In reply: We thank Tuomainen for his interest in our review article, and for sharing with us his data showing an inverse association between fasting serum insulin levels and calculated serum free testosterone levels. We were cautious in our statement about the relationship between hyperinsulinaemia and free testosterone levels, as there are conflicting data in the literature regarding this,1,2 and few studies that directly measure free or bioavailable testosterone. Moreover, there is ongoing controversy about the calculation of free testosterone levels using total testosterone and sex hormone-binding globulin concentrations, with the validity and assumptions of some of these widely used estimation equations being called into question.3,4
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