Approximately 90% of the all circulating cortisol is bound to Corticoid Binding Globulin (CBG, Transcortin), while 7% is bound to albumin. This leaves less than 3% free (unbound), which represents the active form of Cortisol.
In saliva, there is a diurnal fluctuation of the cortisol concentration in adult humans, reaching a peak between 30-60 minutes after waking. Levels begin to fall lower levels throughout the afternoon, reaching their lowest levels prior to bedtime. This response can be either enhanced or diminished during periods of chronic stress, exhaustion, depression as well as other clinical disorders.
Salivary Cortisol measurement is relevant in diseases with abnormal glucocorticoid production, such as Addison’s Disease and Cushing Syndrome.