rs727428 — SHBG SHBG +1091 C>T

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 women¹¹ 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 men²² 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 men³³ 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 men⁴⁴ 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 women⁵⁵ 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 women⁶⁶ 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).