Effects of food deprivation on the feedback regulation of the hypothalamic-pituitary-thyroid axis of the rat.

We have studied the effects of food deprivation on the plasma concentrations of T4, T3, and TSH, the TSH MCR, the pituitary TSH content, and the pituitary TSH response to TRH in female rats. Two days before the beginning of the experiment all rats were prepared with chronic intraatrial catheters through which TRH (100 ng/100 g BW) or [125I]iodo-TSH was administered, and sequential blood samples were obtained. The plasma concentrations of T4 and T3 in food-deprived rats were significantly (P less than 0.05) reduced compared to those in control rats 6, but not 2, days after food removal. Despite the fall in plasma concentrations of T4 and T3, no compensatory rise in the plasma TSH concentration was observed in the food-deprived rats. Plasma TSH concentrations did not differ between groups at any time. The change in the plasma TSH concentration (delta plasma TSH) was significantly greater 15, 30, and 45 min after iv bolus injection of TRH in food-deprived rats than in control rats. To further evaluate the basal TSH secretion rate (MCR X plasma TSH concentration) and the amount of TSH secreted in response to TRH (MCR X area under delta plasma TSH curve), the MCR of TSH was determined in control and food-deprived rats from the plasma disappearance curves of injected [125I]iodo-TSH. No differences were found in the TSH MCR or the calculated basal plasma TSH secretion rate (micrograms per day/100 g BW) of control or 6-day food-deprived rats. However, the change in the amount of TSH secreted in response to TRH was significantly greater in food-deprived rats than in control rats (17.7 +/- 2.1 vs. 11.0 +/- 1.0 micrograms/100 g BW; P less than 0.05). In addition, pituitary weight was significantly decreased in food-deprived rats (8.3 +/- 0.3 vs. 10.8 +/- 0.5 mg; P less than 0.05), but the pituitary TSH concentration (micrograms per mg tissue) was unchanged. These results are consistent with the development of tertiary hypothyroidism (i.e. decreased TSH stimulatory input to the pituitary by TRH or some other agent) during food deprivation in the rat.