Sex Hormone Binding Globulin (SHBG)
Definition - What does Sex Hormone Binding Globulin (SHBG) mean?
Sex hormone-binding globulin (SHBG) is a protein hormone manufactured by the liver and released into the bloodstream that serves to bind the major sex hormones in both men and women.
In men, it binds tightly to testosterone and dihydrotestosterone (DHT); whereas, in women, it binds estrogen. SHBG carries these three hormones in the blood in a bound state. As long as these sex hormones are bound to SHBG, they are inactive and incapable of penetrating the cell membranes to exert their effects. The unbound (not tied to SHBG) or "free" forms of these sex hormones that are the active ones, equal to about 1-2%. The sum of the bound and free testosterone is known as total testosterone. Fluctuations in SHBG levels have an impact on the amount of free hormones available to the body tissues.
FertilitySmarts explains Sex Hormone Binding Globulin (SHBG)
A variety of conditions can cause the SHBG levels in the blood to rise or drop and vice versa. When SHBG levels go up, more SHBG is available to bind more testosterone; hence, the levels of free testosterone fall. In men, a drop in free testosterone can promote enlargement of the male breast tissues, a condition called gynecomastia, such as in liver cirrhosis or reduced functional activity of gonads (termed as hypogonadism).
Women also produce small amounts of testosterone in the ovaries and adrenal glands. This testosterone in females also exists in both the bound and free forms. Normally, SHBG levels should remain on the upper side in females. A drop in SHBG levels liberates more "free" testosterone in the blood. Therefore, in women, features of high blood testosterone or hyperandrogenism (like hirsutism or excess facial and body hair) are observed with a fall in SHBG levels. Conditions/factors that decrease SHBG levels include:
- PCOS (Raised insulin levels stimulate the cells of the ovary to release more androgens and at the same time, decreased the synthesis of SHBG by the liver; obesity is another factor that contributes to androgen excess in females)
- Low thyroid hormone levels (hypothyroidism)
- Cushing's syndrome
- Obesity/high BMI
- Androgen (anabolic steroid) use
Reduced SHBG levels can be considered as predictors for recurrent miscarriages. Low SHBG levels aggravate insulin resistance and increase the risk of diabetes by 11-fold.
Conditions that raise SHBG levels in women include:
- Pregnancy (due to raised estrogens)
- Estrogen-containing hormonal pills (SHBG can remain on the higher side as long as 1 year after stopping)
- Flaxseed use
- High thyroid hormone levels (hyperthyroidism: raises estradiol levels)
- Resistance training
Since high SHBG levels lower the free testosterone, they interfere with the fertility and erectile performance in males. Measurement of SHBG and total testosterone concentrations can help determine the cause of infertility or erectile dysfunction. Likewise, SHBG and total testosterone levels can detect excess androgen production and/or reduced SHBG levels and can be helpful in the assessment of women being suspected of having PCOS. PCOS is the most common cause of anovulatory infertility and recurrent miscarriages. Thus, reduced SHBG levels in women should be evaluated vigilantly.