Follicle-Stimulating Hormone (FSH)

IVF & Controlled Ovarian Stimulation

FSH is the backbone of controlled ovarian stimulation (COS) in IVF. Stimulation protocols typically administer 150-450 IU FSH daily for 8-12 days, aiming to develop multiple mature follicles simultaneously. The individualized approach considers patient age, AMH levels, antral follicle count (AFC), BMI, and prior response to calibrate the starting dose. Nyboe Andersen et al. (2008) demonstrated that recombinant FSH achieves predictable ovarian response with well-characterized dose-response relationships, forming the basis for modern individualized stimulation protocols.

The development of long-acting FSH (corifollitropin alfa, Elonva) extended the duration of action through fusion with the hCG C-terminal peptide, providing 7 days of FSH activity from a single injection and reducing the injection burden during IVF.

Biosimilars & Formulations

The introduction of FSH biosimilars (Ovaleap, Bemfola, Followtrop) has expanded access to fertility treatment. Biosimilar approval requires demonstration of equivalent quality, safety, and efficacy to the reference product. Strowitzki et al. (2016) demonstrated comparable clinical outcomes between biosimilar follitropin alfa and the originator product in a large randomized controlled trial. Follitropin delta (Rekovelle) represents a novel recombinant FSH produced in a human cell line (PER.C6) with a distinct glycosylation pattern, enabling individualized dosing based on AMH and body weight.

PCOS & Ovulation Induction

Polycystic ovary syndrome (PCOS) presents a unique challenge for FSH therapy. PCOS patients have elevated AMH, high antral follicle counts, and exaggerated sensitivity to FSH stimulation, increasing the risk of OHSS and multiple pregnancy. Low-dose step-up protocols (starting at 37.5-75 IU/day with incremental increases every 7-14 days) aim to identify the FSH threshold for monofollicular development. Homburg & Howles (1999) established the low-dose step-up protocol as the standard approach for FSH-based ovulation induction in PCOS, achieving monofollicular development in approximately 70% of cycles.

Male Infertility

In men with hypogonadotropic hypogonadism, FSH combined with HCG restores spermatogenesis. FSH stimulates Sertoli cell proliferation and function, while HCG provides LH-like stimulation of testosterone production by Leydig cells. Dwyer et al. (2019) reviewed the evidence for FSH therapy in male infertility, noting particular benefit in men with pre-treatment testicular volume <4 mL and those with congenital hypogonadotropic hypogonadism who never underwent normal puberty.

Controlled Ovarian Hyperstimulation

Exogenous FSH administration overcomes the natural selection mechanism to stimulate multiple follicular development for IVF. Bosch et al. (2008) demonstrated that individualized FSH dosing based on ovarian reserve markers (AMH, antral follicle count) optimizes oocyte yield while minimizing OHSS risk. Standard protocols use 150-450 IU/day of rFSH for 8-14 days, monitored by serial ultrasound and estradiol measurements. The goal is typically 8-15 oocytes — balancing retrieval number against OHSS risk and oocyte quality.

PCOS Management

Women with PCOS represent the largest anovulatory population requiring ovulation induction. Low-dose step-up FSH protocols (starting at 37.5-75 IU/day with incremental increases) aim to achieve mono-follicular development while avoiding the multi-follicular response that PCOS patients are prone to. Fauser et al. (2012) reviewed strategies for safe gonadotropin use in PCOS, emphasizing the importance of individualized dosing and intensive monitoring to prevent OHSS and high-order multiple pregnancies.

Recombinant vs Urinary FSH

The clinical superiority of rFSH over urinary-derived FSH has been debated extensively. Meta-analyses including van Wely et al. (2011) (Cochrane review) found no significant difference in live birth rates between rFSH and urinary FSH (HP-HMG or uFSH) preparations. Recombinant products offer advantages in purity, batch consistency, and pen-device delivery, while urinary products may provide LH activity that benefits certain patient subgroups (low-LH environments, hypogonadotropic hypogonadism).

Biosimilar Development

The expiration of patents on original rFSH products has enabled biosimilar development. Behre et al. (2014) reported a pivotal randomized controlled trial demonstrating that Ovaleap (follitropin alfa biosimilar) was equivalent to Gonal-F in oocyte yield, fertilization rates, and pregnancy outcomes. Bemfola is another approved biosimilar. These products undergo rigorous comparative testing including physicochemical characterization, preclinical pharmacology, and clinical equivalence trials, and have been shown to provide comparable efficacy with significant cost reduction.

Male Factor Infertility

FSH therapy in men targets oligozoospermia and azoospermia associated with impaired Sertoli cell function. In idiopathic oligozoospermia with normal FSH levels, exogenous FSH supplementation (75-150 IU three times weekly for 3-6 months) has shown improvement in sperm concentration and motility in several trials, though results are heterogeneous. Santi et al. (2015) conducted a meta-analysis of FSH treatment for idiopathic male infertility, finding a significant improvement in spontaneous pregnancy rate (OR 4.5, 95% CI 2.17-9.33) and sperm concentration with FSH therapy compared to placebo.

In hypogonadotropic hypogonadism, FSH is combined with hCG to achieve both spermatogenesis (FSH) and intratesticular testosterone production (hCG), with spermatogenesis typically requiring 6-18 months of combined therapy.

Algorithmic individualized FSH dosing

Follitropin delta introduced fixed daily dosing computed from serum AMH and body weight rather than response-based titration. Phase 3 (ESTHER-1) and subsequent trials show this preserves efficacy while narrowing the response distribution and reducing extreme responses and OHSS-preventive interventions.

• ESTHER-1 (NCT01956110): individualized follitropin delta non-inferior to conventional follitropin alfa for ongoing pregnancy and live birth (PMID 27912901).
• An individual participant data meta-analysis confirmed improved safety while preserving live birth rates (PMID 38750874).
• GRAPE validated the individualized regimen in an Asian population with reduced early OHSS (PMID 34179971).

Stimulation protocol optimization

Comparative trials continue to refine how recombinant FSH is combined with GnRH agonist versus antagonist co-treatment to maximize oocyte yield while controlling OHSS risk.

• BEYOND (NCT03809429) found the GnRH agonist protocol yielded modestly more oocytes than the antagonist protocol with individualized follitropin delta (PMID 38723189).
• Low-dose step-up r-hFSH remains standard for ovulation induction to favor monofollicular development (PMID 24942155).