Test code links throughout this Guide refer to the corresponding test in the User's Guide. The concentration of free testosterone is derived from a mathematical expression based on constants for the binding of testosterone to sex hormone binding globulin. In adult males, testosterone levels show a diurnal variation with the highest levels detected in the early morning and the lowest in the evening.
Levels also increase after exercise and gradually decrease with advancing age.
The conventional radioimmunoassays for testosterone are too insensitive to quantitate the free form. In female infants, values decrease in the first week and remain low throughout early childhood. Levels increase during puberty to adult values, and are related to pubertal stage rather than chronological age. Conditions such as hypogonadism, hypopituitarism, orchiectomy, estrogen therapy, and some cases of Klinefelter's syndrome are associated with decreased levels of testosterone. Clinical applications of serum testosterone tests in pediatrics include detection of precocious puberty, hypogonadism in adolescent boys, pituitary or hypothalamic disease, where both testosterone and gonadotropin concentrations are low, and virilization in girls.