Sex Hormone Binding Globulin (SHBG)
A blood sample drawn from a vein in your arm
Sex hormone binding globulin (SHBG) is a protein that is produced by the liver and binds tightly to the hormones testosterone, dihydrotestosterone (DHT), and estradiol (an estrogen). In this bound state, SHBG transports these hormones in the blood as biologically inactive forms. This test measures the level of SHBG in the blood and is most often used to help evaluate for testosterone deficiency or excess.
In men, about 45% to 65% of testosterone in blood is normally bound to SHBG, with the remainder weakly and reversibly bound to albumin (the main protein in the blood). Only about 2% to 3% of testosterone is immediately available to the tissues as free testosterone, but testosterone that is weakly bound to albumin is also bioavailable and can be readily taken up by the body's tissues.
A slightly increased amount of testosterone (66% to 78%) is bound to SHBG in the blood in women, compared to men. In women, SHBG plays an integral role in regulating the levels of bioavailable male sex hormones (androgens) and estrogens circulating throughout the body. SHBG has a higher affinity for the androgens testosterone and DHT and so, in the setting of low SHBG, women may have signs and symptoms related to androgen excess.
A total testosterone test does not distinguish between bound and unbound testosterone but determines the overall quantity of testosterone. In many cases, this is sufficient to evaluate excess or deficient testosterone production. However, if a person's SHBG level is not normal, then the total testosterone may not be an accurate representation of the amount of testosterone that is available to the person's tissues. An SHBG test may be performed when a person's signs and symptoms do not correlate with the results of a total testosterone test.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
The sex hormone binding globulin (SHBG) test may be used to help evaluate men for low testosterone and women for excess testosterone production. It may be ordered in conjunction with other tests to evaluate the status of a person's sex hormones.
SHBG is a protein that binds tightly to the hormones testosterone, dihydrotestosterone (DHT), and estradiol (an estrogen). In this bound state, SHBG transports these hormones in the blood as biologically inactive forms. Changes in SHBG levels can affect the amount of hormone that is available to be used by the body's tissues.
SHBG in men
SHBG and total testosterone levels may be ordered for an adult male to help determine the cause of infertility, a decreased sex drive, or erectile dysfunction. Measurement of SHBG in addition to testosterone is especially helpful when total testosterone results are inconsistent with clinical signs.
Measurement of total testosterone in the blood does not distinguish between bound and unbound (bioavailable) testosterone but, as the name implies, determines the overall quantity of testosterone in the blood. In many cases, this is sufficient to evaluate excessive or deficient testosterone production. However, if a person's SHBG level is not normal, then the total testosterone may not be an accurate representation of the amount of testosterone that is available to the body's tissues.
Measurement of SHBG helps health practitioners assess bioavailable testosterone with a simple total testosterone measurement. Increased SHBG in men may be associated with symptoms of low testosterone levels (hypogonadism) because less testosterone is available to the body's tissues.
The Endocrine Society professional guidelines recommend measuring total testosterone in the initial screen for testosterone deficiency. If abnormal, the test is repeated on another day. If repeat results are low-normal and/or if SHBG is abnormal, they recommend one of the following:
- Measure bioavailable testosterone (using ammonium sulfate precipitation or SHBG)
- Calculate free testosterone from total testosterone and SHBG
- Measure free testosterone (using a method called equilibrium dialysis)
SHBG in women
In women, small amounts of testosterone are produced by the ovaries and adrenal glands. Even slight increases in testosterone production can disrupt the balance of hormones and cause symptoms such as irregular or missed menstrual periods, infertility, acne, and excess facial and body hair (hirsutism). These signs and symptoms and others are often seen with polycystic ovary syndrome (PCOS), a condition characterized by an excess production of male sex hormones (androgens). SHBG and testosterone testing may be useful in helping to detect and evaluate excess testosterone production and/or decreased SHBG concentrations and in evaluating women suspected of having PCOS.
When is it ordered?
Currently, the SHBG test is not performed frequently or routinely. In many cases, health practitioners feel that the total testosterone, and perhaps free testosterone (as measured by a method called equilibrium dialysis), provides sufficient information. SHBG is ordered primarily when the total testosterone results do not seem to be consistent with clinical signs and symptoms, such as infertility, decreased sex drive, and erectile dysfunction in men or infertility, irregular menstrual periods, and excess facial and body hair in women.
What does the test result mean?
A high SHBG level means that it is likely that less free testosterone is available to the tissues than is indicated by the total testosterone test. A low SHBG level means that more of the total testosterone is bioavailable and not bound to SHBG. This information may be important in the overall evaluation of an individual with signs and symptoms of a condition related to excess or deficient sex hormone production.
Increased SHBG levels may be seen in:
- Liver disease
- Eating disorders (anorexia nervosa)
- Corticosteroids or estrogen use (hormone replacement therapy and oral contraceptives)
- Decreased sex hormone production (hypogonadism)
Decreases in SHBG are seen with:
Is there anything else I should know?
SHBG concentrations are normally high in children of both sexes. After puberty, SHBG levels decrease more rapidly in males than in females. Levels are normally stable in adults and then begin to increase in the elderly male at the same time that total testosterone levels begin to decrease. In postmenopausal women, SHBG, testosterone, and estrogen concentrations decrease as hormone production by the ovaries tapers off.
Are there other uses for the SHBG test?
Not yet, but researchers are evaluating conditions in which there are changes in SHBG levels to see if there may be additional clinical uses for the test. Recent studies have identified that there may be a link between SHBG levels and insulin resistance in type 2 diabetes.
What is a Free Androgen Index?
The Free Androgen Index (FAI) is a calculation that provides information on the amount of testosterone that is not bound to SHBG and is available for the body to use (bioavailable). The results of a total testosterone test and the SHBG are used for the calculation: FAI = Total Testosterone / SHBG. The FAI may be useful in cases where tests for free testosterone or bioavailable testosterone are not available. It offers a reasonable substitute for estimating the concentration of bioavailable testosterone, when SHBG is normal.
On This Site
Elsewhere On The Web
The Hormone Health Network: Polycystic Ovary Syndrome (PCOS)
The Hormone Health Network: Low Testosterone and Men's Health
MedlinePlus Medical Encyclopedia: Polycystic ovary syndrome
Centers for Disease Control and Prevention: Infertility
Mayo Clinic: Testosterone therapy: Potential benefits and risks as you age