treated rats and the decreased growth hormone response to T3 in GH3 pituitary cultures as shown in this study and by the decreased basal metabolic rate found after vitamin A in previous studies. Vitamin A decreased thyroid gland size and increases 125I thyroid uptake. In vitro, vitamin A enhanced T4 to T3 conversion in hepatic homogenates.