Serum but not salivary cortisol levels are influenced by daily glycemic oscillations in type 2 diabetes

Diurnal salivary and plasma cortisol variations are considered valid expression of circadian cortisol rhythmicity. The aim of this study was to assess the reliability of salivary and plasma cortisol and if glycemia and glycemic oscillations may interfere with their concentration. Forty-seven type 2 diabetic patients and 31 controls were studied for glycemic profile and diurnal salivary and plasma cortisol variations on two contemporary samples taken at 08:00 a.m.-11:00 p.m (Late Night, LN). Glucose variability was evaluated in diabetic patients by considering the standard deviation of blood glucose (BGSD) readings, by calculating the mean amplitude of glycemic excursions (MAGEs) and continuous overlapping net glycemic action (CONGA). A significant correlation between LN serum cortisol and morning fasting glycemia (r = 0.78; p = 0.004) was observed in T2DM group but not in the control group (r = 0.09; p = 0.74). While LN serum cortisol significantly correlated with CONGA in diabetic patients (r = 0.50; p < 0.001), LN salivary cortisol did not correlate with any indices of glucose variability. Moreover, a highly significant correlation between LN salivary and LN serum cortisol concentrations was found in control group (r = 0.80; p < 0.001) but not in diabetic patients (r = 0.07; p = 0.62). This study shows for the first time that LN salivary rather than plasma cortisol may give information on the dynamics of adrenal function of type 2 diabetic patients, as it is not significantly influenced by glycemic variations. However, our preliminary results need to be confirmed by further studies with more complete evaluations including many more patients.