Hormones like testosterone and estradiol influence everything from metabolism to reproductive health. These powerful chemical messengers do not travel freely in the bloodstream. Instead, they require specific transport proteins to circulate and reach target tissues efficiently. This system ensures the body maintains a delicate equilibrium, making hormones available in the right amount at the right time.
Understanding SHBG: The Hormone Carrier
Sex Hormone-Binding Globulin (SHBG) is a glycoprotein produced predominantly by the liver, acting as the primary carrier for sex hormones in the plasma. Its main function is to bind tightly to testosterone, dihydrotestosterone (DHT), and estradiol, transporting them in an inactive state. This binding action limits the amount of “free” or unbound hormone that is available to interact with cells and exert a biological effect.
SHBG controls how much of the hormone is biologically active at any given moment. The majority of circulating testosterone, for instance, is bound to SHBG, making it unavailable for tissue use. Only the small fraction of hormone that is unbound or loosely bound to other proteins, such as albumin, can easily enter cells and initiate a response. The concentration of SHBG determines the overall bioavailability of these sex steroids.
Defining “Good” SHBG Levels
A balanced SHBG level depends significantly on sex and age, as a single range does not apply to all adults. For adult men under 50, the reference range for SHBG is typically between 15 and 65 nanomoles per liter (nmol/L). This range tends to increase slightly in men over 50, often rising to between 14 and 78 nmol/L, reflecting the natural decline in testosterone with age.
For non-pregnant adult women, the normal range is considerably wider, typically between 18 and 145 nmol/L. Pre-menopausal women usually have higher SHBG levels than men, which helps regulate hormone balance throughout the menstrual cycle. A “good” level is ultimately one that supports appropriate levels of free hormones, preventing symptoms of deficiency or excess.
Implications of Abnormal SHBG Levels
When SHBG levels stray too high or too low, they disrupt the balance of free hormones, leading to noticeable symptoms. High SHBG binds too much sex hormone, creating a functional deficiency of free testosterone or estradiol, even if total hormone levels appear normal. This can result in symptoms associated with low hormone function, such as diminished sex drive, fatigue, and mood disturbances. Men may experience erectile dysfunction, while women might notice decreased muscle mass or vaginal dryness.
Conversely, low SHBG means that an excessive amount of free hormone is available, which can overstimulate body tissues. In women, this often manifests as symptoms of high androgen exposure, including acne, hirsutism (excessive body and facial hair growth), and irregular menstrual cycles, which are commonly seen in conditions like Polycystic Ovary Syndrome (PCOS). Low SHBG in both sexes is also a strong indicator of underlying metabolic problems, such as insulin resistance.
Factors Influencing SHBG Concentration
The concentration of SHBG is sensitive to metabolic, endocrine, and lifestyle factors. Conditions that promote high estrogen levels, such as pregnancy or the use of certain oral contraceptives, typically cause SHBG production to increase in the liver. An overactive thyroid gland (hyperthyroidism) also stimulates a rise in circulating SHBG levels. Men often see a natural, gradual increase in SHBG as they age.
Numerous factors suppress the liver’s production of SHBG, leading to lower concentrations in the blood. High levels of insulin, which occur with insulin resistance, Type 2 Diabetes, and obesity, are the strongest inhibitors of SHBG synthesis. An underactive thyroid (hypothyroidism) and high levels of androgens also decrease SHBG, often seen in women with PCOS or in individuals using anabolic steroids. High visceral fat accumulation is also correlated with diminished SHBG levels.