Sex Hormone Binding Globulin, or SHBG, is a protein produced predominantly by the liver. Its primary function is to bind to and transport sex hormones—mainly testosterone, dihydrotestosterone (DHT), and estradiol—through the bloodstream. This protein regulates the levels of these hormones throughout the body in both males and females.
The Function of SHBG in Hormone Regulation
SHBG acts as a primary transport vehicle for specific sex hormones circulating in the blood. In the bloodstream, hormones exist in two states: “bound” or “free.” When a hormone is bound to SHBG, it is inactive and cannot be used by the body’s tissues. Only “free” hormones, meaning those unattached to a protein, are biologically active and can enter cells to carry out their functions.
This system allows SHBG to regulate the availability of active hormones. It acts as a reservoir, controlling how much testosterone or estrogen is available for immediate use. By binding to these hormones, SHBG prevents excessive hormonal interaction with cellular receptors, which could otherwise lead to imbalances. The protein can also release hormones at specific tissues, influencing cellular functions where needed.
Factors That Influence SHBG Levels
A variety of factors can influence the concentration of SHBG in the bloodstream. The production of SHBG is affected by other hormones, with estrogens and thyroid hormones increasing its production, while androgens tend to suppress it.
Physiological factors such as age and sex naturally affect SHBG. Levels increase as people get older. After puberty, levels tend to decrease and then stabilize in adulthood, but as men continue to age, their SHBG levels often rise. Metabolic health is another determinant, as a higher body mass index (BMI) and obesity are associated with lower SHBG levels. This is linked to insulin resistance, a condition common in type 2 diabetes, which also tends to decrease SHBG.
Medical conditions, particularly those affecting the liver and thyroid, directly impact SHBG. Since the liver is the primary site of production, diseases like hepatitis and cirrhosis can alter its levels. An overactive thyroid (hyperthyroidism) increases SHBG, while an underactive thyroid (hypothyroidism) decreases it. Certain medications can also change SHBG levels; for example, oral contraceptives containing estrogen and some anticonvulsant drugs can increase SHBG.
Clinical Significance of High and Low SHBG
The level of SHBG in the blood has direct clinical importance because it determines the amount of biologically active hormones available to the body’s tissues. Having levels outside the normal range can lead to various symptoms and is associated with several health conditions.
High SHBG levels mean that more hormones are bound and inactive, reducing the amount of free hormone available to cells. In men, this can lead to symptoms of low testosterone, such as decreased libido or reduced muscle mass, even if their total testosterone measurement appears normal. For women, elevated SHBG can result in effects associated with low estrogen and may be linked to menstrual irregularities.
Conversely, low SHBG levels result in a higher proportion of free, active hormones circulating in the bloodstream. In women, low SHBG is frequently linked with Polycystic Ovary Syndrome (PCOS), a condition characterized by an excess of androgens that can lead to symptoms like acne and hirsutism (excess hair growth). In men, low SHBG means more available free testosterone and estrogen.
How SHBG Is Measured
SHBG is measured through a blood test that quantifies the concentration of the protein in a blood sample. A healthcare professional will draw a small amount of blood from a vein in the arm. This test is not part of a routine check-up but is ordered when symptoms suggest an imbalance of sex hormones, particularly testosterone.
Frequently, a total testosterone test is performed first. If the results of the total testosterone test do not align with the clinical symptoms, an SHBG test is ordered to provide a more complete picture. The results from the SHBG and total testosterone tests are often used together to calculate a “free androgen index” (FAI) or a calculated free testosterone level. This calculation gives a more accurate estimate of the amount of biologically active testosterone.
Reference ranges for what is considered a normal SHBG level can vary between laboratories and depend on factors like age and sex. Men have lower SHBG levels than women, and levels naturally increase with age in men. Therefore, the results must be interpreted by a healthcare provider in the context of an individual’s overall health.
Lifestyle and Dietary Approaches to Support Healthy SHBG
Lifestyle and dietary choices can help in maintaining healthy SHBG levels, primarily by influencing factors like body weight and insulin sensitivity. Making targeted adjustments to daily habits can support the body’s natural hormonal regulatory systems.
Dietary habits are particularly influential, and several strategies can help.
- Consume a diet high in dietary fiber from sources like fruits, vegetables, and legumes.
- Ensure adequate protein intake, as some research indicates that higher protein consumption can help modulate SHBG.
- Limit the intake of sugar and refined carbohydrates to help manage insulin levels, as high insulin can suppress SHBG production.
- Engage in regular physical activity, as exercise can improve insulin sensitivity and help with weight management.
- Maintain a healthy body weight, as excess body fat is known to lower SHBG.