A sex hormone binding globulin (SHBG) is what is known as a glycoprotein, a group of carbohydrate chains attached to polypeptide chains, which 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. Major production of sex hormone binding globulin occurs in the liver, but it may also be produced in the brain, uterus, testes, and in the 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 sex hormones within the bloodstream are biochemically bound to SHBG. Only a tiny portion 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 the body produces.
When produced by the testes, SHBG is instead known as an 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 nurturing developing sperm. High levels of androgen-binding protein in the testes allow sperm cells to mature in a process called spermatogenesis.
Sex hormone binding globulin levels 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 decreases the amount of sex hormone binding globulins in the body. High levels of androgenic hormones also decrease sex hormone binding globulin levels. 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 instance, since 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 a decreased production of thyroid hormones, and polycystic ovary syndrome, a condition in which women produce too many male sex hormones, which is one of the leading causes of female infertility.
Levels of sex hormone binding globulin may be tested to assess levels of male sex hormones in the body. The test is usually performed on men who have a male sex hormone deficiency and on women who have an overabundance of male sex hormones. Testing for levels of SHBG is not a common test; usually, medical professionals will test for testosterone levels first. However, in cases where these tests are inconclusive, such as instances where testosterone levels appear normal in a woman who presents with a number of secondary male sex characteristics, SHBG levels will be tested as well. Higher levels of the binding protein mean that the body has less "free" testosterone available, while lower levels indicate elevated "free" testosterone.