Sex Hormone-Binding Globulin (SHBG) is a protein primarily manufactured by the liver that circulates throughout the bloodstream. Its role is to bind to and transport sex hormones, specifically testosterone, estrogen, and dihydrotestosterone (DHT). This binding action is a regulatory mechanism, ensuring that these potent hormones are delivered throughout the body in a controlled manner. When SHBG levels become elevated, it sequesters a larger portion of these hormones, significantly reducing the amount available for the body’s cells to use. High SHBG creates a state of hormone deficiency at the tissue level, even if total hormone concentration appears normal in a blood test.
The Critical Difference Between Total and Free Hormones
Hormones circulating in the blood exist in different forms, which is why testing only for “Total Hormone” levels can be misleading. Total hormone refers to the sum of all hormone molecules present, including those tightly bound to SHBG, those loosely bound to albumin, and the small unbound fraction. When a sex hormone molecule is firmly attached to SHBG, it is rendered biologically inactive, meaning it cannot interact with cell receptors to exert its effects.
The only fraction capable of immediately entering cells and activating biological processes is the “Free Hormone.” The free hormone, along with the hormone loosely bound to albumin, constitutes what is often referred to as the bioavailable hormone. SHBG levels are the primary determinant of this free fraction; if SHBG is high, it locks up more of the hormone, leaving less free for the body to use. A high SHBG diagnosis often necessitates measuring free or bioavailable hormone levels alongside the total level to accurately assess the functional hormone status.
Primary Factors That Elevate SHBG
Several physiological and external factors can signal the liver to increase its production of SHBG, leading to elevated levels. One common medical cause is hyperthyroidism, where an overactive thyroid gland produces excess thyroid hormones. Thyroid hormones stimulate the liver’s production machinery, specifically promoting the activity of a transcription factor called HNF-4α, which directly results in higher circulating SHBG.
Estrogen also acts as a stimulator of SHBG synthesis in the liver. High estrogen states or the use of estrogen-containing medications, such as oral contraceptives, are frequent causes of elevation. Similarly, the liver is the main production site for SHBG, so conditions like liver disease or cirrhosis can disrupt its normal regulatory processes, sometimes leading to an increase in the protein.
Extreme or rapid weight loss and states of chronic undernutrition, such as anorexia nervosa, are strongly associated with high SHBG. The mechanism here is linked to a state of caloric restriction and altered metabolic signaling within the liver. Furthermore, as individuals age, particularly beyond the middle years, there is a natural, gradual increase in SHBG levels that contributes to the decline in free hormone availability observed in older populations.
Health Consequences of Reduced Bioavailable Hormones
The reduction of biologically active sex hormones due to high SHBG results in a range of symptoms reflecting hormone deficiency, regardless of sex. A common complaint is a decreased libido and general sexual dysfunction because both free testosterone and estrogen are important for sexual desire and function. This shortage of active hormones can also contribute to persistent fatigue and a noticeable reduction in overall energy levels.
For men, low free testosterone makes it difficult to maintain or build muscle mass, often leading to changes in body composition. Women may experience menstrual irregularities, including skipped or absent periods, as estrogen availability is compromised. Over time, a sustained deficiency in bioavailable sex hormones, particularly estrogen and testosterone, can negatively impact skeletal health, increasing the risk of decreased bone density and osteoporosis.
Lifestyle and Dietary Strategies for Reduction
For individuals with high SHBG, the first step involves consulting a physician to identify and treat any underlying medical causes, such as hyperthyroidism. Once medical conditions are addressed, specific dietary and lifestyle adjustments can help moderate SHBG production. Avoiding overly restrictive, very low-calorie diets is important, as chronic caloric restriction is a known factor that increases SHBG synthesis.
Dietary Adjustments
Dietary changes can support a favorable hormonal balance and potentially reduce SHBG levels.
- Increasing the intake of high-quality protein has been shown in some studies to correlate with lower SHBG levels, suggesting that adequate protein consumption supports a favorable hormonal balance.
- Incorporating micronutrients like zinc and magnesium through diet or supplementation may also be beneficial, as these minerals play roles in hormone metabolism and have been investigated for their potential to lower SHBG.
- Eating cruciferous vegetables, such as broccoli and cauliflower, can support the liver in metabolizing excess estrogen, which may indirectly help reduce the estrogen-driven stimulation of SHBG production.