Which imaging or test is the most appropriate next step for diagnosing primary adrenal insufficiency?

The cosyntropin stimulation test is the most appropriate next step for diagnosing primary adrenal insufficiency because it directly assesses the function of the adrenal glands in response to an adrenocorticotropic hormone (ACTH) analog. In primary adrenal insufficiency, the adrenal glands cannot produce adequate cortisol, even when stimulated by ACTH. During this test, cosyntropin is administered, and a lack of significant increase in serum cortisol levels after the stimulation indicates adrenal insufficiency.

This test is specifically designed to differentiate between primary adrenal insufficiency and secondary causes of adrenal insufficiency, which involve insufficient stimulation of the adrenal glands due to low ACTH levels from the pituitary gland. Thus, it provides critical information for a definitive diagnosis of primary adrenal insufficiency.

Other options such as a thyroid ultrasound or measurement of IGF-1 are not relevant for evaluating adrenal function. A dexamethasone suppression test is more appropriate for assessing conditions related to excess cortisol, such as Cushing's syndrome, rather than investigating adrenal insufficiency.