The SHBG gene regulates the production of the protein, sex hormone-binding globulin (SHBG), by the liver.
The role of SHBG in the body is to bind the hormones T, DHT and estrogen to carry them in the blood to target tissue. While bound to SHBG, hormones are inactive.
As a result, high levels of SHBG can lead to low levels of free hormones, whereas, low levels of SHBG can result in high levels of free hormones. SHBG naturally increases with age and also with obesity, increased estrogen levels, hyperthyroidism, and liver disease.
Magnesium is helpful for releasing T from SHBG when SHBG levels are high and T is low. Hypothyroidism causes SHBG levels to decrease, increasing free hormone levels.
The A allele is the variant (−68 GA) is associated with high SHBG levels.