By Pedro Royo Manero (Gynecologist – License No. 312203918). Article updated and medically reviewed on 28/10/2026 by Rut Gómez de Segura (Gynaecologist – Reg. No. 28/2908776).
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.
Having elevated follicle-stimulating hormone (FSH) levels does not mean that a woman cannot get pregnant. When high levels are detected at the beginning of the menstrual cycle, they may indicate a reduced ovarian reserve, meaning fewer eggs are available. However, high FSH does not amount to a diagnosis of infertility, as the chances of pregnancy also depend on factors such as age, egg quality and overall reproductive health.
For this reason, assessing the chances of achieving a pregnancy, either naturally or through assisted reproductive technology, requires an individual assessment. This may include other tests, such as measuring anti-Müllerian hormone (AMH) and counting antral follicles by ultrasound. Together, these results provide a more comprehensive picture of ovarian reserve and can help guide the most suitable reproductive options for each patient.
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 fertilisation (IVF).
Based on these results, fertility specialists can adjust the dose of recombinant FSH used in treatments such as in vitro fertilisation (IVF). During IVF treatment, specialists may administer FSH to stimulate the development of several ovarian follicles. The dose is tailored to each patient, taking into account her age, ovarian reserve and expected response to stimulation. Although starting doses of between 150 and 225 IU per day are used in many treatments, the amount can vary depending on the individual case.
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).
Practice Committee of the American Society for Reproductive Medicine.
Testing and interpreting measures of ovarian reserve: a committee opinion.
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2020;
1151-1157
Revisión sobre las pruebas utilizadas para evaluar la reserva ovárica, incluyendo la FSH basal, la AMH y el recuento de folículos antrales, así como sus aplicaciones y limitaciones:
https://doi.org/10.1016/j.fertnstert.2020.09.134
Sowers MR, Zheng H, McConnell D, Harlow SD, Little R, Matthews KA, Strauss JF.
Follicle Stimulating Hormone and Its Rate of Change in Defining Menopause Transition Stages.
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2008;
3958-3964
Estudio sobre los cambios de la hormona foliculoestimulante durante la transición menopáusica y su relación con las diferentes etapas del envejecimiento reproductivo:
https://doi.org/10.1210/jc.2008-0482
Ferrell RJ, O’Connor KA, Holman DJ, Brindle E, Miller RC, Rodriguez G, Simon JA, Mansfield PK, Wood JW, Weinstein M.
Monitoring reproductive aging in a 5-year prospective study: aggregate and individual changes in luteinizing hormone and follicle-stimulating hormone with age.
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2007;
29-37
Estudio prospectivo de cinco años sobre los cambios de FSH y LH asociados al envejecimiento reproductivo y a la transición menopáusica:
https://doi.org/10.1097/01.gme.0000227859.50473.20
ESHRE Guideline Group on Ovarian Stimulation.
ESHRE guideline: ovarian stimulation for IVF/ICSI.
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2020;
hoaa009
Guía basada en la evidencia sobre la estimulación ovárica para FIV e ICSI, incluyendo el uso de gonadotropinas como la FSH recombinante, la predicción de la respuesta ovárica y la monitorización durante la estimulación:
https://doi.org/10.1093/hropen/hoaa009
ESHRE guideline.
Ovarian stimulation for IVF/ICSI.
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2025;
:
https://academic.oup.com/humrep/article/41/4/498/8495097
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