Cortisol is used to help assess adrenal function in diseases such as Cushing's syndrome and Addison's disease. Plasma cortisol levels are preferably drawn before 10 am (highest level) and after 5 pm (about half 10 am level). Pregnancy or estrogen treatment markedly elevates cortisol levels. Other stimuli such as severe stress may also lead to increased cortisol production. Prescribed medications such as fludrocortisone give positive interference with the test.
Source:
Abbott Alinity Cortisol Package Insert. Ref.: G71180R03. March 2017.