I am looking for solid reference to infer mean (human) plasma cortisol concentration of a time unit (e.g. 12hrs) from measured urinary cortisol concentrations (mean cortisol concentration of a 12hr unit, normalised to creatinine concentration). I need it to use a mathematical cortisol model, of which most published ones only use plasma concentrations, but I only have urinary measures.

Currently I am brute-forcing papers from 50-60 years ago, which doesn't quite help me. Is there anything more recent, or a physiology book that gives me more information about the topic?

Sofar I found that "Urinary free cortisol excretion increases directly in proportion to the plasma unbound cortisol level (Beisel et al, 1964) and provides an integrated measure of plasma free cortisol over time (Burke & Roulet, 1970). (from Carroll76)

Thank you very much.

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