Luteinizing hormone, also known as LH or lutropin, is a hormone secreted by the anterior pituitary gland located in the brain. The function of LH is to stimulate the production of sex hormones, from the ovaries in women and from the testicles, or testes, in men. Luteinizing hormone levels naturally rise and fall during the menstrual cycle but disorders of the ovaries or testes can also affect LH production, and, if the ovaries or testes fail due to abnormal development, injury or disease, high levels may result. If a problem with the pituitary gland or, at a higher level in the brain, the hypothalamus, affects luteinizing hormone production, levels of LH may fall. Certain drugs, such as levodopa, are also associated with changes in luteinizing hormone levels.
Cells inside the pituitary gland called gonadotrophs are responsible for producing luteinizing hormone and another hormone known as FSH, or follicle stimulating hormone. The hypothalamus secretes what is called GnRH, or gonadotrophin-releasing hormone, which stimulates the pituitary to release LH and FSH. Luteinizing hormone acts on the testes and ovaries causing them to produce the sex hormones testosterone and estrogen. While higher levels of sex hormones in the blood normally have a negative feedback effect on the hypothalamus, causing it to reduce GnRH secretion and lower FSH and LH levels, in the middle of the menstrual cycle very high levels of estrogen have a positive feedback effect instead.
Due to the positive feedback effect, a peak in luteinizing hormone levels, called the preovulatory LH surge, occurs at mid-cycle just before ovulation. An egg is then released from a mature follicle, or sac, and the empty follicle develops into a corpus luteum, a body that produces the sex hormones necessary for pregnancy. If the egg is not fertilized, the corpus luteum breaks down and sex hormone production decreases, causing more GnRH to be released and leading to higher FSH and luteinizing hormone levels in the blood, which stimulates egg development for the start of the next cycle.
Abnormally low levels of luteinizing hormone can result from disorders affecting the hypothalamus or pituitary, causing problems such as a low sperm count in men or failure to menstruate in women. High levels may be the result of conditions affecting the ovaries and testes, where fewer sex hormones are produced and the hypothalamus increases GnRH secretion, raising luteinizing hormone levels. Such disorders could include developmental problems, damage from chemotherapy drugs or radiation, and conditions that prevent normal ovulation, such as thyroid disease and ovarian tumors. Treatment in each case will vary, depending upon the underlying illness.