rs858518 — SHBG SHBG intronic regulatory variant

Intronic variant within the SHBG gene that participates in a haplotype (with rs727428) lowering circulating sex hormone-binding globulin levels, increasing bioavailable testosterone and estradiol; lower SHBG is linked to PCOS susceptibility, insulin resistance, type 2 diabetes risk, and female VTE risk mediated by estradiol

Moderate Risk Factor Share

Details

Gene: SHBG
Chromosome: 17
Risk allele: A
Clinical: Risk Factor
Evidence: Moderate

Population Frequency

31%

49%

20%

External

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SHBG rs858518 — The SHBG-Lowering Haplotype Variant

[Sex hormone-binding globulin (SHBG) | A liver-produced transport protein that binds testosterone and estradiol in circulation, controlling how much hormone is biologically active] is a critical gatekeeper for sex hormone action throughout the body. Only 1–2% of testosterone and estradiol circulate as free, bioactive hormones — the rest is bound to SHBG (about 44%) or albumin. The SHBG gene on chromosome 17 encodes this protein, and variants within it directly influence circulating SHBG protein levels. rs858518 is an intronic variant that sits within the SHBG gene and participates in a co-inherited haplotype — together with the nearby intronic variant rs727428 — that reduces SHBG production in the liver.

The Mechanism

Rs858518 (chr17:7,629,707, GRCh38) is located in an intron of the SHBG gene and does not directly alter the SHBG protein sequence. Instead, it likely tags a regulatory element or is in strong linkage disequilibrium with a causal variant that affects SHBG gene expression. Haplotype analysis of 11 tagging SNPs across the SHBG locus showed that rs858518 and the nearby rs727428 act in concert to lower SHBG levels, while this lowering effect is counteracted when rs6259 (Asp356Asn), a coding variant in exon 8, is also present on the same haplotype background¹¹ Haplotype analysis of 11 tagging SNPs across the SHBG locus showed that rs858518 and the nearby rs727428 act in concert to lower SHBG levels, while this lowering effect is counteracted when rs6259 (Asp356Asn), a coding variant in exon 8, is also present on the same haplotype background
Thompson et al. Cancer Epidemiology Biomarkers & Prevention 2008. This three-way interaction illustrates how the SHBG locus functions as a network of co-acting variants rather than a single deterministic SNP.

The practical consequence is straightforward: the A allele at rs858518 participates in a haplotype that reduces the amount of SHBG the liver makes. Lower SHBG means more testosterone and estradiol remain unbound — biologically active — even when total hormone levels appear normal on standard bloodwork. Because SHBG binds testosterone with approximately five times higher affinity than estradiol, the impact is felt most strongly through testosterone bioavailability.

The Evidence

The foundational evidence for rs858518 comes from a 2008 study by Thompson et al. of up to 6,622 breast cancer cases and 6,784 controls, with SHBG level analysis in 1,134 healthy postmenopausal women²² a 2008 study by Thompson et al. of up to 6,622 breast cancer cases and 6,784 controls, with SHBG level analysis in 1,134 healthy postmenopausal women
Thompson DJ et al. Identification of common variants in the SHBG gene affecting sex hormone-binding globulin levels and breast cancer risk in postmenopausal women. Cancer Epidemiology Biomarkers & Prevention 2008. That work identified a haplotype containing rs858518 and rs727428 as the primary SHBG- lowering signal within the SHBG locus, accounting for a substantial fraction of inter-individual variance in circulating SHBG.

