Effect of vitamin A on the hypothalamo-pituitary-thyroid axis.
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.
D000284:Administration, Oral; D000818:Animals; D004305:Dose-Response Relationship, Drug; D007030:Hypothalamo-Hypophyseal System / Q000502:physiology*; D008099:Liver / Q000201:enzymology; D008297:Male; D010902:Pituitary Gland / Q000502:physiology; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013961:Thyroid Gland / Q000502:physiology*; D013974:Thyroxine / Q000378:metabolism; D013997:Time Factors; D014284:Triiodothyronine / Q000378:metabolism; D014801:Vitamin A / Q000008:administration & dosage / Q000494:pharmacology*
D014284:Triiodothyronine; D014801:Vitamin A; D000254:Sodium-Potassium-Exchanging ATPase; D013974:Thyroxine