Testosterone testing has come of age as increased testing for androgen deficiency in men and androgen excess in women has become popular.
Where is Testosterone Produced and What Does it Do?
Testosterone is the principal androgen in men and is required for normal sperm production. LH secreted by the pituitary stimulates the testis to produce testosterone. This entire loop is controlled by by gonadotropin hormone-releasing hormone (GHRH), generated by the hypothalamus. The Leydig cells within the testes produce the majority of the testosterone which secretes the hormone into the seminiferous tubules and complexes with a protein produced by the Sertoli cells. Tissues containing the enzyme 5alpha-reductase converts testosterone to dihydrotestosteron (DHT), the endocrinologically active form of the androgen. DHT influences the prostate, scalp, and beard growth. In addition, with testosterone, it plays a role in the development of the male genital tract and secondary sex characteristics.
Women also produce testosterone and DHT, though in far lesser amounts. The ovary and adrenal glands produce some testosterone, but the majority of the testosterone in women is derived from the peripheral conversion of other steroids.
Only about 2 percent of the total testosterone in the plasma of men is free or nonprotein bound; about 1 percent in women. In most men and women, more than 50 percent of total circulating testosterone is bound to sex hormone-binding globulin (SHBG), and most of the rest is bound to albumin. It is only the free or nonprotein bound testosterone which is the hormonally active form, able to interact with cellular hormone receptors. SHBG-bound testosterone is not readily available for intracellular complex formation because of SHBG's high binding affinity for testosterone.
Testosterone-bound SHBG is considered biologically inactive. SHBG levels are sensitive to changes in estrogen and testosterone. Thus conditions which affect SHBG will directly affect the serum levels and biological activity. Some interfering effects include:
Decreased serum testosterone levels stimulate the production of SHBG by the liver
Increased estrogen levels that occur in pregnancy or with estrogen replacement therapy also increase SHBG production
SHBG levels can also be increased in patients with hyperthyroidism and liver disease
Increased SHBG-bound testosterone can result in total testosterone levels in the normal range, despite a clinical deficiency of bioavailable testosterone
SHBG levels tend to be low in androgen excess states, often resulting in total testosterone levels within normal limits and elevated bioavailable testosterone
UTILITY CHARACTERIZATION Testosterone Deficiency in Adults
Infertility in young adult men
Low testosterone concentrations in these men can be caused by:
Testicular failure (primary hypogonadism)
Inadequate stimulation by pituitary gonadotropins (secondary hypogonadism)
Men with hypogonadism often have high SHBG levels, the measurement of free or bioavailable testosterone has been advocated when total testosterone levels are normal in men with symptoms of androgen deficiency.
Testosterone Excess in Women
First sign of androgen excess in women is the development of male pattern hair growth, which is referred to as hirsutism
Women with more excessive androgen levels may also experience virilization
Many women with slowly progressive androgenic symptoms are diagnosed as having polycystic ovary syndrome
Endocrine screening in 1,022 men with erectile dysfunction: clinical significance and cost-effective strategy.
Buvat J, Lemaire A.
Association pour l'Etude de la Pathologie de l'Appareil Reproducteur et de la Psychosomatique, Lille, France.
J Urol 1997 Nov;158(5):1764-7 Abstract quote
PURPOSE: We reviewed the results of serum testosterone and prolactin determination in 1,022 patients referred because of erectile dysfunction and compared the data with history, results of physical examination, other etiological investigations and effects of endocrine therapy to refine the rules of cost-effective endocrine screening and to pinpoint actual responsibility for hormonal abnormalities.
MATERIALS AND METHODS: Testosterone and prolactin were determined by radioimmunoassay. Every patient was screened for testosterone and 451 were screened for prolactin on the basis of low sexual desire, gynecomastia or testosterone less than 4 ng./ml. Determination was repeated in case of abnormal first results. Prolactin results were compared with those of a previous personal cohort of 1,340 patients with erectile dysfunction and systematic prolactin determination. Main clinical criteria tested regarding efficiency in hormone determination were low sexual desire, small testes and gynecomastia. Endocrine therapy consisted of testosterone heptylate or human chorionic gonadotropin for hypogonadism and bromocriptine for hyperprolactinemia.
