Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH) are gonadotropins, hormones produced and released by the anterior pituitary gland, located at the base of the brain. The pituitary gland responds to signals from the hypothalamus, forming the hypothalamic-pituitary-gonadal axis. LH and FSH regulate the function of the gonads—the ovaries in females and the testes in males. These hormones are fundamental to reproductive health, necessary for processes such as sperm production, egg maturation, and the synthesis of sex steroids like testosterone and estrogen.
The Specific Functions of LH and FSH
Follicle-Stimulating Hormone (FSH) is primarily responsible for encouraging the growth and maturation of ovarian follicles in females. In the early part of the menstrual cycle, FSH stimulates these follicles to develop, preparing an egg for potential release. It also promotes the production of estrogen by these growing follicles.
In males, FSH binds to Sertoli cells within the testes, which support and nourish developing sperm cells. This action is instrumental in spermatogenesis, the creation of sperm. FSH is a direct regulator of fertility in both sexes, focusing on the initial maturation stage of reproductive cells.
Luteinizing Hormone (LH) has a complementary function in the reproductive process. In females, a rapid increase in LH, known as the LH surge, directly triggers ovulation, causing the mature follicle to rupture and release the egg. After ovulation, LH stimulates the remaining follicular tissue to transform into the corpus luteum, which produces progesterone and estradiol.
For males, LH acts on the Leydig cells in the testes. When LH binds to receptors on these cells, it stimulates the synthesis and secretion of testosterone. Testosterone is necessary for the development of male secondary sex characteristics and works alongside FSH to sustain sperm production.
Interpreting Normal Ranges by Context
Defining a “normal” level for LH and FSH depends heavily on the individual’s sex, age, and, for females, the specific phase of the menstrual cycle. Reference ranges vary between testing laboratories but generally use milli-international units per milliliter (mIU/mL). Before puberty, both hormones are kept at low baseline levels, typically ranging from 0 to 5.0 mIU/mL for both LH and FSH in children.
In adult males, LH and FSH levels are relatively stable because reproductive function is continuous, not cyclical. A normal range for LH falls between 1.24 and 7.8 mIU/mL, while FSH ranges from 1.5 to 12.4 mIU/mL. These stable values reflect the consistent signaling needed to maintain testosterone and sperm production.
For females of reproductive age, interpreting results requires knowing the exact day of the menstrual cycle the blood sample was drawn. During the follicular phase (the start of the cycle), FSH levels are elevated to drive follicle growth, often ranging from 2.5 to 10.2 mIU/mL, while LH is lower (1.68 to 15 mIU/mL). The LH surge marks the ovulatory phase, causing LH levels to spike dramatically, sometimes reaching 21.9 to 56.6 mIU/mL. FSH also peaks, though less sharply, between 3.4 and 33.4 mIU/mL.
Following ovulation, the luteal phase begins, and both hormones drop to lower levels (LH typically 0.61 to 16.3 mIU/mL; FSH 1.5 to 9.1 mIU/mL). The most significant shift occurs after menopause, when the ovaries cease to function and produce sex hormones. Without the negative feedback from estrogen, the pituitary gland attempts to overstimulate the non-responsive ovaries, causing both LH and FSH levels to become chronically elevated. Post-menopausal ranges for FSH are high (23.0 to 134.8 mIU/mL), with LH also rising (14.2 to 52.3 mIU/mL).
When Are LH and FSH Levels Tested?
Healthcare providers frequently order LH and FSH tests to investigate issues related to the reproductive system and hormonal balance. A common reason is to evaluate infertility in both women and men to determine if the cause is hormonal. Testing FSH early in the menstrual cycle, often on Day 3, is a standard method for assessing ovarian reserve, an estimate of the remaining egg supply.
The tests are used to diagnose the onset of menopause or perimenopause, especially in women under 45 presenting with symptoms like irregular periods or hot flashes. Chronically high levels of both hormones in a younger woman can suggest Primary Ovarian Insufficiency (POI). In children, measuring LH and FSH is necessary when there are concerns about the timing of puberty, such as precocious (early) or delayed puberty.
These tests also help pinpoint the location of a hormonal problem within the complex feedback system. Testing may identify potential disorders of the pituitary gland or the hypothalamus, which are the control centers for reproductive hormones. Evaluating the ratio between LH and FSH is also a diagnostic tool in certain conditions, such as Polycystic Ovary Syndrome (PCOS).
Implications of Abnormal Results
Results outside the expected normal range provide clues about the underlying cause of reproductive dysfunction. Abnormally high levels of both LH and FSH generally indicate primary gonadal failure. This means the ovaries or testes are not responding correctly to hormonal signals, prompting the pituitary gland to produce excessive amounts of LH and FSH to force a response.
Examples of primary gonadal failure include menopause or Primary Ovarian Insufficiency in females, where the ovaries have stopped functioning. In males, high levels suggest testicular failure, such as damage from infection, chemotherapy, or genetic conditions like Klinefelter syndrome.
Conversely, low levels of LH and FSH often suggest a central, or secondary, failure originating in the brain. This occurs when the pituitary gland or the hypothalamus is not producing enough gonadotropins, leading to insufficient stimulation of the gonads. This condition is termed hypogonadotropic hypogonadism.
Causes for low LH and FSH can include pituitary tumors, severe stress, malnutrition, or excessive exercise. Low levels might indicate a problem with the hypothalamus not releasing Gonadotropin-Releasing Hormone (GnRH), the signal to the pituitary to release LH and FSH. Identifying whether the problem is primary (gonadal) or secondary (central) is a key step in developing an appropriate treatment strategy.