If a dog has a high basal cortisol and normal cortisol levels 4 hours post-ACTH stimulation, what condition is suggested?

In cases where a dog presents with high basal cortisol levels but normal cortisol responses to an ACTH stimulation test four hours post-administration, this finding is suggestive of pituitary-dependent hyperadrenocorticism (Cushing's disease). In this condition, the pituitary gland produces excess adrenocorticotropic hormone (ACTH), leading to an overproduction of cortisol by the adrenal glands.

However, the adrenal glands in this scenario are still capable of responding to ACTH, as evidenced by the high basal cortisol levels. The key point distinguishing pituitary-dependent hyperadrenocorticism in this context is that despite the increased baseline cortisol, the adrenal glands may show a blunted or normal response upon additional stimulation, as seen in post-ACTH cortisol levels that do not significantly rise.

Pituitary-dependent hyperadrenocorticism is the most common form of Cushing's disease in dogs and tends to result in a peculiar pattern in diagnostic tests. In contrast, primary hyperadrenocorticism and adrenal carcinoma would typically produce consistently elevated cortisol levels both at baseline and following stimulation, indicating a deregulated adrenal response. The syndrome of inappropriate ADH secretion is unrelated to cortisol production and would not present with these specific cortisol findings.