rs6166 — FSHR Asn680Ser (N680S)

Affects FSH receptor sensitivity, determining ovarian response to FSH stimulation and influencing fertility treatment dosing in both women and men

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FSHR N680S — The Receptor Sensitivity Variant That Shapes Your Response to Fertility Treatment

The follicle-stimulating hormone receptor (FSHR) sits on the surface of granulosa cells in the ovary and Sertoli cells in the testes, where it receives FSH signals that drive follicle development, oocyte maturation, and sperm production. The N680S variant — a single amino acid change at position 680 from asparagine (N) to serine (S) — sits in the intracellular portion of the receptor and alters how quickly and strongly the receptor responds to FSH11 alters how quickly and strongly the receptor responds to FSH
The variant is in the intracellular signaling domain, not the FSH-binding domain. This difference in receptor kinetics has direct, measurable consequences for ovarian stimulation outcomes and is increasingly being used to personalize IVF treatment protocols.

The Mechanism

When FSH binds to its receptor, the intracellular domain triggers a cascade: it activates a Gs protein, which stimulates adenylyl cyclase to produce cyclic AMP (cAMP), which in turn activates protein kinase A and downstream gene expression. The N680S variant changes the kinetics of this cAMP cascade22 The N680S variant changes the kinetics of this cAMP cascade
Not the peak response, but the time to reach it. Granulosa cells from NN homozygotes reach their cAMP plateau in approximately 45 minutes, while SS homozygote cells take approximately 90 minutes. This slower kinetic response in S-carriers results in lower immediate sensitivity to FSH at any given dose33 lower immediate sensitivity to FSH at any given dose
The receptor reaches full activation later, requiring higher circulating FSH to achieve the same effect over a treatment window. The effect propagates downstream: phospho-ERK1/2 activation, AREG and STARD1 gene expression, and progesterone production are all qualitatively and quantitatively different between NN and SS cells exposed to the same FSH concentration.

The Evidence

The clinical consequence of this receptor kinetic difference is striking. A 2019 study of 586 women undergoing controlled ovarian stimulation44 A 2019 study of 586 women undergoing controlled ovarian stimulation
Alviggi et al. Pharmacogenetics and Genomics found that NN carriers (Asn/Asn) produced significantly more oocytes (16±8) compared to carriers of at least one S allele (11±6) despite receiving 20% lower FSH doses. Critically, none of the women who developed ovarian hyperstimulation syndrome (OHSS) — a potentially dangerous overresponse to FSH — had the CC genotype, while the odds ratio for OHSS in NN carriers was 1.7 (P=0.04). This makes biological sense: the faster, stronger receptor response in NN carriers means the same FSH dose produces a larger follicular cohort response.

A 2014 meta-analysis of 13 studies involving 4,020 women55 A 2014 meta-analysis of 13 studies involving 4,020 women
Yao et al. Journal of Ovarian Research confirmed that the GG (SS) genotype carries an odds ratio of 1.61 for poor ovarian response (though with borderline significance, P=0.08), while N-allele carriers showed a statistically significant increased risk of hyperresponse (OR 1.47, 95% CI 1.05–2.04, P=0.02). This asymmetric risk profile — GG more likely to underrespond, AA more likely to overrespond — has direct implications for starting dose selection.

A key 2025 trial took the logical next step: 475 women were genotyped before IVF and assigned to genotype-matched gonadotropin types66 475 women were genotyped before IVF and assigned to genotype-matched gonadotropin types
Recombinant FSH for NN carriers, urinary FSH for S-allele carriers. The optimally treated group achieved a live birth rate of 40% versus 29% in non-genotyped controls (OR 1.55, 95% CI 1.23–1.96, P<0.001). This establishes that the biological difference translates into meaningful clinical improvements when treatment is personalized.

The variant also matters for men. A pharmacogenetic study of 89 idiopathic infertile men77 A pharmacogenetic study of 89 idiopathic infertile men
Simoni et al. Human Reproduction 2016 found that FSH treatment (using recombinant FSH) improved sperm DNA fragmentation index significantly only in NN homozygous men, not in SS carriers. The mechanism mirrors the female data: NN men have a more FSH-responsive receptor in Sertoli cells.

Population frequencies differ notably by ancestry. In European populations, approximately 30% are NN, 50% NS, and 20% SS. In East Asian women, the AA (NN) genotype is more common at approximately 47%, which may partially explain some population differences in ovarian stimulation response rates reported in clinical studies.

Practical Implications

The clinical applications of FSHR genotyping are clearest in an IVF context. Women who know their FSHR N680S genotype before stimulation can work with their reproductive endocrinologist to:

  • CC carriers (SS): Start with higher FSH doses to overcome reduced receptor sensitivity. Urinary FSH (uFSH, which contains additional gonadotropin components) shows better outcomes than recombinant FSH (rFSH) in S-allele carriers in clinical trials.
  • TT carriers (NN): Use lower starting doses and monitor closely for OHSS. Recombinant FSH (rFSH) shows better outcomes in NN carriers. A "freeze-all" embryo strategy or GnRH agonist trigger should be considered prophylactically.
  • TC carriers (NS): Intermediate response; standard protocols apply but monitoring remains important.

