What laboratory finding is indicative of adrenal insufficiency?

The laboratory finding indicative of adrenal insufficiency is characterized by low cortisol levels paired with elevated ACTH levels. This pattern occurs because, in primary adrenal insufficiency (such as Addison's disease), the adrenal glands fail to produce adequate amounts of cortisol. As a result, the pituitary gland responds by producing more adrenocorticotropic hormone (ACTH) in an attempt to stimulate the adrenal glands, leading to elevated ACTH levels.

In the case of secondary adrenal insufficiency, which is due to a lack of ACTH secretion (often from pituitary dysfunction), cortisol levels would be low, but ACTH levels would also be low—not elevated. This distinction further supports that elevated ACTH paired with low cortisol is a hallmark of primary adrenal insufficiency.

On the other hand, high cortisol levels would indicate that the adrenal glands are functioning properly or are overactive, which is not consistent with adrenal insufficiency. Normal cortisol levels with high aldosterone would suggest that the adrenal glands are functioning correctly, particularly their regulatory mechanisms, while high ACTH levels alone does not provide enough information without the context of cortisol production. Therefore, the combination of low cortisol and high ACTH levels provides the most reliable indication of adrenal insufficiency.