EFFECTS OF NALOXONE INFUSIONS ON PULSATILE THYROTROPIN SECRETION

Endogenous opioids are known to modulate the secretion of some anterio r pituitary hormones, but they are not thought to have significant eff ects on TSH secretion. However, dynamic TSH secretion has not been cha racterized during naloxone infusions. Therefore, we measured TSH level s every 15 min over 24 h in nine healthy young men at baseline and dur ing infusions of naloxone at 2 mg/h. A TRH test was performed after ea ch study. TSH pulses were located by Cluster analysis. Naloxone infusi ons decreased 24-h mean TSH levels by 28%, from 1.68 +/- 0.20 to 1.21 +/- 0.19 mU/L. Mean daytime TSH levels did not change, but nocturnal T SH levels were decreased by 39%, from 2.21 +/- 0.30 to 1.35 +/- 0.21 m U/L. There were no changes in TSH pulse frequency, but naloxone infusi ons decreased 24-h TSH pulse amplitude by 32%, from 2.02 +/- 0.26 to 1 .37 +/- 0.21 mU/L. Daytime TSH pulse amplitude was relatively unaffect ed (1.27 +/- 0.15 us. 1.16 +/- 0.21 mU/L), whereas nocturnal TSH pulse amplitude was decreased by 42%, from 2.72 +/- 0.40 to 1.57 +/- 0.23 m U/L. TSH responses to acute TRH administration were decreased after na loxone infusions (12.38 +/- 1.93 us. 9.17 +/- 1.36 mU/L). Serum T-3 le vels fell by 21% during naloxone infusions, from 1.9 +/- 0.1 to 1.5 +/ - 0.1 nmol/L, whereas other thyroid hormone levels and cortisol levels were unchanged. These findings suggest that endogenous opioids have s ignificant stimulatory effects on TSH secretion, predominantly during the nocturnal TSH surge.