Free luteinizing-hormone beta-subunit in normal subjects and patients with pituitary adenomas

Most clinically nonfunctioning pituitary adenomas (NFPA.) are found to be gonadotropinomas when assessed by immunocytochemistry. However, they are rarely associated with increased basal plasma levels of FSH, LH and/or alpha-subunit. It has been claimed that the paradoxical free LH beta response to TRH may be a useful clinical tool for determining the gonadotropic nature of NFPA. We used a very specific and sensitive immunoradiometric assay (IRMA) for free LH beta measurement and another specific IRMA to check the absence of free CG beta, to study normal subjects and 26 patients with NFPA. Basal plasma levels of LH beta were undetectable in normal men and premenopausal women in the early follicular phase. In contrast, normal postmenopausal women had increased basal plasma LH beta, parallel to dimeric LH and alpha-subunit levels. In healthy subjects, stimulation with GnRH elicited an increase in LH beta while TRH was ineffective. In patients with NFPA, LH beta hypersecretion was found basally and/or after stimulation with TRH in 3 of 16 men, 3 of 5 premenopausal women, and 1 of 5 postmenopausal women, i.e. 7 of 26 patients (26%). In 3 of these 7 cases, alpha-subunit and/or FSH levels were also increased. The LH beta measurement was thus truly informative on the gonadotropic nature of NFPA in only 4 out of 26 cases (15%). In addition, increased LH beta levels and/or a positive response of free LH beta to TRH was observed in 3 patients with pure prolactinomas but in no patients with GH-secreting adenomas. Thus, using this very sensitive and specific IRMA, free LH beta measurement is rarely helpful for determining the gonadotropic nature of NFPA.