LH and hCG: Their distinct physiological roles and use in ovarian stimulation protocols

Abstract

Luteinizing hormone (LH) and human chorionic gonadotropin (hCG) have been used in diagnostics and therapeutics from biologically purified sources. Though both hormones function via the same receptor (LHCGR), mostly hCG has been used due to its widespread availability. Hence, in the mind of the practising physician, both molecules have been considered equal. The recent availability of recombinant LH has led us to reconsider the specificities of both hormones in terms of actions on the body. LH and hCG play essential roles in the reproductive cycle. LH plays a key role in follicular maturation and the ovulation process, and hCG is the "pregnancy hormone." LH and hCG are different in terms of structure, expression, regulation, and function. LH and hCG fundamentally differ in their expression patterns and have complex and unique aspects. LH and hCG should be considered as hormone mixtures, the composition of which fl uctuates during the course of the ovarian cycle and pregnancy and throughout the lifespan of men and women. Diverse isoforms have distinct functions, refl ected by their relative abundance in normal and aberrant physiologic processes. Quantitative and qualitative distinctions in signaling cascades, activated by LH and hCG have been recently discovered; furthermore, the extragonadal activities are currently under exploration. Availability of recombinant LH and hCG as new therapeutic tools for use in specific clinical pro-fertility conditions could lead us to reconsider the specific indications for each of both molecular entities. The first part of this chapter reviews the current knowledge on both parent molecules, emphasizing their specificities and the consequences at the receptor level.