Luteinizing hormone (LH) is a chemical messenger produced by the pituitary gland, located at the base of the brain. Present in both males and females, it plays a direct role in regulating the reproductive system by signaling the primary sex organs—the ovaries and testes.
The Role of Luteinizing Hormone in the Female Reproductive System
In the female reproductive system, luteinizing hormone drives the menstrual cycle. Its primary function is triggering ovulation. Midway through the menstrual cycle, a significant increase in LH levels, known as the “LH surge,” acts on the ovaries, causing a mature follicle to rupture and release an egg.
Following ovulation, LH stimulates the remnants of the ruptured follicle to transform into a temporary structure called the corpus luteum. The corpus luteum then produces progesterone, which prepares the uterine lining, or endometrium, for a potential implantation.
The maintenance of the corpus luteum by LH connects ovulation and early pregnancy. If fertilization and implantation occur, the corpus luteum will continue its progesterone production to support the uterine environment. If pregnancy does not occur, the corpus luteum degrades, leading to a drop in progesterone and the onset of menstruation.
The Role of Luteinizing Hormone in the Male Reproductive System
In males, luteinizing hormone’s function is centered on the testes. LH acts on the Leydig cells within the testicular tissue. This interaction signals these cells to produce and secrete testosterone, the principal male sex hormone.
The testosterone produced under LH’s influence supports spermatogenesis, the process of sperm production. Testosterone also maintains sex drive (libido) and is responsible for male secondary sexual characteristics. These include a deeper voice, muscle mass, and the growth of facial and body hair.
Unlike the cyclical nature of LH in females, its secretion in males is more constant, ensuring a relatively stable level of testosterone production. This steady hormonal environment is necessary for continuous sperm production and male reproductive health.
Regulation of Luteinizing Hormone Production
The production of LH is controlled by the Hypothalamic-Pituitary-Gonadal (HPG) axis. This system involves the hypothalamus, the pituitary gland, and the gonads (ovaries or testes). The hypothalamus releases Gonadotropin-releasing hormone (GnRH) in pulses, which signal the pituitary gland to secrete LH into the bloodstream.
This system operates on a negative feedback loop to maintain hormonal balance. When levels of sex hormones—estrogen and progesterone in females, testosterone in males—rise, they signal the hypothalamus and pituitary gland. This feedback inhibits the release of GnRH and LH, reducing stimulation of the gonads.
Conversely, when sex hormone levels are low, the lack of negative feedback allows the hypothalamus and pituitary to increase GnRH and LH secretion. This action stimulates the gonads to produce more hormones, ensuring levels are kept within a specific range.
Clinical Significance of Luteinizing Hormone Levels
Measuring luteinizing hormone levels is a diagnostic tool for assessing reproductive health. Abnormally high or low levels of LH can provide insight into the cause of infertility, menstrual irregularities, and other hormonal imbalances.
High LH levels can indicate a problem with the gonads, a condition known as primary hypogonadism. In females, this occurs in conditions like primary ovarian insufficiency or menopause, as the ovaries no longer respond to LH. In males, testicular failure leads to elevated LH as the pituitary tries to stimulate non-responsive cells.
Polycystic Ovary Syndrome (PCOS) is another condition often characterized by consistently high LH levels relative to another hormone, FSH.
Low LH levels point toward a problem with the pituitary gland or hypothalamus, a condition called hypogonadotropic hypogonadism. If these glands fail to signal properly, the gonads do not receive stimulation. This can result in a lack of ovulation in women and low testosterone in men.
LH levels are measured through blood tests in a clinical setting. For women trying to conceive, at-home ovulation predictor kits (OPKs) measure LH in urine. These tests detect the LH surge, helping to identify the most fertile window in the menstrual cycle.