This test is used to investigate rare causes of hypogonadism (hyposecretion of the genital glands) in adults, pseudohermaphroditism (congential disorder of sexual differentiation) in children, and certain forms of hirsutism (excessive hair growth) or virilism (male secondary sex characteristics) in women.
In the vast majority of clinical cases, the levels of total testosterone or of fractions of free/bioavailable testosterone provide an adequate picture of testosterone activity in the body tissues. In order for testosterone to be active in the cell, it has to be converted into dihydrotestosterone. The enzyme responsible for this activation is called 5-alpha reductase. For genetic reasons, 5-alpha reductase levels are abnormally low in some children, which can result in an absence of (pseudohermaphroditism) or delay in sexual development despite the presence of normal testosterone levels. Simultaneous measurement of serum testosterone and dihydrotestosterone levels can be particularly useful in these cases.