Endocrine Abstracts

Many laboratories offer a simple ratio of total testosterone (T):SHBG as an index (FAI) of free androgens. Methods for measuring free T (fT) are time consuming (equilibrium dialysis, ED) or reveal concentrations at odds with accepted values (RIA). The validity of FAI has been questioned and an alternative assessment by computation of fT recommended (1). We use a simpler calculation (2) producing results comparable to established literature. We have compared results obtained by these two calculations in a series of 128 healthy men and in an unselected series of both male (191) and female (646) patients. Both methods of calculation involve measuring T & SHBG. Samples were taken with Ethical approval and informed consent. The 2 formulae are previously published (1,2).

Calculated fT showed a strong correlation between the two formulae for patients and controls (r = 0.995, men, r = 0.996, women, r = 0.995, controls). Linear regression analysis revealed the intercepts and slopes for men, women and controls were significantly different from each other (formula 1 = 0.997xformula 2 - 35.6, formula 1 = 0.87xformula 2 - 4.0, formula 1 = 1.06xformula 2 - 66.0, respectively). The mean difference between calculated fT by formula 1 & 2 was comparable for the men and controls (36, 95%CI 33.7-8.3pmol per litre, and 39, 95%CI 36.3-42.1 pmol per litre, respectively) and much lower for the women (9, 95%CI 8.6-9.4 pmol per litre). If fT < 200pmol per litre is taken as the lower reference limit, 2 men with normal T had fT less than this cut-off using either formula. A further 25 men had low T & fT by either formula. An additional 9 cases had low fT calculated by formula 2 but not formula 1 and of these 4 had normal T.

Calculation of fT offers a quick and simple alternative to ED or RIA.

1.Vermeulen A, Verdonck L, Kaufman JM. J Clin Endocrinol Metab 1999;84:3666-72.

2.Nanjee MN, Wheeler MJ. Ann Clin Biochem 1985;22:387-90.