FSHB rs11031006 — The FSH Gonadotropin Locus Governing Reproductive Timing and Twinning
Follicle-stimulating hormone (FSH) is the master regulator of follicle development in women and
spermatogenesis in men. It is produced by the pituitary gland when the beta-subunit gene FSHB |
Located on chromosome 11p14.1, encoding the hormone-specific subunit that confers biological
activity is transcribed and translated. rs11031006
is a G-to-A variant located approximately 26 kilobases upstream of the FSHB transcription start
site, sitting within a conserved regulatory enhancer rather than the coding sequence of FSHB
itself. This locus has emerged as one of the most robustly replicated genetic determinants of
circulating FSH levels, female reproductive timing, dizygotic twinning propensity, and PCOS
susceptibility — and it also influences male spermatogenic function through its effect on FSH
production.
Note on variant identity: rs11031006 is a GWAS lead SNP at the FSHB locus; it is distinct from
rs10835638, the -211G>T proximal promoter variant studied in many clinical male infertility
trials. Both variants affect FSHB regulation, but through different mechanisms and at different
distances from the gene. The two are not in strong linkage disequilibrium. Studies of male
infertility citing "FSHB c.-211G>T" refer to rs10835638, while studies citing the 11p14.1 GWAS
locus and twinning associations predominantly discuss rs11031006.
The Mechanism
The rs11031006 variant sits within a ~450 base-pair region that is highly conserved across
placental mammals, a hallmark of functional regulatory elements. In vitro luciferase assays
demonstrate that this region acts as a transcriptional enhancer of FSHB | The enhancer
augments activin- and GnRH-stimulated FSHB transcription in gonadotrope cell
models. The minor A allele creates a stronger
binding site for Steroidogenic Factor 1 (SF1) | A nuclear receptor transcription factor
essential for gonadotrope cell identity and FSH gene expression,
increasing enhancer activity approximately 1.5-fold compared to the major G allele in cell
culture experiments.
This in vitro finding presents a mechanistic paradox: the A allele increases FSHB transcription
experimentally, yet population data consistently show that individuals carrying the A allele
have lower circulating FSH levels, higher LH/FSH ratios, and altered reproductive phenotypes
compared to G-allele carriers. The discrepancy likely reflects the complexity of pituitary
negative feedback regulation in vivo — the G allele may be associated with higher FSH in
part because its carriers have faster hypothalamic-pituitary-gonadal axis dynamics overall.
Mouse models partially reconcile this: female mice homozygous for the A-equivalent mutation
show fewer litters and abnormal estrous cycling | Despite no reduction in baseline FSH
measured in the deletion model, the point mutation itself disrupts reproductive
cycling, suggesting the locus affects
reproductive cycling through mechanisms beyond steady-state FSH levels.
The Evidence
The most robust evidence comes from multiple GWAS. A 2016 study of mothers of spontaneous
dizygotic twins (n~95,000 births in Iceland plus replication cohorts) | Mbarek et al.,
American Journal of Human Genetics identified
rs11031006-G as a genome-wide significant twinning variant (p=1.54×10⁻⁹), with each copy of
the G allele increasing the likelihood of a mother delivering fraternal twins by approximately
18% (OR 1.18 per copy). The same G allele was associated with higher serum FSH levels, earlier
age at menarche, earlier age at first child, higher lifetime parity, lower PCOS risk, and
earlier age at natural menopause — a constellation that collectively points to a more
"fast-cycling" reproductive phenotype.
In polycystic ovary syndrome genetics, the 11p14.1 locus containing rs11031006 was identified
in European PCOS GWAS as associated with altered LH levels and LH/FSH ratio. Women carrying
copies of the A allele show higher LH/FSH ratios | Consistent with the gonadotropin
imbalance characteristic of PCOS, a pattern
distinct from G-allele carriers who tend to have higher FSH relative to LH.
The male fertility significance was established in a 2022 GWAS of 760 idiopathic infertile
men (validated in 1,140) | Schubert et al., Journal of Clinical Endocrinology & Metabolism.
The 11p14.1 locus (represented by rs11031005, in high LD with rs11031006) was the top
genome-wide significant hit for serum FSH levels, explaining 4.65% of FSH variance overall
and 6.95% of variance in the oligozoospermic subgroup specifically — a larger effect than
the well-studied proximal promoter variant rs10835638 (which explains ~3.6% of FSH variance).
