By Dr. Pedro Royo Manero (Gynecologist – License No. 312203918).
FSH (follicle-stimulating hormone) is a gonadotropic hormone that acts on the gonads—meaning the ovaries in women and the testes in men—to regulate their function. It is produced by the anterior pituitary gland (adenohypophysis), which plays a crucial role in both male and female reproductive systems.
Among its main functions, FSH stimulates the development of ovarian follicles in women and Sertoli cells in men, which are essential for sperm maturation. It also induces the production of estrogens to prepare the uterus for possible embryo implantation, and regulates the first half of the menstrual cycle, known as the follicular phase, in preparation for ovulation.
The presence and concentration of follicle-stimulating hormone (FSH) in the blood provide important clues about hormonal health and the function of the reproductive organs. Therefore, normal FSH levels vary considerably according to sex, age, and in the case of women, the phase of the menstrual cycle.
It is therefore important to measure hormone levels to detect possible fertility alterations or hormonal disorders.

FSH levels are a key indicator of reproductive system function and can reveal important details about hormonal health. High levels may indicate diminished function of the reproductive organs, with the body trying to compensate. In women, this may be associated with menopause or premature ovarian insufficiency, since the ovaries produce fewer hormones. In polycystic ovary syndrome, FSH can be normal or low, but the LH/FSH ratio is often altered, with higher LH levels. In men, high FSH can indicate testicular damage or dysfunction, which can affect sperm production and fertility.
Low levels of FSH may be due to disorders such as hypopituitarism (when the pituitary gland or hypothalamus produces insufficient hormones). In women, this may be linked to irregular menstrual cycles or lack of ovulation, while in men, it may have an impact on sperm production.

To determine FSH levels, basal FSH is usually measured on the third day of the menstrual cycle. This helps estimate ovarian reserve, though the result provides only limited information and can vary between cycles or laboratories. For this reason, FSH is not interpreted in isolation, but alongside other values such as anti-Müllerian hormone (AMH), antral follicle count (AFC), basal estradiol, and age, to obtain a more complete assessment of reproductive potential.
With these results, fertility specialists can adjust the dose of recombinant FSH in treatments such as in vitro fertilization (IVF). Typically, around 150 IU/day is administered for "mono-responders," while in women with low ovarian reserve or elevated FSH, the dose is usually increased to 225–450 IU/day.
During the cycle, ultrasound monitoring and estradiol measurement allow the dose to be adjusted to optimize the number of eggs without risks such as ovarian hyperstimulation syndrome (OHSS).