For the fertility-relevant phenotype of SHBG levels in men with and without infertility, rs727428 — the strong LD partner of rs858518 — provides the most precise effect estimate. In a study of 1,505 men (540 young controls, 641 infertile patients, 324 pregnant women's partners)³³ study of 1,505 men (540 young controls, 641 infertile patients, 324 pregnant women's partners)
Grigorova et al. Genetics of Sex Hormone-Binding Globulin and Testosterone Levels in Fertile and Infertile Men of Reproductive Age. J Endocrine Society 2017, each copy of the rs727428 T allele (the SHBG-lowering allele) was associated with −3.74 nmol/L lower SHBG (SE 0.57, P=7.3×10⁻¹¹). Notably, free testosterone did not differ significantly across genotypes, suggesting compensatory total testosterone adjustments maintain free hormone homeostasis in healthy men. No direct associations with male infertility parameters were detected for this variant, consistent with the view that rs858518/rs727428 acts on the SHBG set-point rather than spermatogenesis directly.

For metabolic disease, a 2019 Uighur population case-control study comparing 114 men with T2DM to 173 healthy controls⁴⁴ 2019 Uighur population case-control study comparing 114 men with T2DM to 173 healthy controls
Quan et al. Association between sex hormone binding globulin gene polymorphism and type 2 diabetes mellitus. Int J Clin Exp Pathol 2019 found that the four-SNP haplotype rs858518-rs3760213-rs1799941-rs6257 with sequence TCGC was significantly more frequent in T2DM cases (P=0.033), placing rs858518 within a broader SHBG haplotype block associated with diabetes risk in this population.

The most recent evidence implicates rs858518 specifically in female VTE risk. A 2024 two-sample Mendelian randomization using UK Biobank and FinnGen data⁵⁵ 2024 two-sample Mendelian randomization using UK Biobank and FinnGen data
Tian et al. The genetic effects of hormones modulated by the pituitary-thyroid/adrenal/gonadal axis on the risk of developing VTE. BMC Cardiovascular Disorders 2024 identified rs858518 as the only SHBG SNP associated with increased female VTE risk, operating through an estradiol-mediated pathway. The authors proposed rs858518 as a "potential prevention and treatment target for female VTE."

Practical Implications

Lower SHBG from this haplotype means higher bioavailable testosterone and estradiol. In women, elevated free androgen is the biochemical hallmark of PCOS — hyperinsulinemia and this genetic tendency toward lower SHBG may act together to amplify the free androgen excess that drives PCOS symptoms. In men, the effect size (-3.74 nmol/L per allele) is moderate in absolute terms; free testosterone is largely maintained through compensatory mechanisms in metabolically healthy individuals, but the SHBG-lowering allele shifts the balance toward higher free-to-total hormone ratios that matter most in clinical edge cases (borderline hypogonadism, anabolic sensitivity, metabolic disease).

For the diabetes-SHBG connection: low SHBG is itself a biomarker of insulin resistance — insulin suppresses SHBG production via HNF4A downregulation. Carrying the rs858518 A allele sets a genetically lower SHBG baseline, potentially compounding the SHBG suppression that accompanies metabolic dysfunction. Monitoring fasting glucose, insulin sensitivity, and SHBG levels together provides the clearest picture.

Interactions

rs727428 (SHBG intron 4, +1091 C>T): The primary LD partner of rs858518. These two variants are co-inherited and likely represent the same haplotype signal — the SHBG- lowering effect reported for rs858518 in haplotype analyses is quantitatively validated through the rs727428 direct association (−3.74 nmol/L per T allele). A compound action for the rs858518 AA + rs727428 TT double-homozygous state (lowest SHBG haplotype) could capture the full extent of SHBG suppression from this intragenic haplotype block.

rs6259 (SHBG Asp356Asn, p.Asp356Asn): This coding variant in exon 8 neutralizes the SHBG-lowering effect of the rs858518/rs727428 haplotype when present on the same chromosomal background, based on Thompson 2008 haplotype analysis. Users who carry AA at rs858518 and the D356N variant at rs6259 may not show the expected SHBG reduction.

rs1799941 (SHBG promoter): This promoter variant independently increases SHBG and is already tracked in the platform's hormones-sleep category. The rs858518 and rs1799941 variants exert opposing effects on SHBG — carrying the rs858518 AA (lowering) and rs1799941 AA (raising) genotypes simultaneously represents a tug-of-war at the SHBG set- point, with net effect depending on additional haplotype context.

