SHBG +1091 C>T — The Hormone Bioavailability Switch Downstream of SHBG
Sex hormone-binding globulin is the liver's primary traffic controller for testosterone and
estradiol. Only about 1–3% of testosterone circulates as "free" bioactive hormone — the rest
is bound to SHBG (roughly 44%) or albumin (54%). When SHBG levels rise, more testosterone
gets locked away; when SHBG levels fall, more free testosterone is available to act on tissues.
Rs727428 is one of the best-replicated [genetic regulators | This variant has been identified
as genome-wide significant in multiple independent GWAS and validated across ancestries]
of circulating SHBG levels, located just outside the SHBG gene itself in a downstream
regulatory region.
The Mechanism
Rs727428 sits approximately 1 kb downstream of the SHBG gene on chromosome 17p13.1, in a
[regulatory region | This area contains chromatin elements that influence SHBG gene expression
in the liver; GERP conservation score and Ensembl RegBuild both annotate it as a functional
regulatory feature] outside the protein-coding sequence. The variant does not change the
SHBG protein structure — it acts by influencing how much SHBG the liver produces. The T
allele at this position is associated with reduced SHBG transcription relative to the C
allele. Each copy of the T allele reduces serum SHBG by approximately 3–4 nmol/L
[| Grigorova et al. 2017, PMID 29264510 — P=7.3×10⁻¹¹, effect −3.74 nmol/L per allele],
meaning TT homozygotes produce measurably less SHBG than CC homozygotes. Because SHBG binds
testosterone with roughly five times higher affinity than albumin, small changes in SHBG
concentration have outsized effects on how much testosterone is biologically active in tissues.
The Evidence
The original genome-wide significant association between rs727428 and circulating SHBG was
established in a GWAS of ~1,600 postmenopausal women | Prescott et al. PLoS One, 2012
where the T allele was associated with lower SHBG (β=−0.126 on a log scale, P=2.09×10⁻¹⁶).
This finding was simultaneously replicated in a GWAS of 3,225 European men | Jin et al.
Human Molecular Genetics, 2012, where rs727428
showed genome-wide significant associations with both total testosterone (P=1.26×10⁻¹²) and
dihydrotestosterone (DHT, P=1.47×10⁻¹¹).
A validation study in 1,687 Japanese men | Sato et al. Human Reproduction Open, 2019
replicated the SHBG association (β=0.21, P=3.4×10⁻¹⁰) but did not replicate testosterone
association after multiple testing adjustment, suggesting that in East Asian populations the
effect on SHBG may not translate directly into measurable testosterone differences — possibly
because other loci or lifestyle factors modulate free testosterone differently in this population.
A population-based sibling study of 999 Dutch men | Walravens et al. Journal of Clinical
Endocrinology & Metabolism, 2025 found that
rs727428 T allele homozygotes had 10.8–23.1% lower SHBG and 3.9–21.4% lower total testosterone
compared to CC homozygotes. Notably, calculated and directly measured free testosterone
showed minimal difference across genotypes — suggesting the body compensates for SHBG-driven
total testosterone changes by adjusting LH/FSH feedback to maintain free testosterone
homeostasis in healthy men.
In women, a Mediterranean PCOS case-control study of 1,004 premenopausal women | Martínez-García
et al. Human Reproduction, 2012 found the T allele
was more frequent in PCOS patients than controls (OR=1.29), independent of obesity. A family-based
PCOS study in 758 women | Wickham et al. Journal of Clinical Endocrinology & Metabolism, 2011
confirmed that rs727428 genotype was independently associated with SHBG levels after controlling
for BMI, insulin resistance, and free testosterone. Meta-analyses have shown mixed results: one
2020 meta-analysis (1,660 cases, 1,312 controls) found the association with PCOS susceptibility
was not statistically significant after pooling, while individual studies — particularly in
Mediterranean and Middle Eastern populations — consistently found associations. The discrepancy
likely reflects population-specific allele frequencies and PCOS diagnostic heterogeneity.
Practical Implications
The clinical consequences of rs727428 depend heavily on context. In women, lower SHBG from the
T allele means more free testosterone and a higher [free androgen index | FAI = total testosterone
÷ SHBG × 100; values above 4–5 in women are associated with androgenic symptoms and PCOS].
