Cortisol, 6 Specimens

Cortisol secretion follows a pronounced circadian rhythm driven by the suprachiasmatic nucleus and central hypothalamic–pituitary–adrenal axis. In healthy individuals cortisol peaks within 30–60 minutes of waking — the so-called cortisol awakening response — and then declines progressively across the day. Morning cortisol reflects the zenith of this rhythm and is the most sensitive single time point for detecting primary adrenal insufficiency, though it lacks specificity unless very low.

The 08:00 cortisol is the anchor of the six-specimen series. Values below approximately 100 nmol/L (3.6 µg/dL) raise concern for adrenal insufficiency and typically prompt dynamic testing, while values above 500 nmol/L (18 µg/dL) make the diagnosis unlikely. Isolated morning elevation may be physiological stress or early Cushing syndrome; serial sampling across the day reveals whether the normal diurnal decline is preserved.

By midday the initial morning peak has begun to decline, with cortisol levels typically 30–50 per cent lower than the 08:00 value. This time point captures the descending limb of the diurnal rhythm and adds a reference for the rate of decline.

The midday specimen helps distinguish a preserved diurnal rhythm from pathological flattening. In Cushing syndrome cortisol may remain inappropriately elevated at noon, while in primary adrenal insufficiency the already low morning level falls further or remains flat.

The late-afternoon cortisol lies on the descending slope of the circadian curve, typically 40–60 per cent of the morning peak. It provides an intermediate checkpoint before the evening nadir.

A 16:00 cortisol that remains high relative to the morning value suggests loss of diurnal rhythm, a hallmark of autonomous cortisol secretion. Conversely, an inappropriately low afternoon level in the setting of symptoms points to inadequate adrenal reserve.

Evening cortisol nears the diurnal nadir, typically less than 30 per cent of the morning peak. The normal evening decline is an important marker of hypothalamic–pituitary–adrenal integrity.

Loss of the evening nadir is an early biochemical sign of Cushing syndrome — many guidelines use a late-night salivary cortisol for this purpose, but serum 20:00 cortisol serves a similar role in a six-point series.

By 23:00 cortisol is at or near its circadian nadir, reflecting quiescence of the HPA axis before the overnight rise that will culminate in the next morning peak. This is one of the most sensitive time points for detecting loss of rhythm.

A 23:00 cortisol that remains elevated is highly specific for Cushing syndrome when the specimen is properly timed and the patient is at rest. It is the serum analogue of late-night salivary cortisol.

Cortisol begins to rise in the early hours of sleep, driven by pulsatile ACTH release, so a 03:00 specimen captures the very start of the ascending limb toward the morning peak. In normal physiology it remains low but is beginning to climb from the nadir.

An inappropriately elevated 03:00 cortisol confirms that the HPA axis is active throughout the night, a pattern seen in Cushing syndrome. Conversely, a persistently low or flat overnight curve may reflect blunted adrenal reserve.

The diurnal cortisol slope is a derived metric calculated from the six time points, quantifying the rate and completeness of the circadian decline. A steep slope indicates robust HPA rhythm; a flat or positive slope signals pathological loss of diurnal variation.

The slope integrates all six specimens into a single interpretable metric. Flattening of the curve is seen in Cushing syndrome, chronic stress, depression, and sleep disorders, while a preserved steep slope argues against autonomous hypercortisolism.