Outside of ART, the variant affects basal reproductive hormone levels. Women with GG (SS) have measurably higher day-3 FSH levels88 Women with GG (SS) have measurably higher day-3 FSH levels
9.2 vs 6.2 mIU/ml for TT carriers, P=0.011 — the body compensates for reduced receptor sensitivity by secreting more FSH. This can make CC carriers appear to have "diminished ovarian reserve" on a simple FSH test even when their actual reserve is normal. An anti-Müllerian hormone (AMH) test, which is not affected by FSH receptor sensitivity, provides a more genotype-independent measure of ovarian reserve.

Interactions

rs6165 (FSHR Thr307Ala): This variant in the same gene is in very high linkage disequilibrium with N680S99 very high linkage disequilibrium with N680S
D'=0.997, r²=0.82–0.99 across populations. They are almost always inherited together and form a haplotype (GG = Ala307/Ser680, AA = Thr307/Asn680). Most studies of "FSHR polymorphisms" have examined both variants together; their effects are nearly inseparable in clinical research.

LHCGR rs2293275 (N312S): The LH receptor N312S variant interacts with FSHR N680S in determining IVF outcomes. Women who are SS at both FSHR N680S and LHCGR N312S positions ("4S") had a 62% live birth rate across three IVF cycles versus 43–47% for other combined genotypes1010 Women who are SS at both FSHR N680S and LHCGR N312S positions ("4S") had a 62% live birth rate across three IVF cycles versus 43–47% for other combined genotypes
Adjusted HR 1.89, P=0.049. This interaction between FSH and LH receptor sensitivity defines a pharmacogenetic profile that appears to respond particularly well to ART, potentially because enhanced sensitivity to both FSH and LH creates an optimally responsive gonadal axis.

Compound implication for FSHR GG + LHCGR SS: Women who carry GG at rs6166 and also carry the serine-serine genotype at rs2293275 (LHCGR N312S) may have a combined receptor sensitivity profile that unexpectedly improves IVF outcomes despite individual poor-response signals. These women may represent a distinct pharmacogenetic subgroup that deserves specific protocol design.

Genotype Interpretations

What each possible genotype means for this variant.

CT Standard Responder Intermediate

One copy of the Ser variant — intermediate FSH receptor sensitivity and standard ovarian response

The heterozygous state produces a mixed population of receptor molecules (some with Asn680, some with Ser680), giving intermediate signaling kinetics. Clinical studies generally show NS carriers cluster between NN and SS groups in oocyte yield, FSH dose requirements, and OHSS incidence.

The 2014 meta-analysis found NS carriers had no significant association with either poor or hyperresponse (OR 0.93 for poor response, P=0.75; OR 1.10 for hyperresponse, P=0.50), confirming they represent the "typical" responder phenotype. Standard IVF protocols were designed with this majority genotype in mind.

CC Reduced Responder Reduced

Two copies of the Ser variant — reduced FSH receptor sensitivity requiring higher FSH doses for ovarian stimulation

The GG (SS) genotype creates a receptor that responds to FSH more slowly and with lower cAMP output per unit time. The meta-analysis of 4,020 women found CC carriers had an OR of 1.61 for poor ovarian response (borderline significant, P=0.08).

A critically important clinical implication: higher baseline FSH on day-3 testing can mimic the pattern of diminished ovarian reserve (DOR), leading to misdiagnosis. In GG women, elevated FSH is often a compensatory response to reduced receptor sensitivity rather than true loss of follicle count. AMH testing, which is not mediated through the FSH receptor, gives a receptor-independent estimate of ovarian reserve that is not distorted by this genotype.

For men, the CC genotype affects Sertoli cell response to FSH in the testes. A 2016 pharmacogenetic study found that FSH treatment improved sperm DNA fragmentation only in NN men — CC carriers did not benefit from standard recombinant FSH treatment. Urinary FSH (which contains additional gonadotropin components) may be more effective for men with this genotype.

In the 2025 genotype-guided IVF trial, urinary FSH (uFSH) rather than recombinant FSH (rFSH) was the preferred treatment for S-allele carriers. The uFSH/rFSH distinction produced live birth improvements of 40% vs 29% compared to non-genotyped controls.

TT High Responder High

High FSH receptor sensitivity — respond strongly to FSH with more oocytes but greater OHSS risk

The NN genotype produces a receptor with faster intracellular cAMP kinetics. In a clinical study of 586 IVF patients, NN women produced 16±8 oocytes at doses 20% lower than NS/SS carriers. Despite receiving less FSH, none of the women who developed OHSS carried the CC genotype — all cases were in NN or NS individuals.

For fertility treatment, this genotype is the strongest argument for starting with conservative FSH doses and having a OHSS prevention plan. Recombinant FSH (rFSH) appears to be the preferred gonadotropin type for NN carriers based on a 2025 trial showing improved live birth rates with genotype-matched treatment.

Outside of IVF, NN women typically have lower baseline FSH levels (around 6 mIU/ml on day 3) because the pituitary doesn't need to compensate with elevated FSH to achieve the same ovarian effect. This can make the ovary appear "normally responsive" on standard hormonal panels.