Is SHBG produced mainly by the liver and suppressed by insulin but increased by thyroid hormone?

Sex hormone–binding globulin (SHBG) is a critical glycoprotein responsible for the transport and bioavailability of sex steroids. Its production is primarily a function of the liver, and its circulating levels serve as a sensitive marker for hepatic metabolic and endocrine health.

Hepatic production and origin

Scientific consensus identifies the liver as the primary source of circulating SHBG. While the SHBG gene is expressed in other tissues such as the testes, prostate, and brain, these sites contribute minimally to the systemic pool.

• Anatomical site: Hepatocytes synthesize and secrete SHBG into the blood, driven by specific promoters (P_L and P_T) that are highly active in the liver.
• Regulatory hub: The liver acts as a central integrator of nutritional and hormonal signals to modulate SHBG levels. A key mediator is Hepatocyte Nuclear Factor-4 alpha (HNF-4α), a transcription factor essential for maintaining high levels of SHBG gene expression.

Suppression by insulin

The relationship between insulin and SHBG is strongly inverse, often used clinically as a proxy for insulin sensitivity.

• Mechanistic pathway: Insulin suppresses SHBG production by downregulating HNF-4α. In hyperinsulinemic states—such as obesity or type 2 diabetes—high insulin levels and increased hepatic lipogenesis (driven by monosaccharides like fructose) reduce HNF-4α availability. This allows repressors like COUP-TFI to bind to the SHBG promoter, silencing its transcription.
• Clinical evidence: Patients with low insulin (e.g., Type 1 Diabetes) typically exhibit elevated SHBG, whereas hyperinsulinemia is a consistent predictor of low SHBG levels. Studies demonstrate that intraperitoneal insulin delivery suppresses SHBG more effectively than subcutaneous delivery, highlighting the direct effect on the liver.

Stimulation by thyroid hormone

Thyroid hormones (T3 and T4) are potent stimulators of hepatic SHBG production, and SHBG is frequently used as a biomarker for peripheral thyroid hormone action.

• Transcriptional activation: T3 (the active form) binds to thyroid hormone receptors (specifically TRβ) in hepatocytes. This complex interacts with HNF-4α and Retinoid X Receptors (RXR) to bind the proximal promoter of the SHBG gene, significantly increasing its transcriptional activity.
• Clinical context: SHBG levels rise in hyperthyroidism and decrease in hypothyroidism. Research shows that switching patients from T4 to the more potent T3 monotherapy results in a more pronounced increase in SHBG, reflecting the strength of this hormonal axis.

Bottom line

The claim is fully supported. SHBG is primarily produced in the liver, where its synthesis is inversely regulated by insulin (via HNF-4α suppression) and positively regulated by thyroid hormones (via TRβ-mediated transcriptional activation).