Compound action proposal for rs858518 AA + rs727428 TT: Both variants on the SHBG- lowering haplotype, homozygous at both positions, represent the lowest-SHBG genotypic state within this locus. The combined recommendation would be: monitor free testosterone (not just total), monitor fasting insulin and HOMA-IR annually, and for women of reproductive age, screen for PCOS-related free androgen excess if symptoms are present. Evidence level: moderate (derivable from Grigorova 2017 and Thompson 2008 haplotype data).

Genotype Interpretations

What each possible genotype means for this variant:

GG “Standard SHBG Haplotype” Normal

Two copies of the reference allele — SHBG levels at the baseline set-point for this locus

The GG genotype at rs858518 represents the reference-allele homozygous state. The G allele is the GRCh38 plus-strand reference (chr17:7,629,707 G). Without the A allele's participation in the SHBG-lowering haplotype, SHBG production from the rs858518 locus is unaffected. SHBG levels are still regulated by other variants in the SHBG gene (particularly the promoter variant rs1799941), by hepatic insulin signaling (HNF4A activity), by adiposity, and by thyroid function — so a GG genotype here does not guarantee high SHBG in the presence of metabolic dysfunction.

Globally GG is the less common genotype (~20%), with the A allele being the majority allele in European and East Asian populations.

AG “Low SHBG Haplotype — Heterozygous” Intermediate Caution

One SHBG-lowering allele — modestly lower SHBG with intermediate effects on sex hormone bioavailability

The additive inheritance pattern of this variant means one copy produces half the effect of two copies. In Grigorova 2017, the effect was expressed per allele (−3.74 nmol/L), implying AG heterozygotes sit approximately halfway between AA and GG in terms of SHBG concentration. For most healthy individuals, this intermediate shift does not produce clinically detectable effects on free testosterone (compensatory mechanisms are intact), but it shifts baseline SHBG into a range that has less buffering capacity against metabolic SHBG suppression.

For women with borderline hyperandrogenism or PCOS risk factors, the intermediate SHBG lowering from one A allele may be enough to push the free androgen index above diagnostic thresholds, particularly if hyperinsulinemia co-exists.

AA “Low SHBG Haplotype — Homozygous” Decreased Warning

Two copies of the SHBG-lowering allele — lowest SHBG from this locus, raising bioavailable testosterone and estradiol

The SHBG-lowering haplotype containing rs858518 and rs727428 operates on SHBG expression in hepatocytes. Lower hepatic SHBG output means less circulating binding capacity for testosterone and estradiol — shifting the equilibrium toward higher free (bioactive) fractions of both hormones. SHBG binds testosterone with ~5-fold higher affinity than estradiol, so the impact falls disproportionately on testosterone bioavailability.

In the Grigorova 2017 study of 1,505 men, the rs727428 T allele (the co-inherited LD partner) showed −3.74 nmol/L SHBG per copy (P=7.3×10⁻¹¹), with no significant difference in free testosterone across genotypes in this healthy young cohort. This suggests the male reproductive system compensates for lower SHBG by adjusting total testosterone production, buffering free testosterone. However, this homeostasis operates within limits — in metabolic disease, obesity, or aging, the compensatory capacity diminishes.

The Tian 2024 Mendelian randomization found rs858518 specifically associated with increased female VTE risk via estradiol, suggesting that the shift in estradiol bioavailability from this variant has measurable downstream cardiovascular consequences in women. This is clinically relevant for women considering combined oral contraceptives (which increase VTE risk independently) or postmenopausal hormone therapy.

For metabolic risk, the Quan 2019 SHBG haplotype study in Uighur men found that a four-SNP haplotype containing rs858518 was significantly more common among men with T2DM. Low SHBG is both a genetic risk factor for and a biomarker of insulin resistance — the feedback loop (insulin suppresses SHBG → lower SHBG enables more free androgen → androgens worsen insulin signaling in women) can be initiated at either end.