Follicle-Stimulating Hormone (FSH) is a hormone produced and released by the pituitary gland. This hormone plays a central, regulatory function in the female reproductive system. FSH is the primary driver of the initial phase of the menstrual cycle. Understanding when FSH levels rise and fall provides insight into the biological processes required for ovulation and fertility.
The Two Primary Peaks of FSH
The highest sustained levels of Follicle-Stimulating Hormone occur at the beginning of the menstrual cycle, during the early follicular phase, which starts on Day 1 with the onset of the period. This initial rise is the most significant peak in magnitude and duration across the cycle. The elevated FSH levels are a direct result of the breakdown of the corpus luteum from the previous cycle, which removes strong hormonal suppression. This release from negative feedback allows FSH concentrations to rise, initiating the development of a new set of ovarian follicles.
While the early follicular phase marks the highest sustained concentration, FSH experiences a secondary, transient peak around the middle of the cycle. This rise occurs simultaneously with the mid-cycle surge of Luteinizing Hormone (LH), which triggers ovulation. The FSH peak at this stage contributes to the final maturation of the dominant follicle and ensures the subsequent release of the egg. Typical ranges for FSH during the follicular phase are between 1.4 to 9.9 mIU/mL, while the ovulatory peak can briefly reach 6.2 to 17.2 mIU/mL.
FSH’s Role in Follicle Selection
The primary purpose of the initial FSH peak is to stimulate the growth of a cohort of early antral follicles within the ovaries. FSH binds to receptors on the granulosa cells surrounding the egg, prompting them to grow and begin producing the enzyme aromatase. Aromatase converts androgens into the estrogen known as estradiol, leading to a progressive rise in systemic estrogen levels.
As the cohort of follicles matures, they collectively produce increasing amounts of estrogen, which signals back to the pituitary gland. This rising estrogen level, along with inhibin B, begins to exert a negative feedback effect, causing the pituitary to reduce its output of FSH. Because FSH levels decline, only the single, most sensitive follicle—the one that has developed the most FSH receptors—can continue to thrive and grow, becoming the “dominant follicle”. The remaining follicles, unable to survive on the lower FSH levels, wither away in a process called atresia.
The Hormonal Suppression After Ovulation
Following the release of the egg at ovulation, the remnants of the dominant follicle transform into a temporary endocrine gland known as the corpus luteum. The corpus luteum begins to secrete high levels of two primary hormones: progesterone and inhibin A. This shift marks the start of the luteal phase, which lasts approximately 14 days.
The elevated progesterone and inhibin A suppress the pituitary gland’s secretion of FSH and LH, driving both hormones down to their lowest levels of the cycle. This negative feedback mechanism ensures that no new follicles are recruited while the body awaits a potential pregnancy. If fertilization does not occur, the corpus luteum naturally degrades toward the end of the luteal phase. The subsequent decline in progesterone and inhibin A then releases the pituitary from its suppression, allowing FSH to rise again and initiate the next cycle.
Clinical Interpretation of FSH Levels
The pattern of FSH fluctuation is used in clinical settings to assess a woman’s ovarian reserve. Physicians typically measure a “baseline” FSH level, usually on Day 2, 3, or 4 of the menstrual cycle, when levels are naturally at their highest sustained peak. This measurement provides a snapshot of how hard the pituitary gland is working to stimulate the ovaries.
A normal baseline FSH level is below 10 mIU/mL, suggesting the ovaries are responding appropriately to hormonal signals. Conversely, a consistently elevated baseline FSH, particularly above 15 mIU/mL, suggests diminished ovarian reserve. High FSH indicates the ovaries are no longer producing enough estrogen and inhibin B to suppress the pituitary, forcing the gland to secrete more FSH. This pattern is also characteristic of the transition toward menopause.