Luteinizing Hormone (LH) is a reproductive hormone produced by the pituitary gland in the brain. LH is a messenger, signaling the ovaries to perform functions necessary for the menstrual cycle and the potential for pregnancy. The menstrual period, known medically as menses, is the first phase of the reproductive cycle, marked by uterine bleeding. Understanding the relationship between LH levels and this bleeding phase is often a source of confusion. This article clarifies the role of LH and its specific level during the menstrual period.
Understanding the Function of Luteinizing Hormone
Luteinizing Hormone plays a regulatory role within the female reproductive system, working in coordination with Follicle-Stimulating Hormone (FSH). Both LH and FSH stimulate the ovaries to begin follicular maturation, preparing an egg for release. LH also stimulates the theca cells to produce androgens, which are then converted into estrogen by the granulosa cells. This interaction promotes the growth of egg-containing follicles during the first half of the cycle.
LH’s primary function is to trigger the release of a mature egg from the ovary, known as ovulation. After ovulation, LH supports the newly formed corpus luteum, which develops from the remnants of the ruptured follicle. The corpus luteum then produces progesterone, a hormone necessary for preparing the uterine lining for a possible pregnancy.
LH Levels During the Menstrual Period
LH levels are typically at their lowest during the menstrual period itself. The menstrual period is the beginning of the follicular phase, which starts on cycle day one with the onset of bleeding. During this time, the body is essentially resetting from the previous cycle, as the unfertilized egg and the prepared uterine lining are shed.
The reason LH levels are low is directly related to the low levels of other hormones that precede menstruation. Both estrogen and progesterone levels have significantly dropped at the end of the previous cycle, which signals the pituitary gland to maintain a low output of LH. Normal LH levels during the follicular phase generally range between \(1.68\) to \(15\) International Units per Liter (\(\text{IU}/\text{L}\)) in the blood. Low LH output during this time ensures that only a select group of follicles begins to develop, setting the stage for ovulation.
The Mid-Cycle LH Surge and Ovulation
While LH is low during the menstrual period, it experiences a rapid increase, known as the LH surge, in the middle of the cycle, well after the bleeding has stopped. This surge is directly caused by the rising levels of estrogen produced by the dominant, maturing follicle. When estrogen levels cross a certain threshold, they switch from suppressing LH output to stimulating a positive feedback loop.
This positive feedback mechanism causes the pituitary gland to release a large amount of LH, with blood levels potentially reaching \(21.9\) to \(56.6 \text{ IU}/\text{L}\). The LH surge is the direct biological signal that triggers the final maturation and release of the egg. Ovulation usually occurs approximately \(24\) to \(36\) hours after the LH surge begins. This distinct and short-lived peak in LH marks the most fertile window of the menstrual cycle.
How LH Testing Aids Cycle Tracking
The predictable nature of the LH surge makes it an effective target for at-home cycle tracking tools, commonly known as ovulation predictor kits (OPKs). These kits work by detecting the presence of a high concentration of LH in the urine. LH is always present in the urine, but the test is designed to register a positive result when the level exceeds a set threshold, indicating the start of the surge.
A positive result on an LH test signals that ovulation is imminent, usually happening within the next one to two days. This information allows individuals who are trying to conceive to time intercourse to maximize the chance of fertilization. The kits offer a more precise prediction of the fertile window compared to calendar-based methods.