Human Luteinizing hormone (LH) is a glycoprotein hormone secreted by the anterior pituitary. This hormone is composed of α and β subunits. The amino acid sequence of α-LH is essentially identical to that of other hormones including follicle stimulating hormone (FSH), thyroid stimulating hormone (TSH) and human Chorionic Gonadotropin (HCG). It is β subunit of LH that confers the biological and immunochemical specificity of the hormone.
LH and FSH together with other steroid hormones are known to play important roles in regulating the ovulation and ovarian functions during the menstrual cycle. Maturation of an ovarian follicle and its oocyte begins during the end of the preceding menstrual cycle. In response to FSH released by the pituitary, the follicle undergoes rapid growth. As follicles develop, estradiol secretion begins to rise slowly and is followed by a rapid increase. This increase of estradiol level is generally believed as the trigger for the rapid rise and peaking of LH activity at the mid-cycle (LH surge). Approximately 24-48 hours after the LH surge, the wall of the enlarged follicle ruptures at ovulation and the mature ovum is extruded. After ovulation, LH returns to its base line level within two days with the concomitant increase of progesterone level to initiate luteal phase. The luteal phase lasts predictably about 14 days. Unless pregnancy occurs, a new follicle begins the selection procedure for maturation in the next menstrual cycle. In view of the characteristic variation of LH during the menstrual cycle, rapid and sensitive measurement of LH is an important tool in the diagnosis and management of infertility in females. Detection of the LH surge can aid in predicting the time of ovulation. The onset of the LH surge precedes ovulation by approximately 30 hours. The analysis of LH has been used successfully to time oocyte retrieval for in vitro fertilization and would similarly assist timing of artificial insemination.