In primary adrenal insufficiency (Addison disease), what would be the expected cortisol level following ACTH stimulation?

• A. Increase from baseline
• B. Decrease from baseline
• C. No change from baseline
• D. Increase briefly then fall

Answer: (C)

The key idea is that the ACTH stimulation test checks whether the adrenal cortex can mount a cortisol response to ACTH. In primary adrenal insufficiency, the adrenal glands are damaged and cannot produce cortisol effectively, so giving ACTH does not provoke a significant rise. The cortisol level after stimulation remains essentially the same as baseline, indicating a blunted or absent response. This contrasts with normal function or with secondary adrenal insufficiency, where the adrenals can respond and cortisol increases after ACTH. So a lack of change from baseline best fits Addison disease, while options suggesting an increase, a decrease, or a transient rise would not reflect the impaired adrenal reserve.