Which set of baseline labs is obtained in suspected adrenal insufficiency?

• A. TSH, Free T4, TSH receptor antibodies
• B. CBC, CMP, LFTs
• C. ACTH, cortisol, renin
• D. Glucose, insulin, C-peptide

Answer: (C)

When adrenal insufficiency is suspected, you want to assess both the cortisol-producing capacity and the regulation of the adrenal axis, including mineralocorticoid function. Morning cortisol tells you how much cortisol the adrenals can produce, and ACTH shows whether that production is driven by the pituitary or if the adrenals themselves are failing. In primary adrenal insufficiency (Addison disease), the adrenals fail and cortisol is low while ACTH is elevated; in secondary or tertiary causes, cortisol is low with inappropriately normal or low ACTH. Measuring renin adds crucial information about the mineralocorticoid pathway: in primary adrenal failure, aldosterone production drops and renin activity rises due to loss of volume and sodium, whereas in secondary causes, renin is typically not elevated because the aldosterone axis is preserved. So, baseline testing that best characterizes the suspected adrenal problem includes ACTH, cortisol, and renin.