LH

LH is one of the pituitary's two messengers to the gonads, and it acts as the trigger for key reproductive events. In women, a sudden burst of LH is what actually causes the egg to be released each month; in men, LH keeps testosterone production running.

LH is a glycoprotein hormone secreted by gonadotroph cells of the anterior pituitary in response to pulses of GnRH from the hypothalamus. It travels in the blood to the ovaries and testes, where it tells specific cells to make sex steroids and to carry out their reproductive tasks.

In the ovary, rising estrogen near mid-cycle flips feedback from negative to positive and provokes a sharp LH surge; this surge triggers ovulation and then transforms the leftover follicle into the corpus luteum, which secretes progesterone (the “luteinizing” in its name). In the testis, LH stimulates Leydig cells to produce testosterone.

LH and its partner FSH are jointly called the gonadotropins. Measuring them helps doctors locate the source of reproductive problems: low levels point to the hypothalamus or pituitary, while high levels suggest the gonads themselves are failing to respond.