Circadian variation in serum free and non‐SHBG‐bound testosterone in normal men: measurements, and simulation using a mass action model

Abstract

Increasing saturation of the binding proteins following rises in testosterone production, and the small but significant changes in protein concentration, probably related to postural changes, were implicated as the major factors in the rhythm amplitude. However, the early morning decline in the non-SHBG-bound fraction was not explained by these factors. The rise in cortisol concentration at this time is a probable cause. Alternatively, simulation suggests that a substance appearing in the early morning and competing with testosterone for albumin binding sites may be responsible.