Lower FSH in men impairs Sertoli cell function and reduces sperm production; the FSHB locus
was identified as a potential etiologic factor in approximately 28% of men with idiopathic
infertility.
Practical Implications
The practical implications of this variant differ by sex. In women, the A allele (associated
with lower FSH and higher LH/FSH ratio) may contribute to longer menstrual cycles, slightly
delayed folliculogenesis, and a reproductive axis phenotype that overlaps with some PCOS
features — although rs11031006 alone is not diagnostic of PCOS. Women carrying the AA
genotype (approximately 2% of European-ancestry individuals) may wish to discuss FSH and
LH panel interpretation with a reproductive endocrinologist if experiencing irregular cycles,
delayed conception, or unexplained subfertility.
In men, the A allele is associated with measurably lower FSH levels at the population level,
which may impair spermatogenesis. Men with low-normal FSH and idiopathic infertility who
carry variants at this locus represent a distinct etiologic subgroup | Defined as functional
secondary hypogonadism with isolated FSH deficiency, this group responds to exogenous
FSH treatment with improved sperm parameters.
Semen analysis combined with FSH measurement is the key initial investigation for male
carriers, particularly for the AA homozygous genotype.
Interactions
rs10835638 (FSHB -211G>T, proximal promoter): This is a separate variant located 211 bp
upstream of the FSHB mRNA transcription start site. Both rs11031006 and rs10835638 affect FSHB
expression but at different positions and through distinct mechanisms. In men, rs10835638 T
allele has been extensively studied and reduces FSH by ~0.51 IU/L per allele and testicular
volume by ~3.2 ml. These two variants are not in strong LD and may have partially independent
effects; their combined impact on FSH levels in men with idiopathic infertility is additive and
warrants separate genotyping.
rs6166 (FSHR N680S): The FSH receptor sensitivity variant interacts functionally with
FSHB variants. In men, the effect of FSHB locus variants on FSH-driven spermatogenesis is
modulated by FSHR genotype — men with lower FSH production (FSHB A allele) and reduced FSH
receptor sensitivity (FSHR GG) have a compounded spermatogenic disadvantage. This interaction
has been documented for the proximal FSHB variant, and the same pathway logic applies to
rs11031006. A compound action may be warranted when both unfavorable genotypes co-occur.
All Genotypes
Two copies of the common G allele — typical FSH production at this locus
You carry two copies of the G allele at rs11031006, the most common genotype (approximately 74% of people of European ancestry). This genotype is associated with typical FSH levels at this locus and is the reference point against which A-allele effects are measured. GG carriers have the highest FSH production at this locus, which is reflected in population associations with earlier age at menarche, higher lifetime parity, greater likelihood of dizygotic twinning, and lower PCOS risk.
One A allele — mildly lower FSH signaling potential; generally adequate fertility, worth noting in unexplained subfertility workup
You carry one copy of the A allele and one copy of the G allele at rs11031006. This heterozygous genotype is carried by approximately 24% of people of European ancestry, making it relatively common. Heterozygous carriers have FSH levels between those of GG and AA homozygotes — the effect is additive and proportional to allele count. For most AG carriers, FSH levels remain in the physiological range and fertility is not significantly impaired. The variant is most clinically relevant when AG appears alongside other factors that reduce FSH action — such as an unfavorable FSHR genotype, elevated BMI (which suppresses gonadotropin pulsatility), or unexplained subfertility.
Two copies of the low-FSH variant — may have meaningfully lower FSH production affecting fertility in both sexes
You carry two copies of the A allele at rs11031006, the rarest genotype at this locus (approximately 2% of people of European ancestry). This genotype is associated with lower circulating FSH levels in population studies, a higher LH/FSH ratio, and altered gonadotropin axis dynamics compared to G-allele carriers. In men, lower FSH impairs Sertoli cell function and spermatogenesis, potentially contributing to reduced sperm concentration or motility. In women, a lower FSH-to-LH ratio may contribute to longer menstrual cycles, delayed follicular maturation, or a PCOS-like hormonal pattern. The absolute magnitude of effect per allele is modest, but the homozygous state carries twice the dosage.