Urinary free cortisol
What it measures, and what changes the number.
Cortisol is a steroid hormone made by the adrenal glands. Most cortisol in blood is bound to carrier proteins. The small unbound, or free, portion is filtered by the kidneys into urine, and this test measures that free cortisol in a urine sample.
It is usually interpreted together with blood cortisol and ACTH, which drives cortisol production. Urine creatinine is often measured in the same sample so the cortisol can be expressed relative to how concentrated the urine is.
Cortisol follows a daily rhythm, so a single timed or spot sample reflects only part of the day, and some labs use a 24 hour collection instead. Immunoassays can cross-react with other steroids, while mass spectrometry methods are more specific, so results from different methods are not directly comparable. Reports may call it Cortisol libre en orina.
Often read alongside: Cortisol, ACTH, Urinary free cortisol (24h), Creatinine