Cortisol midnight plasma

The midnight plasma cortisol determination and combined dexamethasone suppression plus CRH test are used less commonly. 

The presence of hypercortisolism can be established with a midnight plasma cortisol, late-night (11 PM) salivary cortisol, 24-hour urine free cortisol, and/or low-dose dexamethasone suppression test. 

Most of the cortisol in the plasma is carried by its transport protein, transcortin and albumin. The remainder is in the free state and this is the physiologically active fraction. Cortisol levels in the plasma exhibit a circadian rhythm being lowest at about midnight and highest at about 9 a.m. 

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