The Hypothalamic-Pituitary-Testicular Axis in Thyrotoxicosis

Abstract

The hypothalamic-pituitary-testicular axis was evaluated in seven men with thyrotoxicosis due to Graves' disease. Loss of libido and decreased potency were present in 71% and 56%, respectively. All patients had normal testicular volume (25 ml in all) and gynecomastia was detected in two of seven patients. Total sperm counts were less than 40 million in four of the five men tested. There was an inverse correlation between basal serum 17 β-estradiol (E₂) levels and total sperm count (r = −0.87; P < 0.05). Mean (±sc) total testosterone (T) and E₂ levels (1008 ± 104 ng/100 ml and 104 ± 16 pg/ml) were significantly higher than in normal men (P < 0.05). Free T (13.6 ± 2.4 ng/100 ml) was indistinguishable from normal (15.3 ± 1.5 ng/100 ml). The mean (±se) response of serum T to hCG administration was blunted (80 ± 40%) compared to controls (193 ± 19%; P < 0.02). Basal plasma LH levels (15.5 ± 1.5 mlU/ml) were significantly higher (P < 0.05) than in normal men (9.1 ± 0.6 mlU/ml) and hyperresponded to 100 μg LRH iv in five of seven patients. Basal plasma FSH levels and the FSH response to LRH were normal.

These results suggest that men with hyperthyroidism have 1) partial Leydig cell failure, 2) impairment of spermatogenesis, and 3) blunting of the feedback effects of E₂.