Testosterone is the principal male sex steroid hormone. It is produced mainly in the testes and is responsible for the development of male sex characteristics. In males, it is the hormone which develops secondary sex characteristics at puberty. Later in adulthood, testosterone affects sex drive, bone density, muscle size and strength, and red blood cell production. It also has an effect on mood, metabolism, and the cardiovascular system.
Testosterone is produced in smaller amounts by the ovaries in women. In both sexes, there is contribution from the adrenal cortex in the synthesis of other androgens which are precursors for testosterone. Assessment of androgen status is important in men for the diagnosis of hypogonadism and in women for the diagnosis of a variety of disorders including polycystic ovarian syndrome (PCOS). In both sexes, androgen status is important in the evaluation of adrenal disorders, particularly adrenal tumors and certain types of congenital adrenal hyperplasia.
Most clinical laboratories measure total testosterone by utilizing an immunoassay for the assessment of androgen status. Immunoassays are readily available with multiple manufacturers’ reagents and instruments. When the total testosterone level assessment does not support a patient’s clinical symptoms, the sample is usually referred to a laboratory that performs free testosterone by alternate methodology such as liquid chromatography/mass spectrometry (LC/MS), as most hospital laboratories have neither the instrumentation nor the technical expertise to perform mass spectrometry.
The referral process for the analysis of free testosterone is time-consuming and costly. An alternative to measuring free testosterone is the calculation of the free androgen index (FAI). This is a simple calculation of the ratio of total testosterone and sex hormone binding globulin (SHBG) which provides an estimate of the bioavailable testosterone—that is, testosterone that is free (unbound to any protein) plus testosterone that is loosely bound to albumin.
Binding proteins and availability of hormone
SHBG is a high-affinity binding protein for androgens and estrogens. The majority of testosterone is bound to SHBG, which renders testosterone unavailable to tissue receptors. In normal adults about 55 percent of testosterone in men or estradiol in women is tightly bound to circulating SHBG and, therefore, unavailable to tissues. Most of the remaining hormone is loosely bound to bulk carrier proteins, primarily albumin, and is considered available to react with receptors. Only a small fraction of testosterone (about two percent in males, one to two percent in females) is “free” or unbound to any protein1 and also available to tissues. Thus, both free and albumin-bound hormones are considered “bioavailable.” Unbound or free testosterone is the only form of the hormone that mediates its biological action at the target tissues in both sexes via receptors.
According to the “free hormone hypothesis,” SHBG modulates the bioactivity of sex steroids by limiting their diffusion into target tissues.2 The free hormone hypothesis states that the biological activity of hormones is determined by their free (that is, non-protein-bound) concentrations. In the case of androgens and estrogens, free hormone concentrations and bioactivity are believed to be determined by SHBG. SHBG is a liver-secreted homodimeric glycoprotein with high affinity to both steroid hormones.
The FAI can be considered an estimate of the bioavailable hormone. With the availability of immunoassays for both testosterone and SHBG, laboratories can perform these assays and provide an estimate of bioavailable testosterone via the calculation for FAI. This calculation for bioavailable testosterone may show a better correlation with clinical symptoms than total testosterone levels alone.
Utilization in men
The most common use of testosterone assays in males is to diagnose hypogonadism. Hypogonadism is defined as biochemically low testosterone levels along with clinical symptoms, which may include reduced sexual desire (libido) and activity, decreased spontaneous erections, decreased energy and depressed mood, reduced bone and muscle mass, and increased body fat. The term “andropause” is used to describe decreasing testosterone levels related to aging men. In general, older men tend to have lower testosterone levels than younger men. Although this is not universal, testosterone levels gradually decline throughout adulthood in males, on the average of about one to two percent a year.3 In addition to the decline in testosterone, an associated rise in SHBG levels has been documented.4 This rise in SHBG contributes to the decrease in bioavailable testosterone. A calculated FAI can aid in the assessment of andropause-related symptoms. In a retrospective study, Ring et al concluded that adding SHBG to total testosterone testing facilitated a more accurate diagnosis of male infertility associated with hypoandrogenism.5
According to the American Urological Association Position Statement on Testosterone Therapy, testosterone therapy is appropriate treatment for patients with clinically significant hypogonadism, including those with idiopathic clinical hypogonadism that may or may not be age-related.6 In recent years there has been an increased interest in testosterone therapy to reduce the symptoms associated with the decline in testosterone levels. For these patients, regular monitoring of testosterone is essential for management of dosage and symptom abatement. SHBG and FAI could add to the better management of andropause.