RESULTS: Testosterone was less than 3 ng./ml. in 107 patients but normal in 40% at repeat determination. The prevalence of repeatedly low testosterone increased with age (4% before age 50 years and 9% 50 years or older). Two pituitary tumors were discovered after testosterone determination. Most of the other low testosterone levels seemed to result from nonorganic hypothalamic dysfunction because of normal serum luteinizing hormone and prolactin and to have only a small role in erectile dysfunction (definite improvement in only 16 of 44 [36%] after androgen therapy, normal morning or nocturnal erections in 30% and definite vasculogenic contributions in 42%). Determining testosterone only in cases of low sexual desire or abnormal physical examination would have missed 40% of the cases with low testosterone, including 37% of those subsequently improved by androgen therapy. Prolactin exceeded 20 ng./ml. in 5 men and was normal in 2 at repeat determination. Only 1 prolactinoma was discovered. These data are lower than those we found during the last 2 decades (overall prolactin greater than 20 ng./ml. in 1.86% of 1,821 patients, prolactinomas in 7, 0.38%). Bromocriptine was definitely effective in cases with prolactin greater than 35 ng./ml. (8 of 12 compared to only 9 of 22 cases with prolactin between 20 and 35 ng./ml.). Testosterone was low in less than 50% of cases with prolactin greater than 35 ng./ml.
CONCLUSIONS: Low prevalences and effects of low testosterone and high prolactin in erectile dysfunction cannot justify their routine determination. However, cost-effective screening strategies recommended so far missed 40 to 50% of cases improved with endocrine therapy and the pituitary tumors. We now advocate that before age 50 years testosterone be determined only in cases of low sexual desire and abnormal physical examination but that it be measured in all men older than 50 years. Prolactin should be determined only in cases of low sexual desire, gynecomastia and/or testosterone less than 4 ng./ml
CHARACTERIZATION Total Testosterone
Most use automated methods based on immunoassay
Must first displace bound testosterone from SHBG and albumin
Plasma levels of DHT are only about one-tenth of testosterone levels, and the cross-reactivity is typically less than 5 percent.
Androgen Index Calculation
Concentration of testosterone in the various free and bound forms is essentially a function of total testosterone concentration and the relative concentrations of SHBG and albumin.
Index is typically calculated as the ratio of total testosterone divided by SHBG and multiplied by 100 to yield numerical results comparable in free testosterone concentration.
Free Testosterone by Equilibrium Dialysis or Equilibrium Ultrafiltration
Concentration of free testosterone is very low, typically less than 2 percent of the total testosterone concentration.
Measurement of free testosterone by these methods is not available in most clinical laboratories due to the complicated nature of the testing and the requirement of a scintillation counter to measure the tritiated testosterone concentration. The results of equilibrium dialysis and centrifugal ultrafiltration methods have been shown to be quite comparable
Equilibrium dialysis is often considered to be the gold standard but centrifugal ultrafiltration is somewhat simpler to perform and may theoretically be more accurate due to the fact that the equilibrated sample is not diluted with dialysis buffer
Free Testosterone by Analog Tracer Immunoassay
Analogue method to correlate better with total testosterone levels than with bioavailable testosterone determined by the ammonium sulfate precipitation method
Analogue-free testosterone method correlated well with free testosterone by equilibrium dialysis but did not correspond with a free testosterone calculated from total testosterone and SHBG.
Sum of circulating free testosterone and albumin-bound (weakly bound) testosterone
Steroid levels in saliva are thought to reflect the free levels in the blood but it has not gained widespread acceptance for routine clinical applications
Very low, especially in women.
Not been shown to be sensitive enough to produce diagnostically accurate results for the clinical assessment of individual patients, especially women
Henry JB. Clinical Diagnosis and Management by Laboratory Methods. Twentieth Edition. WB Saunders. 2001.
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Last Updated 2/21/2002
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