What does the concentration of urinary free cortisol mainly reflect?

What does the concentration of urinary free cortisol mainly reflect?

• A. Total serum cortisol
• B. Conjugated cortisol
• C. Unbound serum cortisol
• D. Protein-bound serum cortisol

Answer: (C)

Urinary free cortisol reflects the amount of cortisol that is not bound to carrier proteins in the bloodstream. In blood, most cortisol is bound to cortisol-binding globulin and albumin, with only a small free (unbound) fraction circulating. This unbound cortisol is filtered by the kidneys and excreted into urine, so the urinary free cortisol level tracks the circulating free cortisol concentration and overall adrenal cortisol production. Conjugated cortisol (a metabolized, water-soluble form) is excreted separately as conjugates, and protein-bound cortisol does not significantly enter urine. Thus, the urinary free cortisol concentration mainly indicates unbound serum cortisol.

Urinary free cortisol reflects the amount of cortisol that is not bound to carrier proteins in the bloodstream. In blood, most cortisol is bound to cortisol-binding globulin and albumin, with only a small free (unbound) fraction circulating. This unbound cortisol is filtered by the kidneys and excreted into urine, so the urinary free cortisol level tracks the circulating free cortisol concentration and overall adrenal cortisol production. Conjugated cortisol (a metabolized, water-soluble form) is excreted separately as conjugates, and protein-bound cortisol does not significantly enter urine. Thus, the urinary free cortisol concentration mainly indicates unbound serum cortisol.