SHBG, or sex hormone-binding globulin, is as the name suggests, a protein that binds to sex hormones in the blood. It is produced in the liver and is the most important carrier protein for the sex hormones testosterone, dihydrotestosterone and estradiol. SHBGs’ function is to transport the sex hormones to their target tissues, increasing the solubility and protecting them from degradation along the way.
About 97-98% of all sex hormones are bound to SHBG in blood. When bound, the sex hormones are unable to exert their biological effects. This means that only 2-3% of the circulating sex hormones are able to affect its target organs.
As a result, SHBG regulates the bioavailability of sex hormones.
The unbound amount of circulating testosterone is often referred to as “free” or “bioactive” testosterone.
Normal levels of SHBG indicate that your circulating sex hormones are in balance. Normal levels also suggest that your liver function is normal and that your thyroid hormone levels are in balance.
Measuring SHBG is mainly of interest when evaluating the balance of sex hormones in the body. For example when wanting to estimate the amount of free, bioactive testosterone. This is of particular interest when evaluating possible testosterone deficiency, since SHBG can help determine whether low testosterone levels are due to a problem with testosterone production or an increase in the SHBG binding to testosterone.
Testosterone bound to SHBG is not considered active and will therefore not exercise any effects. Another common example of SHBG use is when monitoring and evaluating testosterone therapy, since SHBG can help assess the effectiveness of the treatment.
Low levels of SHBG mean that there is a lower production of SHBG. This is often due to a decreased need for SHBG (as a result of decreased amounts of circulating sex hormones) seen with increasing age. Another reason for lower SHBG levels is when the levels are suppressed due to other underlying mechanisms that cause SHBG to decrease. Some of these are:
- Obesity and high levels of insulin (in type 2 diabetes)
- Hypothyroidism, since thyroid hormones increase the production of SHBG
- Taking anabolic steroids.
The implication of low levels of SHBG (if not due to lower levels of testosterone), is that less testosterone is bound. This results in an excess in circulating free, bioactive testosterone, with the risk of causing symptoms and deviating cholesterol levels.
High levels of SHBG can be a result of several factors, including:
- Dehydration, causing the test to show a false increased level due to concentrated blood
- Hyperthyroidism
- Liver disease, affecting the production of SHBG
Showing high levels of SHBG, could, in turn, imply a low testosterone levels, since more circulating SHBG means more bound (inactive) testosterone.