Cortisol, Free Unit Converter

Free cortisol is the small fraction of cortisol in your blood that circulates unbound to proteins. Only about 3–5% of total blood cortisol is free; the rest is bound to corticosteroid-binding globulin (CBG) and albumin.

Because only the unbound fraction can enter cells and exert biological effects, free cortisol is considered a more accurate reflection of true adrenal function than total cortisol. This is especially important when protein levels are abnormal, such as during pregnancy, critical illness, or use of oral contraceptives.

Cortisol follows a strong diurnal rhythm, peaking in the early morning and declining throughout the day. Reference ranges for free cortisol are therefore specific to the time of collection.

Cortisol is produced by the zona fasciculata of the adrenal cortex, the outer layer of the adrenal glands located above each kidney. Its release is controlled by the hypothalamic-pituitary-adrenal (HPA) axis: the hypothalamus secretes CRH, which stimulates the pituitary to release ACTH, which in turn signals the adrenals to produce cortisol.