Cortisol is a glucocorticoid hormone (gluco=stimulates levels of glucose (blood sugar) and corticoid=secreted by the adrenal cortex) that is involved in many critical body functions. For most of us, the peak levels of cortisol in our body are reached between 6:00 AM and 8:00 AM, and then it gradually declines until about midnight (so we can get to sleep) and then starts gradually increasing through the night until 8:00 AM (so we can get our day started).

Cortisol affects many different body systems. It plays a role in:
Bones

Circulatory system

Immune system
Metabolism of fats, carbohydrates, and protein
Nervous system

Stress responses

A cortisol test measures the amount of cortisol in the blood and may be ordered when a person has symptoms like:
High blood pressure
High blood sugar

Muscle weakness
Osteoporosis

Irregular menstrual periods

Children developing slower than normal

Weight loss

Fatigue

Cortisol and its functions in the body is further discussed here.

NORMAL RESULTS

If the blood test is done at 8:00 AM the normal values are 6 to 23 micrograms per deciliter (mcg/dL).

HIGHER THAN NORMAL RESULTS

If the results of the cortisol test are higher than normal this may indicate:

Cushing’s Disease

A tumor of the adrenal gland

LOWER THAN NORMAL RESULTS

If the results of the cortisol test are lower than normal this may indicate:

Addison’s Disease
Hypopituitarism