Do TMPRSS6 variants that lower hepcidin increase risk of hematocrit rise with testosterone therapy?

Testosterone replacement therapy often leads to a significant rise in hematocrit, a phenomenon driven by the suppression of hepcidin, the body's primary iron regulator. This process increases iron bioavailability to support enhanced red blood cell production. Genetic variations that independently lower hepcidin signaling, particularly those involving the TMPRSS6 gene, are theorized to sensitize individuals to this effect by creating a more permissive environment for iron mobilization.

Clinical and Mechanistic Evidence

• Hepcidin Suppression: Clinical trials demonstrate that testosterone suppresses serum hepcidin levels by approximately 41% to 57% in men. This suppression occurs via the direct transcriptional downregulation of the HAMP gene in the liver, independent of the erythroferrone or erythropoietin pathways.
• Iron Mobilization: By reducing hepcidin, testosterone prevents the degradation of ferroportin (the primary iron exporter). This leads to increased iron export from macrophages and hepatic stores, resulting in elevated serum iron and transferrin saturation. This mobilized iron is then rapidly incorporated into new erythrocytes.
• TMPRSS6 and Hepcidin Regulation: The protease TMPRSS6 is a critical negative regulator of hepcidin. Genetic variations in TMPRSS6, such as the rs855791 polymorphism, are well-established modifiers of baseline hepcidin levels and iron status in human populations.
• Pathway Convergence: Both testosterone and TMPRSS6 act on the same regulatory axis; testosterone suppresses hepcidin transcription via BMP/Smad signaling, while TMPRSS6 inhibits hepcidin by cleaving the BMP co-receptor hemojuvelin. Consequently, genetic factors that lower baseline hepcidin likely amplify the erythropoietic drive initiated by testosterone.

Safety and Practical Considerations

• Erythrocytosis Risk: Secondary erythrocytosis is a common side effect of testosterone therapy, occurring in up to 66% of patients. Identifying genetic markers like TMPRSS6 variants could eventually help predict which patients are at higher risk for excessive hematocrit rises.
• Therapeutic Corroboration: The importance of this pathway is further supported by observations that TMPRSS6-targeted therapies, which raise hepcidin levels, successfully reduce hematocrit in other polycythemic conditions.

Bottom line

The claim is biologically plausible and supported by strong mechanistic evidence. While direct clinical data confirming that TMPRSS6 variants specifically increase testosterone-associated hematocrit rise are currently limited, the convergence of these two factors on the hepcidin-iron-erythropoiesis axis provides a compelling explanation for individual variability in hematologic responses to testosterone.