Insulin resistance in PCOS reduces SHBG and promotes endometrial overgrowth and abnormal uterine bleeding.

Polycystic Ovary Syndrome (PCOS) involves a complex interplay between metabolic dysfunction and hormonal dysregulation. Research confirms that insulin resistance is a primary driver of the endocrine abnormalities that lead to endometrial issues and irregular bleeding.

Mechanistic pathways

The link between insulin and reproductive hormones is mediated primarily through the liver.

• Hepatic Suppression: High levels of circulating insulin (hyperinsulinemia) act directly on the liver to inhibit the synthesis of Sex Hormone-Binding Globulin (SHBG). This occurs via the down-regulation of the transcription factor HNF-4α, which is necessary for SHBG gene expression.
• Hormonal Bioavailability: SHBG acts as a carrier protein that sequesters testosterone and estradiol. When SHBG levels drop, the fraction of these hormones that is "free" or bioavailable increases. Even if total hormone levels are relatively stable, the reduction in SHBG significantly raises the Free Androgen Index (FAI) and the amount of free estradiol available to bind to tissue receptors.

Clinical evidence and effectiveness

The impact of these hormonal shifts on the uterine lining is well-documented in clinical cohorts:

• Endometrial Hyperplasia: Elevated bioavailable estradiol stimulates continuous endometrial proliferation. In the absence of ovulation (and thus the absence of progesterone to "oppose" the estrogen), the endometrium becomes excessively thick. Research indicates that approximately 9% of women with PCOS have non-atypical hyperplasia, while up to 15% may have atypical hyperplasia.
• Bleeding Patterns: This overgrown, thickened endometrium is structurally unstable. Clinical data shows that an endometrial thickness of ≥8.5 mm in PCOS patients is a strong predictor of both hyperplasia and abnormal uterine bleeding (AUB).

Clinical implications

Managing the metabolic aspects of PCOS is often as critical as addressing the hormonal symptoms.

• Metabolic Intervention: Interventions that improve insulin sensitivity—such as specific dietary protocols (e.g., ketogenic diets) or pharmacological agents—have been shown to increase SHBG levels, thereby reducing the pool of bioavailable sex hormones.
• Risk Mitigation: Because SHBG serves as a reliable surrogate marker for liver insulin resistance, monitoring its levels can help clinicians assess the risk of endometrial overgrowth and guide treatment to prevent long-term complications like endometrial malignancy.

Bottom line

In PCOS, insulin resistance suppresses SHBG production in the liver, directly increasing bioavailable estrogen and androgens. This "unopposed" estrogen drives endometrial overgrowth (hyperplasia), which manifests clinically as abnormal and heavy uterine bleeding.