Testosterone is present in the circulation both in protein bound (96%) and in non-protein bound ('free' or unbound) formats. In males, about 44% is bound to Sex Hormone Binding Globulin (SHBG), 50% to albumin and 2-3% 'free' (1).
Until recently it was believed that only the 'free' fraction of testosterone could be taken up by tissues and the protein-bound testosterone complex was inactive. It has now been demonstrated that the albumin bound fraction of testosterone readily dissociates and is absorbed up by the tissues along with the 'free' fraction. Together, these two fractions are referred to as the bioavailable testosterone (Bio-T) fraction.
Clinical Relevance of Bio-T
Whilst age related decreases in total testosterone are debated, studies have shown consistent reductions in bioavailable testosterone of up to 50% in elderly males (2). In some patients, pathologically low levels have been seen in association with declining libido and poor sexual performance. Diminished levels may also be a contributing factor to symptoms of andropause, the male equivalent of menopause. Recent studies have also associated decreased levels with depression in the elderly (3) as well as osteoporosis (4).
In females, increased levels have been associated with some cases of hirsutism. Reduced levels have been found in patients with osteoporosis (4).
Indications for ordering Bio-T
Total testosterone and 'free' testosterone assays have been traditionally ordered. Until further knowledge is gained on the assay, these tests should continue to be ordered, along with bioavailable testosterone.
- Suspected hypogonadism, as seen in a decline of libido or sexual performance
- Andropause ("male menopause")
- Evaluation of hirsutism
Sample requirements & Test Availability
- Samples should be collected before 10:00 a.m.
- Collect one full SST-tube; allow to clot for 30 minutes. Centrifuge, separate serum and freeze immediately. Sample must be transported to the laboratory frozen.
- This test may not be insured by provincial health plans but it may be covered by a patient's insurance plan.
- Please refer patient to a LifeLabs Patient Service Centre for collection.
Results are available within 7-10 days.
Manni A, Pardrige WM, Cefalus W, Nisula BC, Bardin CW: Bioavailability of albumin-bound testosterone. J Clin Endocrinol Metab 61:705, 1985.
Ferrini RL and Barrett-Connor E: Sex hormones and age. Am J Epidemiol 147(8):750-4, 1998.
Barrett-Connor E, Von Muhlen DG and Kritz-Silverstein D. Bioavailable testosterone and depressed mood in older men: the Rancho Bernardo study. J Clin Endocrinol Metab, 84(2): 573-7, 1999.
Greendale GA, Edelstein S, Barrett-Connor E, Endogenous sex steroids and bone mineral density in older women and men: the Rancho Bernardo Study. J Bone Miner Res 12(11): 1833-43, 1997.