Comparison of the effects of pulsatile and continuous TRH infusion on TSH release in men.

Pulsatile secretion of hypothalamic releasing factors modulates the release of pituitary hormones. To compare the effects of pulsatile and continuous administration of TRH on TSH secretion, we studied six healthy euthyroid 20- to 38-year-old men by obtaining blood samples every 20 minutes for 12 hours (8 AM to 8 PM) during five days of study. TRH was administered according to the following schedule: day 1 (no TRH, control); day 2 and subsequent day 3 (20 micrograms IV bolus of TRH every 96 minutes); 6 to 17 days rest; then consecutive days 4 and 5 (continuous infusion of 20 micrograms TRH/96 minutes) for 12 hours on and 12 hours off. The highest mean serum TSH levels occurred on the first day of pulsatile TRH. Serum TSH on pulsatile days 1 and 2 and continuous day 1 was significantly greater than on the control day. Similarly, the mean TSH on each day of pulsatile TRH was greater than the mean TSH on the corresponding days of continuous TRH administration. The highest serum T4 and T3 levels were observed on pulsatile day 2, suggesting that the decrease in serum TSH on this day was due to thyroid hormone negative feedback at the pituitary. The mean T4 and T3 values on continuous day 1 and 2 did not differ significantly, suggesting that other factors, including "down-regulation" of the pituitary TRH receptors by the continuous TRH infusion may be involved in the further decline of TSH levels on continuous day 2. We conclude that pulsatile TRH infusion releases more TSH, T3, and T4 than the corresponding amount of TRH administered continuously.(ABSTRACT TRUNCATED AT 250 WORDS)