In the context of adrenal disorders, what does a low level of ACTH indicate?

A low level of adrenocorticotropic hormone (ACTH) indicates adrenal insufficiency because it suggests that the adrenal glands are not producing adequate amounts of cortisol. When the adrenal glands are functioning normally, ACTH, which is produced by the pituitary gland, stimulates these glands to release cortisol. If there is low ACTH, it may point to a primary adrenal problem where the adrenal glands themselves are not responsive or are compromised, leading to insufficient hormone production.

In contrast, conditions associated with high cortisol levels, such as Cushing's syndrome, typically show elevated or normal levels of ACTH depending on whether the cause is primary (adrenal) or secondary (pituitary). Normal adrenal function would reflect normal levels of both ACTH and cortisol, indicating that the feedback loop between the pituitary gland and adrenal glands is working effectively. Thyroid overactivity is not directly related to ACTH levels; it's more associated with TSH (thyroid-stimulating hormone) and thyroid hormone production. Therefore, the correlation between low ACTH and adrenal insufficiency highlights the dysfunction that occurs in the adrenal glands themselves.