Testosterone in Serum – Lab Test
Testosterone is the most important male sex hormone. The hypothalamic-pituitary-gonadal hormone axis controls the release of testosterone [fig. control of testosterone]. 60% of the testosterone is bound to the sex hormone binding globulin (SHBG), due to the strong bound it is considered biological inactive. Only 1–3% of total testosterone are unbound in plasma (free testosterone). The remaining portion is albumin bound, which is considered to be weak bound. Free testosterone and albumin-bound testosterone are called bioavailable testosterone, because both can exert biological activity.
fig. regulation of testosteron: the hypothalamus, the pituitary gland and the testes form the hypothalamic-pituitary-gonadal hormone axis. The control of hormone release (arrows) is subject to multiple negative feedbacks (dashed lines).
Depending on age 12–35 nmol/l (3,5–10 ng/ml) in the morning. The testosterone circadian rhythm peaks early in the morning, concentrations are 40% lower in the evening (Trottmann, 2010).
Standard Value of Free Testosterone:
>0,25 nmol/l (>7,2 ng/dl).
Laboratory test method:
Competitive immunoassay for the testosterone concentration. Free testosterone is calculated with the Vermeulen formula using testosterone, SHBG and albumine (Vermeuten et al, 1999).
The testosterone concentration is an important diagnostic parameter in andrological diseases. Further indications are the control of antiandrogenic therapy by GnRH analogues in prostate cancer and the evaluation of hormone status in patients with a testicular tumor. LH, FSH and SHGB are determined in case of abnormalities of testosterone. Free testosterone is helpful to assess inconclusive low concentrations of testosterone.
Elevated levels are seen due to anticonvulsants, barbiturates, clomiphene, estrogens or oral contraceptives. Low levels are caused by androgen medication (or abuse), cyproterone, GnRH analogues, GnRH antagonists, digitalis, glucocorticoids, halothane, ketoconazole, metoprolol, phenothiazines, spironolactone and tetracyclines.
Differential Diagnosis of Hypogonadism
Hypogonadism is an endocrine dysfunction of the testes with testosterone deficiency. Primary hypogonadism is caused by a failure of the testosterone production in the testes, the releasing hormones are compensatorily increased (hypergonadotropic hypogonadism). Secondary hypogonadism is caused by a defect of the pituitary hormone release (hypogonadotropic hypogonadism). Tertiary hypogonadism is caused by a defect of the hypothalamus (Koehn, 2004). For details please see section hypogonadism.
Differential Diagnosis of Hypergonadism
Hypogonadotropic hypergonadism is more common in men: testosterone abuse, increased SHBG, hormone-active testicular tumors (e.g. Leydig cell tumor) or adrenal tumors. Hypergonadotropic hypergynadism is seen in the androgen insensitivity syndrome (see disorders of sex development).
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