Sex Hormone Binding Globulin (SHBG) Blood Test

What Does This Test Measure?

SHBG is a liver-produced protein that binds testosterone and estradiol in the blood. Only unbound (free) hormone is biologically active. SHBG levels determine how much total testosterone is available as free hormone; high SHBG reduces free testosterone; low SHBG increases it.

Why Your Doctor Ordered This Test

Doctors order an SHBG blood test to evaluate unexplained androgen-related symptoms in both men and women. In men, the test is commonly part of a workup for low libido, erectile dysfunction, fatigue, or infertility, where it helps determine whether a normal total testosterone could be accompanied by low free testosterone due to elevated SHBG. In women, SHBG is checked when there are signs of androgen excess (such as hirsutism, severe acne, or irregular menstrual cycles) suggesting polycystic ovary syndrome (PCOS). Because SHBG binds testosterone and estradiol tightly, its level directly affects the amount of hormone available to tissues. A doctor wants to know if a patient's symptoms can be explained by altered free hormone concentrations rather than a primary gonadal disorder. In both sexes, SHBG is used together with total testosterone (and sometimes albumin) to calculate free testosterone or the free androgen index, giving a more accurate picture of bioactive hormone status and guiding therapy.

Additionally, SHBG is a surrogate marker for insulin resistance and metabolic syndrome. Low SHBG levels are strongly associated with obesity, prediabetes, and type 2 diabetes because elevated insulin suppresses hepatic SHBG production. A physician may order SHBG not only in a hormone context but also during cardiometabolic risk assessment, particularly in women with PCOS who are at higher risk for insulin resistance. The test is also used to monitor the effects of hormone therapy: estrogen therapy (including oral contraceptives and menopausal hormone therapy) raises SHBG, potentially reducing free testosterone in women who report low libido; conversely, androgen therapy lowers SHBG. In transgender hormone therapy, SHBG helps guide dosage adjustments to achieve target free hormone levels. By measuring SHBG, a doctor can distinguish whether symptoms are due to inadequate or excessive hormone dosing versus the body's binding protein response.

What High Sex Hormone Binding Globulin Means

High SHBG: reduces free testosterone. Causes: aging (SHBG rises with age, reducing free testosterone), liver disease (increased hepatic production), hyperthyroidism, estrogen therapy or pregnancy (estrogen stimulates SHBG), HIV, and prolonged fasting/anorexia. High SHBG can cause low free testosterone even when total testosterone is normal.

What Low Sex Hormone Binding Globulin Means

Low SHBG: increases free testosterone. Causes: obesity and insulin resistance (insulin suppresses SHBG production; the most common cause), hypothyroidism, androgen use, Cushing syndrome, and nephrotic syndrome. Low SHBG in the setting of insulin resistance is part of the metabolic syndrome cluster and tends to normalize with weight loss.

How to Prepare and What Affects the Result

SHBG testing typically requires an overnight fast of 8 to 12 hours, particularly if the blood draw also includes glucose, insulin, or a lipid panel that mandates fasting. Although SHBG itself is relatively stable after meals, the strong relationship between SHBG and insulin means that recent food intake can subtly shift insulin and thus SHBG levels; therefore, a fasting sample ensures the result reflects the basal hepatic production environment. Blood should be drawn in the morning, ideally between 8 and 10 AM, because SHBG can exhibit a minor diurnal variation, although it is much less pronounced than testosterone's.

Certain medications and supplements can dramatically alter SHBG levels and must be noted at the time of testing. Oral contraceptives, estrogen replacement, and tamoxifen can raise SHBG 2- to 4-fold, while androgens, anabolic steroids, glucocorticoids, and danazol lower it. Insulin-sensitizing drugs such as metformin may increase SHBG indirectly by reducing hyperinsulinemia over weeks. Over-the-counter supplements like DHEA, androstenedione, or high-dose biotin can interfere with the immunoassay, leading to falsely elevated or lowered results. The sample is collected in a serum separator tube (SST) and should be centrifuged within two hours; serum is stable for up to seven days at 2–8°C and for longer if frozen. Hemolyzed or repeatedly thawed samples should be rejected because they can degrade SHBG and introduce error.