Women with TT genotype benefit most from monitoring free androgen index rather than total
testosterone, and from strategies that support SHBG production — particularly insulin-sensitizing
approaches, since insulin is a potent suppressor of hepatic SHBG synthesis. In men, lower SHBG
generally keeps more testosterone available in tissue, but the Walravens 2025 data suggest the
body's hormonal feedback loop largely compensates in healthy young men. Where this compensation
may fail is in older men, men with metabolic syndrome, and men undergoing testosterone monitoring
for hypogonadism — in these contexts, SHBG genotype should inform how total testosterone is
interpreted.
Interactions
rs1799941 (SHBG promoter G-68A): This variant in the SHBG promoter acts independently from
rs727428 to regulate SHBG levels. The A allele of rs1799941 increases SHBG by 15–25%, opposing
the T allele effect at rs727428. The Grigorova 2017 study (PMID 29264510) found these variants
replicate independently — both are significant GWAS hits with distinct mechanisms (one alters
promoter transcription factor binding; the other affects downstream regulatory architecture).
A person carrying T at rs727428 AND G at rs1799941 has two additive SHBG-lowering variants;
a person carrying T at rs727428 AND A at rs1799941 has opposing forces that may partially cancel.
This is a strong candidate for a compound action given the opposite-direction effects in the same pathway.
rs6259 (SHBG Asp327Asn): A missense variant in the SHBG protein (p.Asp327Asn) that reduces
testosterone binding affinity by approximately 10%, thereby increasing free testosterone
bioavailability even when total SHBG concentration is unaffected. In men, the A allele of rs6259
was associated with increased free testosterone in the Walravens 2025 study. Compound carriers of
both the rs727428 T allele (less SHBG protein) and the rs6259 A allele (SHBG protein with lower
binding affinity) would have a compounded increase in free testosterone — relevant for interpreting
androgen status in both sexes.
Compound action proposal for rs727428 TT + rs1799941 GG: Women carrying TT at rs727428 AND
GG at rs1799941 carry two independent SHBG-lowering variants — one reducing transcription rate
(promoter), one reducing downstream regulatory expression. The combined recommendation: measure
free androgen index (FAI = total testosterone ÷ SHBG × 100); track SHBG as a metabolic risk
biomarker; implement insulin-sensitizing strategies (inositol supplementation has evidence for
PCOS with low SHBG); and discuss combined SHBG genotype result with clinician before interpreting
any sex hormone panel. Evidence level: moderate (individual effects well-established; combined
effect inferred from independent pathway data).
Alla genotyper
One C and one T allele — SHBG levels in the mid-range, modest effect on free hormone bioavailability
You carry one copy of each allele at rs727428 — the most common genotype globally, present in approximately 49% of people. Your SHBG levels sit between the high-SHBG CC genotype and the low-SHBG TT genotype. The effect on free testosterone and free estradiol is modest and unlikely to be clinically significant in isolation for healthy individuals. The heterozygous state is worth noting in the context of fertility evaluation or when interpreting sex hormone panels at the margins — a borderline free androgen index in a PCOS workup or a borderline low free testosterone in a hypogonadism workup may be partially explained by this genotype.
Two T alleles — SHBG runs below average, releasing more free testosterone and estradiol into circulation
You carry two copies of the T allele at rs727428, the genotype associated with the lowest SHBG levels among the three genotypes. Each T allele reduces SHBG by roughly 3–4 nmol/L, so TT homozygotes produce measurably less SHBG than CC homozygotes. Approximately 19% of people of European descent share this genotype; it is more common in East Asian (29%) and South Asian (37%) populations. Lower SHBG means a greater fraction of your circulating testosterone and estradiol is unbound and biologically active. In women, this increases the free androgen index and is associated with higher PCOS risk and androgenic symptoms. In men, lower SHBG typically keeps more testosterone available to tissues but is also a recognized marker of metabolic syndrome risk when SHBG falls below 30–40 nmol/L.
Two C alleles — SHBG runs above average, binding more testosterone and estradiol
You carry two copies of the C allele at rs727428, the genotype associated with the highest SHBG levels of the three genotypes. Your liver produces more SHBG than the T allele groups, which means a greater proportion of your circulating testosterone and estradiol is protein-bound and less is biologically active ("free"). About 32% of people of European descent share this genotype globally. In healthy young people, the body typically compensates — the pituitary adjusts LH output to maintain free testosterone in the normal range. But this genotype becomes clinically relevant when free testosterone is borderline low, when you are older (SHBG naturally rises with age), or when interpreting total testosterone on lab results without a corresponding SHBG measurement.