Does the SHBG rs1799941 A allele increase SHBG and reduce free testosterone in men?

The SHBG gene variant rs1799941 plays a major role in regulating sex hormone-binding globulin (SHBG) and androgen biodistribution in men.

Genetic and molecular mechanisms

• Transcriptional regulation: The rs1799941 variant is a G→A single nucleotide polymorphism (SNP) located in the proximal promoter/5'UTR region of the SHBG gene.
• Hepatic expression: This variant acts as a functional cis-regulatory expression quantitative trait locus (eQTL) that enhances hepatic SHBG gene transcription, resulting in increased synthesis and secretion of the protein from the liver.

Circulating SHBG and testosterone dynamics

• Genotype-dependent SHBG elevation: Genetic evidence shows a clear dosage effect where circulating SHBG levels rank AA > GA > GG. Carriers of the AA genotype exhibit approximately 24% to 25% higher SHBG levels than GG homozygotes. In young obese men, the GA genotype increases SHBG by ~3.3 nmol/L, whereas the AA genotype increases it by ~12.5 nmol/L.
• Hormonal binding and free testosterone: Elevated SHBG binds testosterone with high affinity, which triggers a compensatory increase in circulating total testosterone. Because of this parallel rise, directly measured free testosterone (via equilibrium dialysis) often remains relatively stable. However, the biologically active, calculated free and bioavailable fractions relative to the total testosterone pool are lower in A-allele carriers due to increased protein-bound sequestration.

Bottom line

• The rs1799941 A allele is robustly associated with a stepwise increase in hepatic SHBG expression and circulating levels (up to 25% higher in AA homozygotes). Its association with lower free testosterone is plausible, though this reduction is primarily observed in calculated bioavailable fractions rather than directly measured free hormone levels.