Salivary cortisol: A useful measurement in the diagnosis of Cushing's syndrome and the evaluation of the hypothalamic-pituitary-adrenal axis

One of the hallmarks of the hypothalamic-pituitary-adrenal (HPA) axis is a distinctive circadian rhythm with a peak in cortisol at 8 a.m. and a nadir around midnight. Because patients with en dogenous hypercortisolism have an increase in the nadir in the circadian rhythm of cortisol, the ability to demonstrate elevated late night cortisol secretion has proved useful in the diagnosis of Gushing's syndrome. Salivary cortisol is in equilibrium with, and is an excellent reflection of, free, biologically active cortisol in t he plasma. Sampling saliva is simple and can be performed by patients at ho me, and at ages ranging from infants to the very old. Therefore, assessment of salivary cortisol may prove useful in screening for, and the diagnosis of, endogenous hypercortisolism. Several studies have demonstrated that an elevated salivary cortisol late in the evening (e.g., 11 p.m.) suggests the presence of Gushing's syndrome. Other studies have also used salivary cort isol measurement to identify and characterize patients with intermittent Gu shing's syndrome and with endogenous depression, as well as to document act ivity of the HPA axis in a wide array of studies in normal subjects (e.g., psychoneuroendocrinology). The simplicity of sampling salivary cortisol wit hout the inconvenience of collecting timed urine samples and the stress of blood sampling makes it an ideal and indispensable tool in the evaluation o f patients with suspected Gushing's syndrome.