The luteinizing hormone surge—the final stage in ovulation induction: modern aspects of ovulation triggering

To compile updated information regarding gonadotropin secretion, specifically the physiology of the midcycle LH surge, in natural cycles and under various ovulation induction protocols. Studies that deal with the clinical aspects of LH surge manipulation or substitution were identified through literature and Medline searches. Three major regulatory factors have been identified as participants in the induction of the midcycle gonadotropin surge. These are hypothalamic GnRH secretion, ovarian and adrenal steroids, and less well-characterized ovarian peptide hormones. Gonadotropin-releasing hormone pulsatility is regulated by a complex mechanism that integrates multiple neurotransmitters and sex steroids. Estradiol plays a central role in the pituitary secretion of LH, which also is influenced by P concentrations. Gonadotropin surge attenuating factor also has been implicated in the regulation of timing and amplitude of the LH surge. Human chorionic gonadotropin is used extensively as a LH surrogate, but its use is associated with a number of disadvantages. Induction of an endogenous LH surge through use of the flare effect of GnRH analogues has been examined more recently and has been found to have several advantages. Recombinant human LH is in the final stages of clinical testing. Although much is known about the physiology of the midcycle LH surge and its variations under different clinical conditions, new approaches to the induction or substitution of the LH surge currently are being examined and learned. The introduction of recombinant gonadotropins into clinical practice is likely to influence ovulation induction and IVF practice to a significant degree in the near future.