Which test is used for initial screening of adrenal insufficiency?

The correct choice for the initial screening of adrenal insufficiency is the low-dose ACTH stimulation test. This test assesses the adrenal glands' ability to respond to adrenocorticotropic hormone (ACTH). In cases of primary adrenal insufficiency (such as Addison's disease), the adrenal glands are unable to produce sufficient cortisol, leading to a blunted or absent response when stimulated with ACTH.

In the context of evaluating adrenal function, the low-dose ACTH stimulation test measures the increase in serum cortisol levels after administration of a low dose of synthetic ACTH. A normal response suggests intact adrenal function, whereas a lack of response indicates possible adrenal insufficiency.

Other tests mentioned, like the corticotropin-releasing hormone stimulation test and the high-dose ACTH stimulation test, are used for specific scenarios or differential diagnoses but are not suitable for initial screening. The 24-hour urinary free cortisol test measures cortisol excretion, which is more useful in diagnosing Cushing's syndrome rather than adrenal insufficiency.

Thus, the low-dose ACTH stimulation test serves as an effective and appropriate initial screening tool for adrenal insufficiency by directly assessing the functionality of the adrenal glands in response to ACTH stimulation.