Cortisol is a steroid hormone produced by the adrenal gland in the adrenal cortex. The cortisol test is a preventive measure that can show problems with the pituitary or adrenal glands. Overactive adrenal glands, indicated by high levels of cortisol in the blood, are a sign of Cushings syndrome and may indicate an adrenal tumor. Low levels can be an indication of Addison's disease or hypopituitarism. Cortisol levels increase as the pituitary gland releases the adrenocorticotropic hormone. Therefore, high cortisol levels may also indicate a problem with the pituitary gland. Without proper levels of cortisol, the body may not be able to process sugar and fat correctly or to manage stress adequately. 

It is important to maintain optimal levels of cortisol, as the hormone affects many different body systems. Cortisol is important to bone health, the circulatory system, metabolism, the immune system and the nervous system. 

Cortisol levels typically are highest in the morning and lower at night a few hours before the body is ready for sleep. The test should ideally be performed once in the morning and once in the evening to get an ideal reading. Note that high stress levels, injury, activity and some prescription medications can affect the results of the cortisol blood test.