Luteinizing hormone (LH) is a glycoprotein hormone produced by the pituitary gland. This hormone fundamentally regulates the reproductive system. During the menstrual cycle, LH levels remain low until a rapid, temporary spike occurs mid-cycle, known as the LH surge. Tracking this event is a common method for identifying the most fertile window and maximizing the chances of conception. This often raises the question of whether a larger, more dramatic LH surge translates to better fertility outcomes.
The Essential Function of the Luteinizing Hormone Surge
The LH surge begins with the maturation of an ovarian follicle. As the dominant follicle grows, it produces increasing amounts of estrogen (estradiol). Once estrogen reaches a concentration threshold, it stimulates the pituitary gland, triggering the release of LH.
The surge provides the precise signal required to finalize the egg’s maturation process. Its function is to initiate the rupture of the ovarian follicle wall, releasing the mature egg into the fallopian tube. This event, known as ovulation, typically occurs about 24 to 36 hours after the LH surge begins.
Following the release of the egg, the ruptured follicle transforms into the corpus luteum. LH stimulates this temporary structure to produce progesterone. Progesterone is the hormone responsible for preparing the uterine lining for potential implantation and supporting a possible early pregnancy.
Surge Sufficiency Versus Peak Height
The effectiveness of the LH surge depends on reaching a necessary minimum level, known as sufficiency, rather than the absolute peak height. The surge acts as a biological “on” switch for ovulation; once the hormonal threshold is met, egg release is initiated. A higher peak does not confer a better quality egg or increase the likelihood of conception.
Scientific research shows that the LH levels needed to trigger ovulation vary significantly between individuals. Peak LH levels have been observed ranging from 6.5 mIU/mL to over 101 mIU/mL, though the median is often cited around 44.6 mIU/mL. A moderate, clearly detectable surge is functionally equivalent to a very high one, provided it crosses the individual’s threshold for ovulation.
For those tracking their cycle, the goal is to confirm that a surge has occurred, not to achieve a record-high reading. Variability in peak height is normal, and obsessing over a higher number is unnecessary if ovulation is confirmed. Fertility relies on the successful release of an egg, which a sufficient surge achieves, not the magnitude of the signal.
Common Causes of High or Low LH Readings
When LH readings appear consistently outside the expected range, it can point to underlying physiological issues.
Causes of High LH
Persistently high LH levels outside of the mid-cycle window may indicate conditions where the ovaries are not responding adequately to the pituitary’s signal. This is seen in primary ovarian insufficiency, where the ovaries are declining prematurely, or in perimenopause and menopause. Polycystic Ovary Syndrome (PCOS) also causes chronically high baseline LH levels. This high baseline can make it difficult to detect a true surge or may result in a surge that fails to trigger ovulation.
Causes of Low or Undetectable LH
Consistently low or absent LH surges can result from issues originating in the brain, such as functional hypothalamic amenorrhea. This condition is often linked to excessive exercise, low body weight, or malnutrition.
Testing Factors
A non-physiological reason for a low reading is missing a naturally short surge window. Testing with diluted urine can also cause a false negative result, as the hormone concentration is artificially lowered. Conversely, some ovulation predictor kits may yield a false positive result if they cross-react with human chorionic gonadotropin (hCG), which has a similar molecular structure to LH.