Serum or plasma mass concentration of free (unbound) cortisol

Why This Test Matters

Free cortisol is important because it indicates how effectively your adrenal glands are producing cortisol, a hormone that helps regulate stress responses, metabolism, and immune function. Unlike total cortisol, free cortisol isn’t influenced by protein binding, so it provides a clearer picture of your body’s active hormone levels.

How It’s Used in Diagnosis

Doctors may order this test to diagnose conditions like Cushing’s syndrome (too much cortisol), adrenal insufficiency (too little cortisol), or to evaluate cortisol metabolism disorders. It can also help confirm or monitor treatment effectiveness in hormone-related disorders.

Sample Collection and Preparation

This test requires a blood sample drawn from a vein, typically in the morning when cortisol levels are highest. Your blood will be separated into serum or plasma to measure the free (unbound) fraction of cortisol. Avoiding caffeine or stress before the test can help ensure accurate results.

What to Expect Before the Test

No special preparation is needed, but inform your healthcare provider about any medications you’re taking, especially steroids or supplements that may affect cortisol levels. If you have a history of bleeding disorders or are on blood thinners, let your provider know to discuss precautions.