What is the purpose of the cosyntropin (ACTH) stimulation test and how is its result interpreted?

• A. It assesses adrenal cortex reserve; Insufficient cortisol rise (<18–20 µg/dL) after ACTH suggests adrenal insufficiency; a normal rise excludes primary insufficiency.
• B. It measures aldosterone response.
• C. It tests pituitary function.
• D. It assesses catecholamine secretion.

Answer: (A)

The test is used to check how well the adrenal cortex can respond to ACTH by measuring its cortisol output after giving a synthetic ACTH dose. In a normal adrenal response, cortisol should rise to a peak above about 18–20 µg/dL after stimulation. If the cortisol fails to rise to that level, it indicates adrenal insufficiency, meaning the adrenal cortex isn’t able to produce adequate cortisol. If the cortisol does rise normally, it suggests the adrenal cortex is capable and makes primary adrenal disease unlikely, pointing toward a pituitary or hypothalamic issue as the source of insufficiency rather than the adrenal glands themselves. The procedure typically involves baseline cortisol measurement, administration of cosyntropin (ACTH), and a check of cortisol 30–60 minutes later to assess the peak response.