SHBG: what this protein is and how it binds testosterone

SHBG is sex hormone-binding globulin: a blood protein that binds sex hormones, primarily testosterone. A significant portion of testosterone in the blood is bound to SHBG and albumin.

What sex hormone-binding globulin is

SHBG (sex hormone-binding globulin) is a protein that binds testosterone in the blood. All testosterone exists in two forms: bound and free, and most circulates bound — to SHBG and albumin. The body uses the free fraction more easily; bound hormone is used up more slowly, and this protects against excessive use.

What SHBG does and how it is linked to testosterone

The protein keeps part of testosterone bound: the more is bound, the less free testosterone remains available to tissues. Tests for free and bioavailable testosterone are done less often, but sometimes they help find the cause of an abnormality.

When testosterone is checked

Indications for which a doctor checks testosterone:

• in men — signs of low testosterone: decreased libido, erectile dysfunction, infertility, breast enlargement, loss of muscle mass, bone thinning, or anemia without a clear cause;
• in women — signs of high testosterone: excessive growth of dark coarse hair, male-pattern hair loss, voice change, irregular or absent menstruation, acne;
• monitoring: a previously abnormal testosterone result, taking medications that affect its level, hormone therapy;
• in children and adolescents — puberty that is too early or too late.

Blood for a testosterone test is usually taken in the morning, between 7 and 10 a.m. A low result is often rechecked with a second test.

What testosterone level depends on

Total testosterone decreases in chronic diseases, including liver and kidney disease, pituitary disorders, reduced thyroid function, testicular disease, excess body fat, sleep-disordered breathing, chronic stress from excessive exercise, and alcohol dependence. In women, an extremely low level occurs in disorders of the adrenal glands, pituitary, and ovaries.

Total testosterone increases in tissue resistance to male hormones, ovarian tumors, testicular cancer, congenital adrenal hyperplasia, and taking medications and supplements that raise testosterone. In women, high testosterone may be associated with polycystic ovary syndrome.

Testosterone declines naturally with age — this is not a disease.

Testosterone in men: what to pay attention to

In men, most testosterone is produced by the testicles, and most of the hormone in the blood is bound to proteins. For complaints of low libido, erectile dysfunction, or infertility, a doctor usually starts with a total testosterone test; free and bioavailable testosterone are measured less often and in selected cases.

Approximate reference intervals for total testosterone in men: ages 17–50 — 249–836 ng/dL (8.6–29.0 nmol/L), over 50 — 193–740 ng/dL (6.7–25.7 nmol/L). Laboratories use different units, so the intervals on the report form may differ.

When to see a doctor

An abnormal result on a report form is not a diagnosis. It makes sense to see a doctor if the number is accompanied by complaints: decreased libido, erection problems, infertility, hair loss, irregular cycle, voice change. The doctor considers the complaints, examination, and results of several tests together. Do not start or stop medications on your own: some drugs and supplements themselves change testosterone levels.

A change in SHBG is not in itself a reason to call an ambulance; 103/112 — for life-threatening conditions: severe injury, signs of stroke or heart attack.

What to remember: hormone numbers are discussed with a doctor, not based on a single indicator from a report form.

This material is for reference: it explains the meaning of the test but does not replace a doctor's consultation.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer