Does elevated free testosterone increase androgen signaling in tissues?

In clinical endocrinology, the "free hormone hypothesis" remains the primary model for understanding hormone delivery, positing that only the unbound fraction of testosterone can exert biological effects.

Biological activity and cellular entry

Free testosterone is defined as the biologically active fraction because its unbound state allows it to leave the circulation and enter target cells.

• Passive Diffusion: Unlike testosterone bound to sex hormone-binding globulin (SHBG), which is restricted from crossing lipid bilayers, free testosterone enters cells via passive diffusion.
• Receptor Activation: Once inside the cell, free testosterone binds to the cytoplasmic androgen receptor (AR). This binding triggers nuclear translocation, where the hormone-receptor complex regulates gene expression.
• Animal Models: Transgenic research shows that even if total testosterone is high, biological activity in target organs is diminished if the free fraction is kept stable, confirming that free testosterone is the actual driver of physiological action.

Tissue signaling and clinical effects

Elevated levels of free testosterone lead to a direct, dose-dependent increase in androgen signaling across various tissues.

• Dose-Response Relationship: In women, research on testosterone replacement therapy shows that supraphysiologic peaks in serum levels (100–250 ng/dL) are strongly correlated with increased tissue signaling. For instance, studies comparing different pellet doses found that higher doses led to a significantly higher incidence of androgenic side effects (57.6% vs. 14.8%).
• Clinical Indicators: High androgen signaling manifests as physical changes such as hirsutism (measured by Ferriman-Gallwey scores), androgenic alopecia, and vocal cord edema.
• Alternative Pathways: While most entry occurs via diffusion, some tissues (like the kidney or prostate) may utilize receptor-mediated endocytosis (megalin/cubilin system) to internalize protein-bound hormones, though this remains an exception to the general rule of free hormone dominance.

Bottom line

Free testosterone is the biologically active fraction that enters cells via passive diffusion to activate androgen receptors. Consequently, elevations in free testosterone directly increase androgen signaling, leading to dose-dependent physiological and clinical effects.