Follicle-Stimulating Hormone (FSH) is a glycoprotein hormone produced and secreted by the anterior pituitary gland. FSH plays a direct role in reproductive function in both males and females, supporting the development of eggs in the ovaries and the production of sperm in the testes. Because reproduction requires precise timing, FSH levels are not static; they fluctuate significantly in response to internal biological signals.
The Hormonal System Governing FSH Release
The mechanism controlling FSH secretion is known as the Hypothalamic-Pituitary-Gonadal (HPG) axis. This system links the hypothalamus, the pituitary gland, and the gonads. The hypothalamus initiates this process by releasing Gonadotropin-Releasing Hormone (GnRH) in a pulsatile manner, which stimulates the pituitary to synthesize and release FSH and Luteinizing Hormone (LH).
A feedback loop ensures the proper balance of these hormones. Active ovaries or testes produce sex hormones like estrogen and testosterone, along with a peptide called inhibin. These substances act as negative feedback signals, traveling back to the pituitary and the hypothalamus to suppress the release of FSH and GnRH. Inhibin plays a major role in selectively inhibiting FSH secretion from the pituitary.
This system manages reproductive cycles precisely. In women, growing ovarian follicles release inhibin and estrogen, which reduce FSH output. This reduction ensures that only one dominant follicle is selected for ovulation. The interplay between stimulatory signals from the brain and inhibitory signals from the gonads drives the hormone’s fluctuation.
FSH Fluctuation During the Monthly Cycle
FSH levels change predictably across the female reproductive cycle, rising and falling in coordination with ovarian activity. The cycle begins with the follicular phase, where FSH levels are relatively low but gradually rise to recruit a cohort of ovarian follicles. This initial increase in FSH, often measured between 1.4 to 9.9 milli-international units per milliliter (mIU/mL), stimulates the growth and maturation of the eggs.
As one follicle becomes dominant, it increases its production of estradiol (a form of estrogen) and inhibin, causing a temporary suppression of FSH. Just before ovulation, FSH experiences a rapid surge, often alongside a larger surge of LH. This peak completes the final maturation of the egg and triggers its release from the ovary.
Following ovulation, the cycle enters the luteal phase, and FSH levels drop to their lowest point. This trough is maintained by the high levels of progesterone and estrogen produced by the corpus luteum. If pregnancy does not occur, the corpus luteum dissolves, hormone levels decline, and FSH begins to rise again toward the end of the cycle to prepare for the recruitment of the next cohort of follicles.
Age-Related Changes and Long-Term Variability
FSH levels exhibit long-term variability tied directly to reproductive aging. The most significant shift occurs as a woman approaches perimenopause and menopause. As the ovarian reserve diminishes, the ovaries contain fewer follicles to respond to hormonal stimulation.
Fewer follicles result in less production of the inhibitory hormones, estrogen and inhibin. With the negative feedback signal weakening, the pituitary gland attempts to compensate by increasing its output of FSH. This effort leads to a progressive elevation of the baseline FSH level over several years before the final menstrual period. In postmenopausal women, FSH levels stabilize at a high plateau.
In men, long-term variability is less pronounced because sperm production is not cyclical. FSH helps maintain spermatogenesis, and while levels generally remain steady, they may show a slight increase with age. A sustained elevation in a man’s FSH often indicates a problem with testicular function, as the pituitary attempts to overcome the failure of the testes to produce sperm or sufficient inhibin. Conditions like Primary Ovarian Insufficiency (POI) in younger women can also lead to premature, high FSH levels, mirroring the hormonal state of natural menopause.
Clinical Interpretation of FSH Levels
Measuring Follicle-Stimulating Hormone levels is a standard procedure in assessing reproductive health and fertility potential. Because FSH levels fluctuate throughout the month, the test is typically performed during the early follicular phase, usually on day two or three of the menstrual cycle, to establish a reliable baseline. This measurement provides an indication of a woman’s ovarian reserve.
The rationale for this timing is that it captures the body’s effort to stimulate the ovaries when sex hormone levels are at their lowest. A baseline FSH result that is higher than the normal range, generally considered to be above 10 mIU/mL in this early phase, suggests that the pituitary is working harder than usual to recruit a follicle. This elevated level is often interpreted as a sign of diminished ovarian reserve.
Conversely, very low FSH levels can suggest an issue at the level of the pituitary gland or the hypothalamus. This indicates that the brain centers are not properly signaling the pituitary to release the necessary gonadotropins. The interpretation of an FSH level is always considered in the context of the patient’s age and other hormone measurements to accurately diagnose the source of a reproductive issue.