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 LHbeta 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 LHbeta measurement and another specific IRMA to check the absence of free CGbeta, to study normal subjects and 26 patients with NFPA. Basal plasma levels of LHbeta were undetectable in normal men and premenopausal women in the early follicular phase. In contrast, normal postmenopausal women had increased basal plasma LHbeta, parallel to dimeric LH and alpha-subunit levels. In healthy subjects, stimulation with GnRH elicited an increase in LHbeta while TRH was ineffective. In patients with NFPA, LHbeta 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 LHbeta measurement was thus truly informative on the gonadotropic nature of NFPA in only 4 out of 26 cases (15%). In addition, increased LHbeta levels and/or a positive response of free LHbeta 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 LHbeta measurement is rarely helpful for determining the gonadotropic nature of NFPA.