Can elevated SHBG produced by the liver lower free testosterone even if total testosterone is normal or high?

The liver is the primary organ responsible for the synthesis and secretion of sex hormone-binding globulin (SHBG). This glycoprotein plays a critical role in regulating the biological activity of androgens by acting as their primary transport vehicle in the bloodstream.

Clinical and physiological evidence

Clinical studies, including the European Male Ageing Study, demonstrate that SHBG levels typically rise with age, often beginning around age 35. This increase has a profound effect on testosterone partitioning:

• Total vs. Free Testosterone: Total testosterone measures the sum of all testosterone in the blood—both the fraction bound to proteins and the "free" fraction. Because SHBG binds testosterone with high affinity, an increase in SHBG concentration sequestered a larger percentage of the hormone.
• Occult Deficiency: Research shows that in aging men, total testosterone levels can remain stable (e.g., around 450 ng/dL) or even appear high, while free testosterone—the biologically active portion—drops significantly as SHBG rises.
• Diagnostic Impact: In a study of Chinese cohorts, researchers found that total testosterone levels did not reliably correlate with symptoms of erectile dysfunction, whereas higher SHBG levels (which lower free testosterone) were strongly associated with clinical symptoms.

Mechanistic explanations

The relationship between SHBG and testosterone is governed by the "Free Hormone Hypothesis," which states that only the unbound fraction of a hormone is free to diffuse into tissues and exert biological effects.

• Hepatic Synthesis: SHBG is produced by hepatocytes in the liver and regulated by transcription factors like hepatocyte nuclear factor 4-alpha (HNF-4α). Its production is highly sensitive to metabolic states, such as insulin levels and liver fat content.
• High-Affinity Binding: SHBG is a homodimer with two specific binding pockets located in its N-terminal domain. It binds testosterone with a high affinity constant (Kd of 1.1 to 2.0 nM).
• Equilibrium Shifting: Biologically, about 40–65% of testosterone is bound to SHBG, and only 1–3% remains free. When liver production of SHBG increases, the chemical equilibrium shifts, forcing more free testosterone into a bound state. This reduces the concentration of the bioavailable hormone even if the total amount of testosterone in the system remains unchanged or elevated.

Bottom line

The claim is strongly supported by clinical and molecular evidence. Elevated SHBG levels, primarily driven by hepatic production, can lead to functional androgen deficiency by lowering free testosterone levels, even when total testosterone measurements appear normal or high.