Utilization in women
Testosterone measurements in women are used for evaluating states of androgen excess to exclude androgen-producing tumors and to aid in the diagnosis of other hyper-androgenic states, the most important being PCOS, the most common endocrine disorder in females. The prevalence of PCOS varies depending on which criteria are used to make the diagnosis, but it is as high as 15 percent to 20 percent when the European Society for Human Reproduction and Embryology/American Society for Reproductive Medicine criteria are used.7 PCOS is a common heterogeneous endocrine disorder characterized by irregular menses, hyperandrogenism, and polycystic ovaries. According to guidelines published in 2013, the diagnosis of polycystic ovary syndrome is made if two of the three following criteria are met: androgen excess, ovulatory dysfunction, or polycystic ovaries.8 In peri-menopausal and post-menopausal women, the authors suggest that a presumptive diagnosis of PCOS can be based upon a well-documented long-term history of oligomenorrhea and hyperandrogenism during the reproductive years. The diagnosis of PCOS in an adolescent girl can be made based on the presence of clinical and/or biochemical evidence of hyperandrogenism (after exclusion of other pathologies) in the presence of persistent oligomenorrhea. Thus the measurement of testosterone is key to diagnosis.
An early study by Escobar-Morreale et al9 measured testosterone and SHBG and identified SHBG and FAI as the best assays for diagnosis with AUC 0.875 and 0.87 respectively. In a more recent study of 122 women with and without PCOS, measurements of total testosterone, SHBG, and calculated FAI were compared. The authors concluded that FAI is a valuable laboratory assessment in the diagnosis of PCOS. In a study comparing tests and outcome, Miller et al found that the calculation of FAI correlates better than the more complex measurement of free testosterone in women for hypoandrogenism.10Al Kindi and colleagues concluded that FAI is superior for the diagnosis of hyperandrogenism in women to total testosterone alone.11 Thus FAI can be an important tool in assessing the androgenic state in the female population and, in particular, women with PCOS.
Utilization in children
In boys, testosterone measurements are used during adolescence in the evaluation of early or delayed puberty or at birth during the evaluation of under-virilized males. In girls, testosterone assays are used to assess and treat disorders of sexual development and in the evaluation of contra-sexual pubertal development. As in women, testosterone determination in children should be carried out only with assays of sufficient sensitivity and in conjunction with appropriate normative data. The measurement of SHBG and calculating FAI may be underutilized in the assessment of androgen status in children.
Utilization of SHBG alone
Until recently, the sole function of SHBG was thought to be transport of sex steroids. As discussed previously, the measurement of SHBG can be used to estimate bioavailable testosterone levels in patients suffering from either too little or excessive androgen exposure.
New information suggests that SHBG may have broader utility in assessing the risk for endocrine diseases and the clinical consequences of the metabolic syndrome, namely, type 2 diabetes and cardiovascular disease. An association between SHBG and insulin resistance is reported across many longitudinal and cross-sectional studies. A recent review discusses the evidence.12 In a recent nested case control study of postmenopausal women from a cohort of the Women’s Health Study and a cohort of men from the Physicians’ Health Study II of men, Ding et al concluded that low circulating levels of SHBG are a strong predictor of the risk of type 2 diabetes in both women and men.13 They suggested that SHBG could be an important target in stratification for the risk of type 2 diabetes and for early intervention.
SHBG has also been associated with the development of gestational diabetes (GD). GD is a common pregnancy complication that is associated with increased maternal and neonatal morbidity. Identifying and treating these women is important to improve outcomes. In a recent case-controlled study, researchers found that patients with GD have lower circulating levels of SHBG than normally glucose-tolerant pregnant women and concluded that circulating concentrations of SHBG represent a potentially useful new biomarker for prediction of risk of GD beyond the currently established clinical and demographic risk factors.14 The authors also established a cut-off value for SHBG which had 90 percent sensitivity and nine percent specificity for diagnosis. Whether SHBG can be used to replace the diagnosis of gestational diabetes is yet to be determined.
Several recent studies have shown a relationship between SHBG and metabolic syndrome. Metabolic syndrome consists of a set of factors that confer increased risk of cardiovascular diseases, including obesity (especially abdominal obesity), insulin resistance, dyslipidemia (increased triglyceride levels and reduced HDL cholesterol levels), and systemic arterial hypertension. In an early study of older men, Chubb and colleagues concluded that low SHBG is more strongly associated with metabolic syndrome than low total testosterone.15 In a subsequent retrospective study, Callou de Sa and colleagues also concluded that low serum levels of SHBG are associated with a higher prevalence of metabolic syndrome among male patients.16
A review of SHBG in children and adolescents has been provided by Ayden and Winters.17 Although more research is needed in children, the authors determined, “Evidence is accumulating that low SHBG levels are an indicator of insulin resistance, and SHBG may be an easy-to-measure and clinically useful biomarker for the early identification of children who are destined to develop obesity-related chronic diseases.”
FAI and SHBG
The calculation of FAI can be an important addition to the laboratory diagnosis of androgen status and may be more beneficial than measuring total testosterone alone. In addition to its use in the calculation of FAI, there is increasing evidence in the literature to suggest that SHBG levels are correlated with multiple medical conditions. It remains to be determined whether SHBG is solely a biomarker, or if it actively participates in the pathogenesis of metabolic disease.
References
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