What a cortisol test can tell you about your health

Cortisol is often associated with stress, but this hormone has several important functions in the body. It helps regulate blood pressure, metabolism and the immune response, while also helping the body respond to stressful situations. When there is a medical reason to check whether the body is producing too much or too little cortisol, a cortisol test can provide useful information.

Cortisol levels naturally change throughout the day. This means that the type of test and the time at which a sample is taken can both affect the result. So what exactly does a cortisol test measure, and when can testing be useful?

What is cortisol?

Cortisol is a hormone produced by the adrenal glands, which are located just above the kidneys. It is sometimes called the “stress hormone” because cortisol production can increase when the body is under physical or emotional stress.

Stress is only one part of its function. Cortisol also helps control blood pressure, affects how the body uses carbohydrates, fats and proteins, and plays a role in regulating the immune system.

The amount of cortisol in the body changes naturally over the course of the day. Levels are usually highest in the morning and become lower later in the day. Doctors take this daily pattern into account when arranging and interpreting cortisol tests.

What does a cortisol test measure?

A cortisol test measures the amount of cortisol in the body. This can be done using a sample of blood, saliva or urine.

There are three common methods:

• Blood testing: measures the amount of cortisol in the blood at a particular time.
• Saliva testing: measures cortisol in saliva and can be useful when samples need to be collected at specific times.
• Urine testing: usually involves collecting urine over 24 hours to measure cortisol production over a longer period.

The most suitable method depends on the reason for testing. In some cases, a doctor may request samples at different times or use more than one type of test.

Why are cortisol levels tested?

Doctors may recommend cortisol testing when symptoms or other test results suggest that the body could be producing too much or too little cortisol.

High cortisol levels can occur in people with Cushing’s syndrome. Low cortisol levels may be associated with Addison’s disease or other forms of adrenal insufficiency.

Symptoms of these conditions vary. Fatigue, muscle weakness, changes in weight and changes in blood pressure can occur, but these symptoms are not unique to problems with cortisol. They can have many other causes.

For this reason, a cortisol test is normally used as part of a wider medical assessment. The result needs to be considered together with a person’s symptoms, medical history and any other relevant tests.

What is the difference between blood, saliva and urine tests?

Each testing method provides slightly different information.

A blood test measures the cortisol level at the time the blood is taken. Because cortisol levels change during the day, the timing of the sample matters. Blood may therefore be taken at a specific time, or samples may be taken at more than one point during the day.

Saliva samples can also be collected at specific times. Some saliva tests are performed later in the day, when cortisol levels would normally be lower.

A 24 hour urine test measures cortisol released into the urine over a full day. Rather than looking at a single moment, it provides information about cortisol production over a longer period.

The method used will depend on what the doctor wants to investigate.

What can affect the result?

Several factors can influence cortisol levels. Physical and emotional stress can raise cortisol, as can vigorous exercise. Sleep patterns and certain medicines may also affect the result.

This makes it important to follow the instructions provided with the test. Someone having a cortisol test may be asked to collect a sample at a particular time or avoid strenuous exercise beforehand.

It is also important to tell the doctor or healthcare professional about any medicines being used. Some medicines can change cortisol levels or interfere with the test. Prescribed medication should not be stopped unless a healthcare professional has advised it.

How are cortisol test results interpreted?

There is no single cortisol level that applies to every test and every person. Reference ranges can differ between laboratories, and the expected result may depend on the time of day and the type of sample being tested.

A result outside the expected range does not automatically reveal the cause. Further testing is often needed to find out why cortisol levels are unusually high or low.

This is also why interpreting a cortisol result without medical context can be misleading. The number itself is only one part of the assessment.

What can a cortisol test tell you?

A cortisol test can help doctors investigate how the adrenal glands and related hormone systems are functioning. Blood, saliva and urine tests can all be used, depending on the information that is needed.

Testing can be valuable when there is a medical reason to investigate abnormal cortisol production. The result is most useful when it is interpreted alongside symptoms, medical history and other test results. If cortisol levels are outside the expected range, a healthcare professional can determine whether further investigation is needed.