free androgen index test (FAI)

Last reviewed 09/2022

  • free androgen index test [FAI = total testosterone (nmol/ L)) x 100 /sex hormone-binding globulin (SHBG) (nmol/ L))

Notes (1)

  • free testosterone and FAI seem to be sensitive for the detection of hyperandrogenaemia (1)
  • in using FAI in the diagnosis of androgenic alopecia in women consider:
    • in women, at least 80% of bound serum testosterone is bound to SHBG. Consequently, free serum testosterone levels are substantially influenced by SHBG levels, which limit the interpretation of free serum testosterone
      • the FAI takes this SHBG dependence into account
    • FAI levels of 5 and above are indicative for polycystic ovary syndrome
      • other disorders presenting with clinical and/or biochemical signs of hyperandrogenism such as congenital adrenal hyperplasia, androgen-secreting tumours or Cushing syndrome should be excluded. For this purpose further laboratory testing, e.g. 17-OH-progesterone, follicle-stimulating hormone, oestradiol, prolactin or cortisol may be necessary
    • it makes sense to take any hormonal level only on the condition that there is no hormonal intake. Oestrogens lead to elevated SHBG levels, whereas testosterone levels may be only slightly changed. Consequently, the FAI can be markedly improved by hormonal contraception
      • therefore, the minimum pause in hormonal contraception has to be 2 months. The measurements should be taken between 08.00 and 09.00 h, ideally between the second and fifth days of the menstrual cycle