Free testosterone concentration is derived by calculation from the
total testosterone
and the
sex hormone binding globulin
using the Vermeulen formula.
Free testosterone makes up approx. 2% of the circulating serum testosterone.
Free testosterone is the active form and is a more accurate index of testosterone availability to tissues.
Total testosterone provides an adequate assessment of androgen status in most men and women.
Free testosterone may be helpful in borderline cases.
Free testosterone levels less than 220 pmol/L are significantly associated with symptoms of sexual dysfunction in men (1)
The laboratory may decline a request for free testosterone if:
-
the total testosterone level in a woman is lower than 1.3 or higher than 5 nmol/L
-
the total testosterone in a man is lower than 7 or higher than 15 nmol/L
In these cases the total testosterone level provides unequivocal information. However, if SHBG testing is still required, please contact the on-call Chemical Pathologist for approval.
(See also
testosterone, total)
References
1. Wu FC, Tajar A, Beynon JM et al. Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med;363:123-135