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Indirect Measurement of Bioavailable Testosterone with the Bayer Immuno 1 System

Departments of
¹ Biochemistry,
² Pathology, and
³ Urology, Queen’s University and Kingston General Hospital, Kingston, Ontario, K7L 3N6 Canada

^aaddress correspondence to this author at: Department of Pathology, Richardson Laboratories, Queen’s University, Kingston, Ontario, K7L 3N6 Canada; fax 613-533-2907, e-mail collier@cliff.path.queensu.ca

Testosterone is the principal circulating androgen in men. A major fraction of testosterone is specifically bound with high affinity and low capacity to sex-hormone-binding globulin (SHBG), whereas most of the remaining testosterone is bound with low affinity to albumin (ALB), leaving only 1–2% to circulate as "free" testosterone (FT) not bound to protein in serum (1). Because the SHBG–testosterone complex dissociates very slowly in vitro (dissociation half-time, 20 s), SHBG-bound testosterone (SHBG-T) is not considered to be available for biologic action in tissues. According to the original "free hormone hypothesis", only FT is able to enter target cells. Tait and Burstein (2), however, suggested that because testosterone is only loosely bound to ALB, this fraction is also available for tissue uptake (dissociation half-time, 1 s). The concept of bioavailable testosterone (BAT), also known as "non-SHBG-bound testosterone" [or both FT and ALB-bound testosterone (ALB-T)], was supported by in vitro kinetic calculations (3), but others (4)(5) have challenged the assumptions used for these calculations. Indeed, if SHBG facilitates entry of testosterone into tissues, as reported by Pardridge (6), then SHBG-T would also be "available" for tissue uptake and action. Controversy remains about the best biochemical measurement for testosterone function in humans.

The initial clinical diagnostic evaluation for men with symptoms of hypogonadism involves measurement of the serum total testosterone. In situations such as hyper- and hypothyroidism, obesity, and the use of antiepileptic drugs, the correlation between clinical symptoms and the serum total testosterone concentration has been reported to be poor. This may be attributable in part to the inactivity of the SHBG-T fraction and to variation in the plasma concentration of SHBG in these situations. The fact that plasma SHBG changes during life and in several pathologic situations is well . . . [Full Text of this Article]

Acknowledgments

References

The following articles in journals at HighWire Press have cited this article:

				W. de Ronde, Y. T. van der Schouw, H. A.P. Pols, L. J.G. Gooren, M. Muller, D. E. Grobbee, and F. H. de Jong Calculation of Bioavailable and Free Testosterone in Men: A Comparison of 5 Published Algorithms Clin. Chem., September 1, 2006; 52(9): 1777 - 1784. [Abstract] [Full Text] [PDF]