Sex hormone binding globulin (SHBG) is a glycoprotein that binds to sex hormones, limiting their levels in the body. It is produced in the liver, brain, uterus, testicles, and placenta. SHBG levels are influenced by hormones such as insulin, growth hormone, estrogen, and thyroxine. Low levels are linked to diabetes, hypothyroidism, and polycystic ovary syndrome. Testing for SHBG levels can assess the levels of male sex hormones in the body.
A sex hormone binding globulin (SHBG) is what is known as a glycoprotein, a group of carbohydrate chains attached to polypeptide chains, that binds to human sex hormones. These binding proteins are most often found bound to the primary male sex hormone, testosterone, and the primary female sex hormone, estradiol, which is a type of estrogen. The major production of sex hormone-binding globulin occurs in the liver, but it can also be produced in the brain, uterus, testicles, and placenta during pregnancy. The function of this protein is to limit the levels of active unbound sex hormones in the body. Very high or very low levels of the protein in the body can indicate a variety of health conditions in both women and men.
Most of the sex hormones in the bloodstream are biochemically related to SHBG. Only a small fraction of sex hormones are “free” and able to enter cells where they bind to hormone receptors. Therefore, the availability of sex hormones in the human body is directly linked to the amount of SHBG produced by the body.
When produced by the testicles, SHBG is instead known as androgen-binding protein, a protein that specifically binds to androgens or male sex hormones. This protein is produced in the Sertoli cells, which are cells found in the seminiferous tubules that are responsible for feeding the developing sperm. High levels of androgen-binding protein in the testicles allow sperm cells to mature in a process called spermatogenesis.
Levels of sex hormone-binding globulin in the body are influenced by a number of different factors, with the protein decreasing or increasing in the presence of a variety of hormones. Insulin is a hormone that controls the body’s metabolism and also reduces the amount of sex hormone binding globulin in the body. High levels of androgen hormones also reduce levels of sex hormone binding globulin. The amounts of the binding proteins increase with high levels of growth hormone, estrogen and thyroxine, which is one of the primary hormones produced by the thyroid gland.
Many health conditions are indicated by increased or decreased levels of SHBG. For example, because pregnancy usually leads to the production of more female sex hormones, such as estrogen, it also leads to higher SHBG levels. Low levels of this binding protein are linked to diabetes, hypothyroidism or decreased production of thyroid hormones and polycystic ovary syndrome, a condition in which women produce too many male sex hormones, which is a leading cause of female infertility .
Levels of sex hormone binding globulin can be tested to assess the levels of male sex hormones in the body. The test is usually done on men who are deficient in male sex hormones and on women who have an overabundance of male sex hormones. Testing for SHBG levels is not a common test; usually, medical professionals will test your testosterone levels first. However, in cases where these tests are not conclusive, such as cases where testosterone levels appear normal in a woman who has a number of secondary male sex characteristics, SHBG levels will also be tested. Higher levels of the binding protein mean the body has less available “free” testosterone, while lower levels indicate elevated “free” testosterone.
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