Luteinizing hormone (LH) is a reproductive hormone that triggers ovulation in women and drives testosterone production in men. It’s made by the pituitary gland, a pea-sized structure at the base of the brain, and works alongside another hormone called follicle-stimulating hormone (FSH) to control sexual development and fertility in both sexes.
How LH Works in Women
LH’s most important job in women is triggering ovulation. During the first half of the menstrual cycle, FSH stimulates a group of follicles in the ovary to grow, with one eventually becoming dominant. Around the middle of the cycle, the body produces a sharp spike in LH, often called the “LH surge.” This surge causes the dominant follicle to rupture and release a mature egg.
After the egg is released, LH has a second critical role: it transforms what’s left of the ruptured follicle into a temporary structure called the corpus luteum. The corpus luteum produces progesterone, the hormone that prepares the uterine lining for a potential pregnancy. If pregnancy doesn’t occur, the corpus luteum breaks down, progesterone drops, and menstruation begins.
How LH Works in Men
In men, LH targets specialized cells in the testes called Leydig cells. When LH binds to receptors on these cells, it kicks off a chain reaction that ultimately converts cholesterol into testosterone. The process starts when LH activates a signaling molecule inside the cell, which shuttles cholesterol into the cell’s energy-producing compartments (mitochondria). From there, enzymes convert cholesterol step by step into testosterone.
This matters for more than just testosterone levels. Testosterone produced locally in the testes is essential for sperm production. This is why men who take external testosterone (for example, as hormone therapy) often see their sperm counts drop. The external testosterone signals the brain to stop releasing LH, which means Leydig cells produce less testosterone internally, and sperm production stalls as a result.
Normal LH levels in adult men range from about 1.42 to 15.4 IU/L and stay relatively stable after puberty, unlike in women where levels fluctuate throughout the menstrual cycle.
The Feedback Loop That Controls LH
LH doesn’t operate on its own. It’s part of a tightly regulated communication chain between three structures: the hypothalamus, the pituitary gland, and the gonads (ovaries or testes).
Neurons in the hypothalamus release a signaling hormone called GnRH into the bloodstream. GnRH travels to the pituitary gland and tells it to produce both LH and FSH. Once LH does its job and sex hormone levels rise (testosterone in men, estrogen and progesterone in women), those hormones signal back to the brain to slow down GnRH production. Less GnRH means less LH. When sex hormone levels drop, GnRH ramps back up, and the cycle continues.
There’s one notable exception to this pattern. During the middle of the menstrual cycle, rising estradiol (a form of estrogen) actually increases GnRH production instead of suppressing it. This positive feedback loop is what creates the LH surge that triggers ovulation. It’s the only point in the cycle where the usual “brake” system flips into an “accelerator.”
LH and Fertility Tracking
Because the LH surge reliably precedes ovulation, it’s the basis of over-the-counter ovulation predictor kits. These urine-based tests detect when LH levels spike above a threshold. Once the test turns positive, ovulation typically occurs within 24 hours. This makes LH testing one of the most practical tools for timing intercourse when trying to conceive.
LH is also measured through blood tests alongside FSH. The two hormones work so closely together that interpreting one without the other gives an incomplete picture. Doctors typically order both when evaluating fertility, irregular periods, or signs of hormonal imbalance.
What High LH Levels Can Mean
Elevated LH in women of childbearing age can point to several conditions. Polycystic ovary syndrome (PCOS) is one of the most common, where chronically elevated LH contributes to irregular ovulation. Other causes include primary ovarian insufficiency (when the ovaries stop functioning normally before age 40), thyroid or adrenal gland disorders, Turner syndrome, or ovaries that didn’t develop normally. After menopause, high LH is expected because the ovaries are no longer producing enough estrogen to signal the brain to slow down.
In men, high LH levels suggest the brain is working overtime to stimulate testosterone production, usually because the testes aren’t responding normally. Causes include physical injury to the testes, Klinefelter syndrome, mumps-related damage, or germ cell tumors. In both men and women, autoimmune disease, radiation therapy, and chemotherapy can damage the gonads and push LH levels up.
In children, elevated LH and FSH before age 8 in girls or age 9 in boys is a sign of precocious puberty, where sexual development begins abnormally early.
What Low LH Levels Can Mean
Low LH typically points to a problem with the brain rather than the gonads. This is called secondary hypogonadism (or hypogonadotropic hypogonadism), where the hypothalamus or pituitary gland fails to send the right signals. Without adequate LH, the testes don’t produce enough testosterone and the ovaries don’t ovulate properly.
Causes of low LH include pituitary tumors, head injuries affecting the pituitary area, and hemochromatosis (iron overload), which can impair pituitary function. The consequences depend on when the deficiency begins. If it starts before birth, it can affect genital development. If it occurs before puberty, it can delay or prevent normal development: muscle growth, voice deepening, and body hair growth may not happen on schedule. In adults, symptoms tend to start with low sex drive, fatigue, and depression, potentially progressing to erectile dysfunction and loss of muscle mass over time.
LH During Puberty and Aging
Before puberty, LH levels are very low in both boys and girls. The onset of puberty is essentially the hypothalamus “waking up” and beginning to pulse GnRH in a pattern that stimulates LH and FSH release. In boys, rising LH drives testosterone production, which triggers the development of male secondary sex characteristics. In girls, LH works with FSH to mature the ovaries and eventually establish the menstrual cycle.
In men, LH levels hold fairly steady throughout adulthood. In women, they fluctuate with every menstrual cycle, peaking dramatically at ovulation. After menopause, LH rises and stays elevated permanently because there’s no longer enough estrogen from the ovaries to complete the feedback loop and tell the pituitary to ease off.