Effect of food deprivation and altered thyroid status on the hypothalamic-pituitary-thyroid axis in the rat.

Propylthiouracil (PTU) was administered to rats for different lengths of time with or without food deprivation on the last 2 days. Within 4 days of PTU treatment peripheral 3,5,3'-tri-iodothyronine (T3) fell to low levels and beta-subunit of thyroid-stimulating hormone (beta-TSH) mRNA increased significantly in the anterior pituitary. Pro-thyrotrophin-releasing hormone (pro-TRH) mRNA in the hypothalamic paraventricular nucleus (PVN) increased significantly in the control group of animals by 8 days and in the food-deprived group by day 12; the increment of pro-TRH mRNA in the food-deprived group on day 12 was significantly less than that in the control group. In a second study, animals were treated with intraperitoneal injections of T3 with or without the food deprivation. After 4 days of T3 treatment, peripheral T3 levels were markedly increased and pro-TRH mRNA in the PVN and beta-TSH mRNA in the anterior pituitary were significantly reduced. Food deprivation had no additional suppressive effect. These studies confirm that the predominant effect of food deprivation on the thyroid axis is at the hypothalamic or suprahypothalamic level and that it can, at least in part, overcome the increase in TRH mRNA due to diminished T3 feedback.