The test for sex hormone binding globulin (SHBG) is primarily ordered in conjunction with other tests to evaluate the status of a person's androgen levels - the male hormones. With men, the issue of concern is testosterone deficiency, while with women the concern is excess testosterone production. A total testosterone may be ordered prior to or along with a SHBG test.
SHBG and testosterone levels may be ordered on an adult male to help determine the cause of infertility, a decreased sex drive, and erectile dysfunction, especially when total testosterone results are inconsistent with clinical signs.
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 lack of menstruation (amenorrhea), infertility, acne, and male pattern hair growth (hirsutism). These symptoms and others are often seen with polycystic ovarian syndrome, a condition characterized by an excess production of androgens. SHBG and testosterone testing may be useful in helping to detect and evaluate excess testosterone production and/or decreased SHBG concentrations.
Sometimes, a total testosterone and SHBG are ordered to evaluate free androgens by calculating the Free Androgen Index (FAI). This equation gives doctors an idea of the quantity of testosterone that is not bound (bioavailable testosterone) to SHBG and is calculated as follows: FAI = Total Testosterone / SHBG. In males, a fall in testosterone causes a rise in SHBG, so an FAI or another measure (bioavailable testosterone) is often ordered to detect low levels of free testosterone, which may cause decreased sex drive, loss of muscle mass, decreased bone mass, or occasionally problems with having erections (erectile dysfunction). An elevated FAI may be useful in the evaluation of androgenic alopecia (balding), hirsutism, and severe acne where testosterone levels may be normal.
Currently, the SHBG test is not performed frequently or routinely. In many cases, doctors feel that the total testosterone, and perhaps free testosterone, provides sufficient information. SHBG is ordered primarily when the total testosterone results do not seem to be consistent with clinical signs, such as decreased sex drive in men or hirsutism in women.
When the SHBG level is increased beyond what is expected, there is likely to be less free testosterone available to the tissues than is indicated by the total testosterone. If the SHBG concentration is decreased, more of the total testosterone is "bioavailable" - 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.
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.
Testing for bioavailable testosterone is becoming an increasingly important concept to both doctors and researchers. It may be measured by removing the SHBG-bound testosterone from the collected blood sample and then analyzing what’s left. An estimate can also be calculated using the Free Androgen Index (see above).
This article was last reviewed on February 7, 2012. | This article was last modified on February 13, 2012.
The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.
The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.