To evaluate whether the concentration of SHBG is affecting the amount of testosterone available to the body’s tissues.
A blood sample taken from a vein in your arm
The Sex Hormone Binding Globulin (SHBG) test measures the amount of SHBG in the blood. SHBG is a protein that is produced by the liver. SHBG binds tightly to the hormones testosterone, dihydrotestosterone (DHT), and oestradiol (an oestrogen) and transports them in the blood in an inactive form. The amount of SHBG in the circulation is affected by age and sex, by decreased or increased testosterone or oestrogen production. It is also affected by certain diseases and conditions such as liver disease, hyperthyroidism or hypothyroidism, by obesity, and by anticonvulsant drugs like phenytoin and phenobarbitone.
Changes in SHBG can affect the amount of testosterone that is available to be used by the body. Normally, about 45% of testosterone in men and about 70% in women, is tightly bound to SHBG, and most of the rest is weakly and reversibly bound to another protein, albumin. Only about 1-3% is not bound and immediately available to the tissues as free testosterone.
A total testosterone test does not distinguish between bound and free testosterone; it measures the overall quantity of testosterone. In many cases, this measurement is sufficient to discover excessive or deficient testosterone production; but, if a patient’s SHBG level is not normal, then the total testosterone may be misleading.
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?
SHBG may be measured to provide additional information about the concentration of testosterone in the blood. In men it is used to look for testosterone deficiency. In women it is used to investigate excess production of testosterone.
SHBG and testosterone levels may be used on an adult man to help discover 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 amenorrhea, infertility, acne, and hirsutism. These symptoms are often seen with polycystic ovarian syndrome, a condition which has an excess production of androgens like testosterone. SHBG and total testosterone testing may be useful in helping to detect and investigate excess testosterone production and/or decreased SHBG levels.
For both sexes, one or more other hormones, such as prolactin, oestradiol, and LH (luteinising hormone) may also be measured to evaluate a patient’s existing balance of hormones.
When testosterone and SHBG are measured together the laboratory will usually estimate the amount of free testosterone. This may be done using a mathematical formula, or by dividing the testosterone result by the SHBG result to give what is called the Free Androgen Index (FAI=Total testosterone/ SHBG).
When is it requested?
In most cases the total testosterone measurement gives sufficient information. SHBG is normally used when the total testosterone results do not seem to fit with clinical signs, such as decreased sex drive in men or hirsutism in women but apparently normal levels. Generally Free Androgen calculations are not recommended as first line tests as do not, in most cases, give more information than testosterone levels alone.
What does the test result mean?
When SHBG levels are increased beyond what is expected, there is likely to be less free testosterone available to the tissues than is indicated by the total testosterone. This could lead to poor libido in men.
If SHBG concentrations are decreased, more of the total testosterone is available to the tissues. In women this could lead to hirsutism.
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 oestrogen concentrations decrease as hormone production by the ovaries decreases.
Some laboratories calculate bioavailable testosterone from the total testosterone and SHBG results. Bioavailable testosterone is the free testosterone plus the testosterone bound to albumin. Since testosterone is loosely bound to albumin it has been suggested that it is also available to tissues and should be also considered. It gives the same information as calculated free testosterone and the FAI.
Although SHBG is not usually used to diagnose or monitor these diseases, increases in SHBG are seen with liver disease, hyperthyroidism, anorexia, and oestrogen use (hormone replacement therapy and oral contraceptives). Decreases in SHBG are seen with obesity, hypothyroidism, androgen use, and Cushing’s disease.
Are there other uses for the SHBG test?
Can I change my level of SHBG?
Not directly and there is no reason to do so. SHBG is present in the healthy person to maintain the correct level of testosterone that enters the tissues. In some males testosterone may be given to correct a deficiency and oestrogen replacement in women may lower the SHBG slightly. People who take excessive amounts of androgens will cause the SHBG to decrease leading to high free testosterone. In such cases there are also significant side effects which may be serious.