Figure 31.2 Major steroid biosynthesis pathway.
Laboratory Assessment of Hyperandrogenemia
Approximately 66% to 78% of circulatory testosterone is bound to sex hormone–binding globulin (SHBG) and is considered biologically inactive. Most of the proportion of serum testosterone that is not bound to SHBG is weakly associated with albumin (20% to 32%). A small percentage (1% to 2%) of testosterone is entirely unbound or free. The fraction of circulating testosterone that is unbound by SHBG has an inverse relationship with the SHBG concentration.
Increased SHBG levels are noted in conditions associated with high estrogen levels. Pregnancy, the luteal phase, use of estrogen (including oral contraceptives), and conditions causing elevated thyroid hormone levels and cirrhosis of the liver are associated with reduced fractions of free testosterone caused by elevated SHBG levels. Conversely, levels of SHBG decrease and result in elevated free testosterone fractions in response to androgens, androgenic disorders (PCOS, adrenal hyperplasia or neoplasm, Cushing syndrome), androgenic medications (i.e., progestational agents with androgenic biologic activities, such as danazol, glucocorticoids, and growth hormones), hyperinsulinemia, obesity, and prolactin.
In hyperandrogenic states, increases in testosterone production are not proportionately reﬂected in increased total testosterone levels because of the depression of SHBG levels that occurs concomitant with increasing androgen effects on the liver. Therefore, when moderate hyperandrogenism, characteristic of many functional hyperandrogenic states, occurs, elevations in total testosterone levels may remain within the normal range, and only free testosterone levels will reveal the hyperandrogenism. Severe hyperandrogenism, as occurs in virilization and that results from neoplastic production of testosterone, is reliably detected by measures of total testosterone. Therefore, in practical clinical evaluation of the hyperandrogenic patient, determination of the total testosterone level in concert with clinical assessment is frequently sufficient for diagnosis and management. When more precise delineation of the degree of hyperandrogenism is desired, measurement or estimation of free testosterone levels can be undertaken and will more reliably reﬂect increases in testosterone production. These measurements are not necessary in evaluating the majority of patients, but they are common in clinical research studies and may be useful in some clinical settings. Because many practitioners measure some form of testosterone level, they should understand the methods used and their accuracy. Although equilibrium dialysis is the gold standard for measuring free testosterone, it is expensive, complex, and usually limited to research settings. In a clinical setting, free testosterone levels can be estimated by assessment of testosterone binding to albumin and SHBG.
Testosterone, that is nonspecifically bound to albumin (AT), is linearly related to free testosterone (FT) by the equation:
AT = Ka [A] × FT, where AT is the albumin-bound testosterone, Ka is the association constant of albumin for testosterone, and [A] is the albumin concentration.
In many cases of hirsutism, albumin levels are within a narrow physiologic range and thus do not significantly affect the free testosterone concentration. When physiologic albumin levels are present, the free testosterone level can be estimated by measuring the total testosterone and SHBG. In individuals with normal albumin levels, this method has reliable results compared with those of equilibrium dialysis. It provides a rapid, simple, and accurate determination of the total and calculated free testosterone level and the concentration of SHBG.
The bioavailable testosterone level is based on the relationship of albumin and free testosterone and incorporates the actual albumin level with the total testosterone and SHBG.
This combination of total testosterone, SHBG, and albumin level measurements can be applied to derive a more accurate estimate of available bioactive testosterone and thus the androgen effect derived from testosterone. Bioactive testosterone determined in this manner provides a superior estimate of the effective androgen effect derived from testosterone (5).