Robustness of the male lactotropic axis to the hyperprolactinemic stimulus of primary thyroidal failure.

Primary hypothyroidism is a presumptive proximate basis for increased serum PRL concentrations. However, most studies to date have been performed in females and do not address the possible effect of sex differences. In the present study we investigated the 24-h patterns of pulsatile PRL release in 10 hypothyroid men (24-h mean +/- SEM: total T4, 35 +/- 8 nmol/L; TSH, 55 +/- 10 mU/L) by sampling blood at 10-min intervals for 24 h. The study was repeated in 5 men, 5-7 months and again 14-36 months after treatment with levothyroxine. The control group consisted of 7 normal age-matched euthyroid men. The mean 24-h serum PRL concentration of 4.9 +/- 0.6 micrograms/L in 10 hypothyroid men was not significantly different from the value of 4.9 +/- 0.5 micrograms/L found in the normal group (P greater than 0.5). Pulsatile features of PRL release were evaluated by Cluster analysis, which revealed normal mean PRL peak frequency (12.6 +/- 0.7 vs 13.4 +/- 0.6 pulses/24 h; P greater than 0.5), maximal peak amplitude (6.1 +/- 0.7 vs. 6.7 +/- 0.6 micrograms/L; P greater than 0.5), peak increment (2.2 +/- 0.3 vs. 2.6 +/- 0.3 micrograms/L; P = 0.3), mean valley (4.2 +/- 0.6 vs. 4.4 +/- 0.4 micrograms/L; P greater than 0.5), and prepeak nadir (3.6 +/- 0.5 vs. 3.6 +/- 0.4 micrograms/L; P greater than 0.5) PRL concentrations. Five to 7 months of T4 therapy in 5 hypothyroid men caused significant decreases in the mean 24-h (2.9 +/- 0.5 vs. 6.1 +/- 1.2 micrograms/L; P less than 0.05), interpulse valley (2.5 +/- 0.4 vs. 5.2 +/- 1.1 micrograms/L; P less than 0.05), and prepeak nadir (2.3 +/- 0.3 vs. 4.6 +/- 1.0 micrograms/L; P less than 0.05) PRL concentrations. These values returned to normal after 14-36 months of treatment. Cosinor analysis revealed preserved circadian PRL rhythmicity during hypothyroidism, with a normal circadian mesor (mean), amplitude, and acrophase. In summary, we have demonstrated normal mean 24-h serum PRL concentrations as well as normal pulsatile and circadian patterns of PRL release in men with primary hypothyroidism. These results stand in contrast to previous reports of increased serum PRL concentrations observed predominantly in hypothyroid premenopausal women. The foregoing disparity suggests the possibility of a role for estrogen in enhancing the effect of hypothyroidism on PRL release.(ABSTRACT TRUNCATED AT 400 WORDS)