Luteinizing hormone (LH) is a reproductive hormone produced by the pituitary gland, located at the base of the brain. LH communicates with the ovaries to regulate the menstrual cycle, which involves rising and falling hormones that prepare the body for potential pregnancy. LH levels do not remain constant but undergo significant fluctuations, with one major event defining its role in the monthly cycle.
Luteinizing Hormone’s Primary Role in the Cycle
The primary function of luteinizing hormone is triggering the release of an egg from the ovary, known as ovulation. As ovarian follicles mature during the first half of the cycle, they release increasing amounts of estrogen. When estrogen reaches a certain concentration threshold, it signals the pituitary gland to abruptly release a massive amount of LH, creating the LH surge.
This surge typically lasts for 24 to 36 hours. Its function is to cause the dominant, mature follicle to rupture and eject the egg into the fallopian tube. LH levels are normally at their lowest during the follicular phase, making this mid-cycle peak the high point of the hormone’s activity. Ovulation occurs around the middle of the menstrual cycle, usually 12 to 16 days before the next period.
The Hormonal Landscape of the Luteal Phase
Following the LH surge and ovulation, the menstrual cycle enters its second half, called the luteal phase, which precedes menstruation. The ruptured follicle transforms into the corpus luteum, which produces large quantities of progesterone and, to a lesser extent, estrogen.
Progesterone becomes the dominant hormone during this phase, preparing the uterine lining for the possible implantation of a fertilized egg. High progesterone and estrogen levels actively suppress the pituitary gland’s release of LH and Follicle-Stimulating Hormone (FSH). This mechanism, known as negative feedback, ensures that no further ovulation occurs. Consequently, LH levels throughout the luteal phase are consistently low, dropping significantly after the mid-cycle surge is complete.
Menstruation is initiated only if fertilization does not occur, leading to the breakdown of the corpus luteum. This degeneration causes a sharp crash in both progesterone and estrogen levels, which removes the negative feedback on the pituitary. This hormone withdrawal triggers the shedding of the uterine lining, marking the start of the menstrual period, not a rise in LH.
Explaining Apparent LH Rises Before Menstruation
Observing an LH rise just before a period is confusing because a true, significant LH surge is unnecessary at this time. One primary reason for this apparent rise relates to the limitations of at-home ovulation predictor kits (OPKs). These tests are designed to detect LH in urine, and highly sensitive versions may occasionally register a positive result due to cross-reactivity with low basal levels of LH or other hormones.
A small, transient elevation of LH, often called a secondary LH peak, can sometimes occur in the late luteal phase of some cycles. This minor peak is a normal, non-ovulatory fluctuation that occurs after progesterone and estrogen levels begin to decline, temporarily lessening their suppressive effect on the pituitary. This subtle rise is usually not strong enough to trigger a positive test but contributes to hormonal instability just before the period begins.
A more compelling scenario involves early pregnancy, where the apparent LH rise is a case of mistaken identity. The early pregnancy hormone, human Chorionic Gonadotropin (hCG), is structurally very similar to LH, particularly in their shared alpha subunit. Since many OPKs are not designed to distinguish perfectly between the two hormones, rising hCG after implantation can cause the test to mistakenly register a positive result, which the user interprets as an LH surge. This cross-reactivity leads to confusion, as the positive test appears just before the expected period date, suggesting a late LH surge when it is actually an indication of a possible conception.