AJP - Endocrinology and Metabolism, Vol 238, Issue 2 174-E179, Copyright © 1980 by American Physiological Society
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.
