FSH (Follicle Stimulating Hormone)
The pituitary hormone that drives egg and sperm development
Plain English
Follicle-stimulating hormone (FSH) is produced by the pituitary gland and drives the development of reproductive cells: egg follicles in women and sperm production in men. In women, FSH is also a critical marker of ovarian reserve. As the ovaries age and follicle count declines, FSH rises as the pituitary pushes harder to get a response. Elevated FSH in a woman of reproductive age signals declining ovarian reserve.
The Mechanism
FSH is released from the pituitary gland in response to gonadotropin-releasing hormone from the hypothalamus, the same upstream signal that drives LH release. In women, FSH rises in the early follicular phase of the menstrual cycle, stimulating a cohort of ovarian follicles to grow and mature. The dominant follicle produces estradiol, which eventually rises high enough to trigger the LH surge that causes ovulation.
As women age and the pool of remaining follicles shrinks, the estradiol response to FSH becomes weaker. The pituitary compensates by producing more FSH. This is why a day-3 FSH level (measured on cycle day 3, when it should be at its lowest) is a standard ovarian reserve marker: values above 10-12 IU/L indicate diminishing reserve, and values above 20-25 IU/L are associated with reduced fertility.
In men, FSH acts on Sertoli cells in the testes to support spermatogenesis. Unlike LH, which drives testosterone production, FSH drives sperm cell maturation. A man with low sperm count but normal testosterone may have FSH dysfunction specifically in the spermatogenesis pathway, while his LH and testosterone remain intact. Elevated FSH in men with poor semen quality points to testicular failure of the sperm production pathway.
Why It Matters
In women, FSH is the earliest measurable signal of ovarian aging.
FSH is the earliest hormonal signal of declining ovarian reserve in women, often rising years before cycles become irregular or symptoms of perimenopause appear. It is also a critical diagnostic marker for distinguishing types of hormonal dysfunction in men: high FSH alongside poor semen analysis points to testicular failure, while low FSH alongside low testosterone points to central signaling failure. In perimenopause, FSH fluctuates erratically before eventually staying elevated, which is why a single FSH reading is less informative than the trend.
Common Misconception
Many women first hear about FSH during fertility workups and assume a single high reading confirms premature ovarian failure. In reality, FSH fluctuates significantly from cycle to cycle, particularly in perimenopause, and a single elevated reading requires confirmation. The Anti-Mullerian Hormone (AMH) test is generally more stable and is now used alongside FSH for ovarian reserve assessment.
Signs It Is Disrupted
- Irregular menstrual cycles with shorter cycle lengths or increased variability
- Hot flashes or night sweats in women under 45, which may indicate premature ovarian insufficiency
- Difficulty conceiving after adequate time trying, prompting fertility evaluation
- In men, abnormal semen analysis alongside elevated FSH suggests spermatogenesis failure
- Absent or significantly delayed puberty in adolescents, where FSH and LH pulsatility fail to initiate
How to Improve It
3 Things to Remember
FSH drives follicle maturation in women and sperm development in men. Elevated FSH in women signals declining ovarian reserve, often years before cycles become irregular.
Day-3 FSH testing is the standard ovarian reserve marker: above 10-12 IU/L indicates diminishing reserve, though it should be paired with AMH for a complete picture.
In men, elevated FSH alongside poor semen analysis points to testicular failure of sperm production specifically, while testosterone and LH may remain normal.
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