Measurement of midnight ACTH levels is useful for the evaluation of midnight cortisol levels.

OBJECTIVE

Elevated midnight cortisol levels induced by non-suppressed ACTH levels may lead to false-positive results for hypercortisolism in patients with adrenal incidentaloma. We investigated whether plasma ACTH-associated high midnight serum cortisol levels are correlated with other endocrinological findings with respect to hypothalamic-pituitaryadrenal function or hypercortisolism status.

METHODS

Two-hundred-forty-six patients with adrenocortical adenoma were evaluated via measurements of midnight ACTH and cortisol levels, a 1-mg dexamethasone suppression test (DST), and a cosyntropin-releasing hormone (CRH) stimulation test. Patients were divided into four groups according to their midnight plasma ACTH levels.

RESULTS

The groups with higher midnight ACTH levels had significantly higher basal ACTH levels. A positive relationship was observed between midnight serum cortisol and serum cortisol in the 1-mg DST for all groups; stronger associations were observed in the group with lower midnight ACTH. In the CRH test, peak, delta, and sigma ACTH had significant inverse relationships with midnight cortisol levels in the lowest and second lowest midnight ACTH groups. Patients with midnight cortisol levels >3.5 μg/dL were further divided into two groups according to whether their midnight plasma ACTH levels were below or above 10.0 pg/mL. There were significantly fewer patients with hypercortisolism in the higher ACTH group; midnight serum cortisol levels were associated with hypercortisolism only in the lower ACTH group.

CONCLUSION

We demonstrated that midnight ACTH-associated cortisol values were not correlated with other endocrinological findings or hypercortisolism state. Measurement of midnight ACTH levels is important, and careful evaluation is needed for patients with higher midnight ACTH levels.