Can high SHBG, increased DHT conversion, and lower DHEA-S reduce testosterone signaling despite normal total testosterone?

High SHBG is a well-established reason an older man may have reduced biologically available testosterone despite a normal total testosterone result. Greater DHT conversion and lower DHEA-S may modify this picture, but their combined independent clinical effect has not been established.

Clinical evidence

• SHBG binds circulating testosterone and reduces the free/bioavailable fraction available for androgen-receptor signaling. Cohorts of men with normal total testosterone but low calculated free testosterone found that these men were older, had higher SHBG, and had more sexual, physical, hematologic, and bone-related features compatible with lower androgen exposure.
• Thus, a “normal” total testosterone does not by itself exclude reduced androgen signaling. This distinction is particularly relevant at age 71, when SHBG tends to be higher and free/bioavailable testosterone may decline disproportionately.

Mechanistic interpretation

• Increased conversion of testosterone to DHT could reduce the testosterone substrate available for testosterone-mediated actions. Directionally, suppressing DHT formation with finasteride increased testosterone by 14%.
• However, DHT is also a potent androgen. Available evidence does not show that a high DHT:testosterone conversion ratio independently lowers free/bioavailable testosterone or produces worse androgen-sensitive outcomes.
• DHEA-S is an adrenal androgen reservoir that can contribute to downstream androgen synthesis. Its age-related decline occurs alongside lower free/bioavailable testosterone and rising SHBG, but this does not establish that low DHEA-S independently causes reduced testosterone signaling when total testosterone is normal.

Clinical implications

• When symptoms suggest androgen deficiency, repeat fasting morning total testosterone and obtain free testosterone by equilibrium dialysis or a validated calculation using total testosterone, SHBG, and albumin; analog free-testosterone immunoassays are unreliable.
• Diagnosis requires compatible symptoms or signs plus consistently low testosterone measures and evaluation for reversible contributors.

Bottom line

• High SHBG can convincingly reduce available testosterone signaling despite normal total testosterone; increased DHT conversion and lower DHEA-S are plausible modifiers, but their independent or combined clinical impact remains unproven.