Luteinizing Hormone (LH) is a key hormone in the female reproductive system, playing a central role in regulating the menstrual cycle. Produced by the pituitary gland in the brain, LH helps to control the timing and sequence of events that prepare the body for potential pregnancy. Its presence and fluctuating levels are instrumental in orchestrating the complex hormonal symphony of a woman’s cycle.
Luteinizing Hormone and Ovulation
Before ovulation, LH works in conjunction with Follicle-Stimulating Hormone (FSH) to mature ovarian follicles, which are small sacs containing eggs. As a dominant follicle develops, it produces increasing amounts of estrogen. This rising estrogen eventually triggers a rapid and significant increase in LH levels, known as the “LH surge.” The LH surge is the immediate trigger for ovulation, causing the mature egg to be released from the ovarian follicle, typically within 24 to 36 hours after the surge begins. This event marks the end of the follicular phase and the start of the luteal phase, where the ruptured follicle transforms into a structure called the corpus luteum.
Hormonal Shifts After Ovulation
Following ovulation, the corpus luteum begins producing progesterone, a hormone important for preparing the uterine lining for a potential pregnancy. If fertilization does not occur, the corpus luteum degenerates after about two weeks. This degeneration leads to a sharp decline in progesterone and estrogen levels, which triggers the shedding of the uterine lining, resulting in menstruation. The drop in these hormones then signals the pituitary gland to increase FSH and LH production, initiating a new menstrual cycle.
Conversely, if fertilization and implantation occur, the hormonal landscape changes significantly. The early embryo produces Human Chorionic Gonadotropin (hCG), detectable around 7-9 days post-fertilization. A primary function of hCG is to “rescue” the corpus luteum, signaling it to continue producing progesterone and estrogen.
This sustained progesterone production maintains the uterine lining and supports early pregnancy. High progesterone levels, maintained by hCG, then suppress LH and FSH production from the pituitary gland. Thus, LH levels fall after the ovulation surge and remain low throughout pregnancy due to sustained progesterone and hCG.
LH Tests Versus Pregnancy Tests
The distinct functions of LH and hCG lead to differences in the tests designed to detect them. LH tests, commonly known as ovulation predictor kits (OPKs), are designed to identify the LH surge before ovulation, aiding in timing intercourse for conception. They detect LH levels indicating impending ovulation.
However, LH and hCG share structural similarities, particularly in their alpha subunits. This structural resemblance means that LH tests can sometimes cross-react with hCG, potentially showing a positive result if a person is pregnant. LH tests are not intended for or reliable in detecting pregnancy. Pregnancy tests are specifically designed to detect hCG and are more sensitive. Relying on an LH test for pregnancy can lead to inaccurate results, as their sensitivity and target hormones differ.