| Abstract: | Quantitative reduction in LH secretion resulting from hypothalamic-pituitary dysfunction is a known cause of impotence. Qualitative abnormalities of secreted LH, however, have not been described under these circumstances. During evaluation of a 39-yr-old man with impotence and a calcified pituitary mass (pituitary stone), we detected a qualitative abnormality of LH characterized by a low ratio of bio- to immunoactivity (B:I). Initial work-up revealed basal morning serum testosterone levels of 2.14, 3.18, 3.97, and 3.11 ng/ml on 4 separate days, low to low normal urinary LH (300, 200, and 478 mIU/h), and normal GH, TSH, PRL, and ACTH secretion after provocative testing. The response of impotence to testosterone but not placebo in a double blind trial confirmed the clinical significance of the borderline low androgen levels. These findings prompted a systematic analysis of 24-h LH pulses as well as clomiphene and GnRH responsiveness. By RIA, mean serum LH levels 9.1 +/- 0.3 (+/- SE) mIU/ml] and all other response parameters were normal. In striking contrast, mean serum LH by bioassay was low (9.9 +/- 0.4 mIU/ml vs. 41.4 +/- 5.7 in normal subjects), as were B:I ratios (1.0 +/- 0.03 vs. control values of 3.1 +/- 0.5 to 5.3 +/- 0.3). Only during maneuvers designed to increase GnRH were B:I ratios increased to 3.3 +/- 0.22 (exogenous GnRH) and 1.8 +/- 0.12 (clomiphene). Mean testosterone levels before and after exogenous GnRH treatment were 3.28 +/- 0.24 and 4.76 +/- 0.16, respectively (P less than 0.001). The results suggest an association between the low LH B:I ratio and the anatomical disruption of the hypothalamic-pituitary portal system by the pituitary stone. The increased B:I ratio during GnRH or clomiphene administration indicates a functional link between pituitary GnRH exposure and the greater potency of the LH secreted. |
