Validity of radioimmunological methods for determining free testosterone in serum

Abstract

Objective: To assess serum free testosterone (free T) concentrations in various groups of women by means of an ultrafiltration technique (UFT) or two RIAs and to investigate the influence of levonorgestrel (LNG) and norethisterone (NET) on free T.

Design: Experimental study with serum samples from various groups of patients.

Setting: Obstetrics and gynecology department of a university.

Patient(s): Samples from 20 normal healthy women, 20 menopausal women, 21 pregnant women, 23 women using oral contraceptives (OC), 20 hyperandrogenemic women, 20 hirsute patients with normal total testosterone (total T), 18 patients with acne (normal total T), and 9 women with adrenogenital syndrome (AGS).

Intervention(s): Blood samples were taken by venipuncture.

Main outcome measure(s): Serum concentrations of free T.

Result(s): Mean free-T levels determined by UFT amounted to 5-6 pg/mL in most groups, except in patients with hyperandrogenemia (8.3 pg/mL) or AGS (10.7 pg/mL) and in women using OC (1.9 pg/mL). The values were twofold to fourfold and threefold to sevenfold higher than those determined with the RIAs. The proportion of free T as determined by UFT was 0.9-1.2% of total T and was 0.55% during OC treatment. The presence of 10 ng/mL of LNG or NET increased free-T levels by only 13%.

Conclusion(s): Serum free-T levels determined with RIA are much lower than those measured with the UFT. The ratio between the UFT and RIA values may vary depending on the endocrine status. In women with normal or elevated sex hormone-binding globulin levels, nortestosterone derivatives increase free T levels only slightly.