Common Misinterpretations

One of the most frequent misinterpretations is evaluating SHBG in isolation without concurrent total testosterone and a clinical picture. A normal total testosterone with markedly elevated SHBG can produce low free testosterone and symptoms of androgen deficiency, leading a clinician to miss the fact that the pituitary-gonadal axis is intact. Conversely, low SHBG in obesity can mask true hypogonadism because the low SHBG keeps free testosterone in the normal range despite low total testosterone. To correctly assess androgen status, SHBG must be combined with total testosterone (and often albumin) to calculate free testosterone by equilibrium dialysis or the Vermeulen equation. Relying solely on total testosterone, especially when SHBG is at the extremes, leads to misdiagnosis and inappropriate hormone therapy.

Another common mistake is assuming that a low SHBG always reflects insulin resistance or metabolic syndrome without considering thyroid status. Hypothyroidism reduces SHBG, and hyperthyroidism elevates it; therefore, SHBG can be a thyroid function mimic. Clinicians sometimes chase insulin resistance with lifestyle changes while overlooking an underlying thyroid disorder that could be driving the SHBG abnormality. Additionally, SHBG levels can be artifactually altered by assay interference from biotin supplements, causing a high or low result that does not reflect true protein concentration. Doctors may then order extensive and unnecessary workups for hyperestrogenism or hypogonadism when simply stopping biotin for 48 hours would normalize the result. Finally, interpreting SHBG in pregnancy or while on hormonal contraceptives without recognizing the expected 5- to 10-fold increase can lead to needless alarm about hyperestrogenism or liver disease.

Frequently Asked Questions

What causes high SHBG levels in men?

High SHBG in men can result from aging (SHBG rises by about 1% per year after age 50), liver cirrhosis because the liver is the main producer, hyperthyroidism where thyroid hormone stimulates production, and estrogen excess, whether from obesity-related aromatization, medications, or rare estrogen-secreting tumors. Prolonged fasting, malnutrition, and HIV infection also elevate SHBG. When SHBG is high, total testosterone may appear normal but free testosterone is low, causing symptoms of low libido, fatigue, and erectile dysfunction.

What causes low SHBG levels in women?

Low SHBG in women is most commonly driven by insulin resistance and obesity; elevated insulin directly suppresses hepatic SHBG synthesis. Polycystic ovary syndrome (PCOS) frequently presents with low SHBG, contributing to high free testosterone and symptoms like hirsutism and acne. Hypothyroidism, Cushing syndrome, and the use of androgens or progestin-only contraceptives also reduce SHBG. Because low SHBG allows more testosterone to be unbound, it can exacerbate androgen excess even when total testosterone is only mildly elevated. Weight loss and insulin-sensitizing treatments often raise SHBG back into the normal range.

How does SHBG affect free testosterone?

SHBG is the primary binding protein for testosterone in the blood. About 60–70% of circulating testosterone is tightly bound to SHBG and biologically unavailable, while 30–40% is loosely bound to albumin, and only 1–2% is free. Because SHBG-bound testosterone cannot enter cells, high SHBG reduces free testosterone even if total testosterone is normal; conversely, low SHBG increases free testosterone. This is why doctors calculate free testosterone or the free androgen index when SHBG levels are abnormal.

Can SHBG be lowered naturally?

Yes, strategies to lower SHBG typically target the underlying cause. For elevated SHBG due to insulin resistance or obesity, weight loss and a diet low in refined carbohydrates can reduce insulin levels, which in turn suppresses SHBG production. Adequate protein and zinc intake may also modestly lower SHBG. However, if high SHBG is caused by hyperthyroidism, liver disease, or estrogen therapy, addressing the specific condition or adjusting medications is necessary. Regular exercise improves insulin sensitivity and can contribute to SHBG normalization.

What is a normal SHBG level for a woman?

Adult women typically have SHBG levels between 18 and 144 nmol/L, but this range is assay-dependent and varies with age and hormonal status. Premenopausal women usually fall in the middle of this range, while postmenopausal women may have lower SHBG due to declining estrogen. Conditions like pregnancy or use of estrogen-containing contraceptives can push SHBG above 200 nmol/L. Because the normal range is broad, a result must be interpreted together with total testosterone and clinical symptoms.

Related Markers: Read Together for Full Context

BloodWorker reads lab values together, not in isolation. These related markers provide essential context for interpreting your Sex Hormone Binding Globulin result: