Effects of naloxone infusions on pulsatile thyrotropin secretion

Endogenous opioids are known to modulate the secretion of some anterior pituitary hormones, but they are not thought to have significant effects on TSH secretion. However, dynamic TSH secretion has not been characterized during naloxone infusions. Therefore, we measured TSH levels every 15 min over 24 h in nine healthy young men at baseline and during infusions of naloxone at 2 mg/h. A TRH test was performed after each study. TSH pulses were located by Cluster analysis. Naloxone infusions 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 TSH levels were decreased by 39%, from 2.21 ± 0.30 to 1.35 ± 0.21 mU/L. There were no changes in TSH pulse frequency, but naloxone infusions 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 unaffected (1.27 ± 0.15 vs. 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 mU/L. TSH responses to acute TRH administration were decreased after naloxone infusions (12.38 ± 1.93 vs. 9.17 ± 1.36 mU/L). Serum T₃ levels 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 significant stimulatory effects on TSH secretion, predominantly during the nocturnal TSH surge.