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

Abstract

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