How does SHBG affect testosterone?

Sex Hormone Binding Globulin (SHBG) significantly affects testosterone availability in the body. Here's how this interaction works:

SHBG and Testosterone

1. Binding Role: SHBG binds to testosterone and other sex hormones, which limits the amount of free testosterone available for the body to use. Only free testosterone can enter cells and activate androgen receptors, thus exerting its biological effects.

2. Free Testosterone Levels: High levels of SHBG can lead to lower percentages of free testosterone. For example, if a man has a total testosterone level of 900 ng/dL but a free testosterone level of just 8 ng/dL, a high SHBG level could be responsible, as mentioned by Peter Attia. A healthy free testosterone level for men is typically about 2% of total testosterone.

3. Influencing Factors: Several factors can affect SHBG levels, including:

   • Hormones: Higher estradiol levels increase SHBG, leading to reduced free testosterone. Conversely, testosterone and insulin can lower SHBG levels.
   • Diet and Metabolism: Low carbohydrate diets can increase SHBG due to reduced insulin levels, further lowering free testosterone, as discussed by Andrew Huberman and Peter Attia.

Practical Implications

• Men and women with high SHBG may experience symptoms of low testosterone due to a lack of available free testosterone, even if total testosterone levels appear normal.
• Addressing factors that increase SHBG—like insulin levels and estradiol—can potentially help improve free testosterone levels.

This dynamics highlights the complexity of hormonal interactions and the importance of measuring both total testosterone and SHBG to understand hormonal health fully.