Background: Under ordinary circumstances, the daily secretion of cortisol displays a characteristic diurnal variation with a peak in the morning and a trough at night. It is important to understand the variation in cortisol secretion when analysing results of cortisol tests.
Patient and method: We present a patient in whom work-up for acute headache revealed a pathologically low morning serum cortisol level. This led to further evaluation of adrenocortical function with an adrenocorticotropic hormone (ACTH) stimulation test and cortisol day profiles in serum and saliva.
Results and interpretation: The analyses showed normal adrenocortical function and diurnal variation with normal amplitudes. The morning rise in cortisol secretion was, however, delayed by 7-8 hours due to a delayed sleep phase. This case illustrates the importance of the diurnal rhythm in the assessment of pituitary-adrenal function. There should be clinical suspicion of a disorder before a test is requested.