Luteinizing Hormone (LH) is produced and released by the pituitary gland. Its primary function is to regulate the reproductive system in both males and females. In the menstrual cycle, LH triggers the release of a mature egg from the ovary, a process known as ovulation. This hormone signals the ovaries to proceed with the final step of the reproductive cycle.
The Mechanism of the Luteinizing Hormone Surge
The process begins with the maturation of a dominant follicle on the ovary. As the follicle develops, it secretes increasing amounts of estradiol, a form of estrogen. When estradiol concentration reaches a sufficient threshold and is sustained for 36 to 50 hours, it signals the pituitary gland. This signal initiates a rapid increase in LH release, known as the LH surge.
This spike in LH concentration directly stimulates the ovarian follicle to rupture and release the egg. The surge lasts for about 24 to 48 hours. Ovulation generally occurs 24 to 36 hours after the LH surge begins. Identifying this brief surge is important for pinpointing the most fertile window of the menstrual cycle.
Defining Standard LH Levels During Ovulation
Luteinizing hormone levels are measured in milli-international units per milliliter (mIU/mL) and fluctuate across the menstrual cycle. During the follicular phase, the time leading up to the surge, LH levels are basal, falling within a range of 2 to 15 mIU/mL. This low concentration supports the initial growth of the ovarian follicles.
The onset of the LH surge marks a sharp increase from baseline values. Peak levels during the surge vary widely among individuals, typically ranging from 25 to 100 mIU/mL. While the average peak is around 45 mIU/mL, some individuals peak as high as 118 mIU/mL or as low as 12 mIU/mL, demonstrating high variability. The absolute peak number is less significant than the rapid, several-fold increase from the person’s baseline.
Immediately after the egg is released, LH levels decline sharply, returning to basal concentrations for the luteal phase. This post-ovulatory drop often falls below 15 mIU/mL, similar to follicular phase levels. The high concentration of LH is a transient event, lasting only long enough to trigger ovulation.
Interpreting At-Home Ovulation Test Results
At-home Ovulation Predictor Kits (OPKs) detect Luteinizing Hormone in urine. These tests turn positive when the LH concentration crosses a sensitivity threshold, often set around 20 to 25 mIU/mL. Reaching this threshold indicates that the LH surge is underway and ovulation is imminent within the next day or two.
A positive result on a traditional strip test is visually confirmed when the test line is as dark as or darker than the control line. The control line confirms the test strip is functioning correctly, and the test line’s intensity corresponds to the LH concentration. If the test line is lighter than the control line, the LH surge has not yet been detected.
It is important to test in the late morning or early afternoon, rather than with the first morning urine, because LH is released in pulses. The hormone takes time to be metabolized and appear in the urine, meaning the peak level may not be reflected in a concentrated morning sample. Some individuals aim to identify their personal “peak,” which is the last and darkest positive test result before the line fades.
Factors That Influence or Skew LH Readings
Several physiological and external factors can cause misleading results when testing for the LH surge. Polycystic Ovary Syndrome (PCOS) is a common condition that causes consistently high baseline LH levels, sometimes in the 10 to 20 mIU/mL range. These elevated levels can lead to multiple false-positive OPK results throughout the cycle, even without ovulation.
Women approaching menopause may experience erratic and sporadic LH surges. As ovarian function declines, the pituitary gland attempts to compensate by sending strong, irregular hormonal signals, resulting in abnormally high LH readings that do not lead to viable ovulation. Certain fertility medications, particularly those containing Human Chorionic Gonadotropin (hCG), can also cause false positives.
This occurs because the hCG hormone is structurally similar to LH, allowing it to cross-react with the antibodies on the test strip. Hydration level also affects the concentration of LH in a urine sample. Over-hydrating dilutes the urine, potentially causing a true LH surge to register as a false negative, while dehydration can concentrate the urine, leading to a false positive reading.