How to Fast Before Test for Cortisol Levels

Cortisol levels are tested to aid in diagnosing Cushing's syndrome and Addison's disease, both of which are adrenal gland diseases. Cortisol levels higher than normal are a sign of Cushing's syndrome and cortisol levels lower than normal are a sign of Addison's disease. In healthy people, cortisol levels are highest upon awakening and lowest before bedtime. Normal morning cortisol levels range from 6 to 23 mcg/dL according to "Mosby's Manual of Diagnostic and Laboratory Tests."

A doctor is analyzing a blood sample. (Image: psphotograph/iStock/Getty Images)

Step 1

Do not eat or drink anything containing calories upon waking up the morning of your test. Water, plain tea, plain coffee and diet soft drinks are the only acceptable beverages.

Step 2

Drink water upon waking up, both to maintain good hydration and to avoid an unpleasant testing experience. Dehydration causes veins to shrink, therefore making it more difficult for the phlebotomist to draw your blood to test your cortisol levels. Drinking adequate amounts of water will keep your veins plump, making it easier for the phlebotomist to locate a suitable vein and successfully draw your blood.

Step 3

Visit the laboratory to have your cortisol levels tested as soon as possible after waking up for the most accurate results. Cortisol levels naturally decrease as the day progresses, so it is essential that your cortisol levels are tested as early in the day as possible so your doctor can evaluate your cortisol levels at their highest to determine if they are in normal range.