Testosterone is secreted by the testes in men. In women, the adrenal glands secrete 25% of the testosterone and the ovaries secrete another 25%; the remainder is produced by peripheral conversion of androstenedione. Most circulating testosterone is bound by sex hormone binding globulin (SHBG) and albumin; approximately 2% of total testosterone is free (not bound to protein).
SHBG-bound testosterone is so tightly bound that it is not biologically active. Both free and albumin-bound testosterone are biologically active, and together are referred to as the bioavailable fraction. Thus, bioavailable testosterone levels depend on albumin levels to a small extent (low binding affinity) and SHBG levels to a large extent (high binding affinity) in addition to rates of testosterone production and clearance.
In utero, testosterone is necessary for the development of male genitalia in 46, XY fetuses. In the absence of testosterone, the fetus tends to develop as a female. Thus, with a disorder of sexual development, a 46, XY newborn may present with external genitalia ranging from nearly normal female to nearly normal male, depending on the severity of the defect. Total testosterone levels can range from absent to increased, depending on the condition. Male infants with hypogonadism or hypopituitarism may display micropenis or cryptorchidism.
Delayed puberty and hypogonadism in boys and men can be associated with primary or secondary testicular failure. Elevated LH and FSH are consistent with primary hypogonadism whereas decreased levels are consistent with secondary or tertiary hypogonadism. As men age, testosterone levels decrease, SHBG levels increase, and these changes result in a decrease in free testosterone levels. Thus, free testosterone measurement offers greater sensitivity than total testosterone for diagnosis of hypogonadism in older men.2
Measurement of free or bioavailable testosterone in females offers greater sensitivity for evaluation of mild androgen excess than total testosterone.3,4 In girls and women, excess androgen production is associated with premature adrenarche (ie, appearance of pubic and/or axillary hair before age 8), oligo/amenorrhea, and clinical features of hyperandrogenism (eg, alopecia, severe acne, hirsutism). These features are associated with polycystic ovary syndrome (PCOS), the most common endocrinopathy in women of reproductive age. Free testosterone levels are elevated in ~70% of PCOS cases.3
Direct immunoassays cannot accurately measure the low serum testosterone levels found in women and children,5 hypogonadal men,6 or patients undergoing antiandrogenic therapies.6 Thus, the Endocrine Society recommends testosterone methods that use extraction and purification prior to measurement.1 Liquid chromatography tandem mass spectrometry (LC/MS/MS) methods meet these recommendations. In addition, because of increased sensitivity and specificity, LC/MS/MS has emerged as the method of choice in these populations.