What laboratory test is commonly used to evaluate adrenal function in patients suspected of having Addison's disease?

The serum cosyntropin test, often referred to as the ACTH stimulation test, is a key diagnostic tool for evaluating adrenal function, particularly in cases of suspected Addison's disease. This test assesses how well the adrenal glands respond to adrenocorticotropic hormone (ACTH), which is released from the pituitary gland. In a healthy individual, administration of cosyntropin, a synthetic form of ACTH, should stimulate the adrenal glands to produce cortisol.

In patients with Addison's disease, however, the adrenal glands are damaged or inactive, resulting in little to no cortisol production after cosyntropin administration. Consequently, cortisol levels will remain low, which confirms the diagnosis of adrenal insufficiency due to primary adrenal failure seen in Addison's disease.

Other tests listed do have roles in evaluating adrenal function but are not specifically used as the first-line test to diagnose Addison's disease. The 24-hour urine cortisol test evaluates cortisol levels over time but is not as direct in assessing the adrenal response to ACTH. Plasma aldosterone testing can be useful in certain contexts of adrenal evaluation, particularly regarding hyperaldosteronism, but it is not primarily focused on Addison's disease. Lastly, thyroid function tests assess thyroid hormone levels and are not