EFFECTS OF METOCLOPRAMIDE ON FASTING-INDUCED TSH SUPPRESSION

Short-term caloric deprivation leads to suppression of TSH secretion i n healthy subjects, but the mechanism of this effect is unknown. Since dopamine inhibits TSH secretion at physiologic levels, increased endo genous dopamine activity may cause the TSH suppression observed during fasting. To test this hypothesis, 11 healthy subjects underwent four studies: (1) Baseline-subjects were allowed ad libitum food. (2) MCP-s ubjects were allowed ad libitum food and received iv metoclopramide (M CP) at 30 mu g/kg/h over 48 h. (3) Easting-subjects received no calori c intake for 56 h. (4) Fasting + MCP-subjects fasted for 56 h, and rec eived iv MCP during the final 48 h of the study. Serum TSH levels were measured every 15 min during the final 24 h of each study, and a TRH stimulation test was performed at the conclusion of each study: 56 h o f fasting decreased 24 h mean TSH levels and TSH pulse amplitude by 40 %, with blunting of the TSH response to TRH. MCP infusions increased 2 4 h mean TSH levels and TSH pulse amplitude 26-34%, with no difference s between the fasting and nonfasting studies. MCP infusions did not no rmalize TSH levels, TSH responses to TRH, or serum T-3 levels during f asting. These data suggest that endogenous dopaminergic activity does not play a major role in fasting-induced TSH suppression in healthy su bjects.