Why is Sex Hormone-Binding Globulin (SHBG) Low? Causes, Symptoms, and What to Do

Low SHBG (sex hormone-binding globulin) means your liver is making less of the protein that carries testosterone and oestrogen in the blood, so a larger share of these hormones is "free" and active. The most common reasons are excess body weight and insulin resistance (including type 2 diabetes), followed by an underactive thyroid, steroid or androgen medicines, and polycystic ovary syndrome (PCOS) in women. Low SHBG is not a disease in itself, and it causes no symptoms of its own; it is a clue that is read together with your testosterone result. A high SHBG has the opposite effect and has its own set of causes, covered below.

What SHBG is and why it matters

SHBG is a protein made mainly in the liver. It binds testosterone, oestradiol and dihydrotestosterone (DHT) tightly and carries them in an inactive form [1]. Only the unbound (free) part of these hormones acts on the body's tissues. That is why SHBG matters: with the same total testosterone, a person with low SHBG has more free testosterone, and a person with high SHBG has less [2][3].

The test is usually ordered together with total testosterone, for example to investigate excess hair growth (hirsutism) or irregular periods in women, or low sex drive, erectile problems or infertility in men, especially when the testosterone result does not fit the symptoms [1][2]. In women, the two results are often combined into a free androgen index (FAI), an estimate of active testosterone; this calculation is not used for men [1].

Typical adult reference ranges (nmol/L)

SHBG is reported in nmol/L. As the table shows, ranges differ considerably between laboratories and assays, and they depend on age and sex. Pregnancy raises SHBG, so pregnant women have their own ranges [1][2]. Compare your result only with the range on your own report.

Causes of low SHBG

Common

• Excess body weight. Obesity is consistently linked with lower SHBG production [1][2][3].
• Insulin resistance and type 2 diabetes. High insulin levels switch off a liver signal (HNF-4α) that drives SHBG production, so the liver makes less [3]. Lower SHBG is also associated with metabolic syndrome and fatty liver [3].
• Polycystic ovary syndrome (PCOS) in women, a hormonal condition linked to insulin resistance [3].
• Medicines: glucocorticoid (steroid) medicines, androgens including anabolic steroids, and progestins [1][2].

Less common

• Underactive thyroid (hypothyroidism). Thyroid hormone normally increases SHBG production, so too little thyroid hormone lowers it [1][2][3].
• Cushing's syndrome, an excess of the body's own steroid hormone cortisol [2].
• Growth hormone excess [1].
• Inherited (familial) low SHBG, in which some people simply make less SHBG [1].

What a high SHBG means

A high SHBG binds more testosterone, so free testosterone can be low even when total testosterone looks normal [2]. Causes include [1][2]:

• pregnancy;
• oestrogen-containing medicines, such as some contraceptive pills and hormone replacement therapy;
• an overactive thyroid (hyperthyroidism);
• liver disease such as cirrhosis;
• older age;
• anorexia nervosa or very low body weight;
• less often, low androgen levels, growth hormone deficiency, a high prolactin level (hyperprolactinaemia) or active porphyria.

Symptoms

Low SHBG itself does not cause symptoms. What people notice comes from either the underlying cause (for example weight gain or an underactive thyroid) or from the higher share of free hormones. In women, low SHBG can mean that total testosterone looks normal while free testosterone is raised [3], which can show up as excess facial or body hair, acne or irregular periods [2]. In men, especially with excess weight, low SHBG often lowers total testosterone; when SHBG is outside its range, measuring free testosterone may give a more accurate picture [5].

What usually happens next

1. Reading SHBG with testosterone. Your clinician will look at total testosterone, and often free testosterone or, in women, the free androgen index [1][2].
2. Checking common causes: weight, blood sugar and diabetes tests, and thyroid tests such as TSH [2][3].
3. Pituitary hormones such as LH and FSH, depending on symptoms and the testosterone result [2].
4. A medicine review, including steroids, anabolic steroids, hormonal contraceptives and hormone therapy [1][2].
5. Repeating the test if needed. When testosterone is measured at the same time, a morning blood sample is usually advised. High-dose biotin supplements can interfere with the test, so mention them [1][2].

Frequently asked questions

What does low sex hormone-binding globulin mean? It means less of your testosterone and oestrogen is bound to its carrier protein, so more is free and active. It most often reflects excess weight, insulin resistance or type 2 diabetes, an underactive thyroid, steroid or androgen medicines, or PCOS [1][2][3].

What causes low SHBG? Obesity, high insulin levels and type 2 diabetes, PCOS, hypothyroidism, glucocorticoids, androgens and progestins, Cushing's syndrome, growth hormone excess, and inherited low SHBG [1][2][3].

Is low SHBG serious? Not in itself. It is a marker rather than a disease. Its importance lies in what it points to, most often insulin resistance and metabolic health, and in how it changes the reading of your testosterone result [3].

What does high SHBG mean? More testosterone is bound, so free testosterone may be low even if total testosterone is normal. Common reasons include pregnancy, oestrogen-containing medicines, an overactive thyroid, liver disease, older age and very low body weight [1][2].

Can diet or weight affect SHBG? Weight is one of the strongest factors linked with SHBG: obesity and insulin resistance are associated with lower levels [1][3]. Discuss with your doctor what is likely to matter in your case.

Do I need to fast for an SHBG test? Fasting is not needed for SHBG alone. If testosterone is measured at the same time, a morning sample is usually recommended [2].

When to see a doctor

A single low SHBG result is not urgent. It is worth discussing with your doctor soon if your result is outside the range together with an abnormal testosterone result, or if you have symptoms such as new excess hair growth, irregular or absent periods, reduced sex drive, erectile problems, difficulty conceiving, unexplained weight change, or tiredness that might suggest a thyroid problem. If you take hormones, steroids or anabolic steroids, mention them, as they change the result.

Sources

1. Sex Hormone Binding Globulin (SHBG) — Gloucestershire Hospitals NHS Foundation Trust Pathology
2. Sex Hormone Binding Globulin (SHBG) Test — Testing.com
3. Sex Hormone-Binding Globulin (SHBG) as an Early Biomarker and Therapeutic Target in Polycystic Ovary Syndrome — International Journal of Molecular Sciences, via PubMed Central (NIH)
4. Sex Hormone Binding Globulin — Royal Cornwall Hospitals NHS Trust Pathology
5. Lowered testosterone in male obesity: mechanisms, morbidity and management — Asian Journal of Andrology, via PubMed Central (NIH)

Decisions about your health should always be made in consultation with your healthcare provider.