A man presents with bitemporal hemianopia and facial changes. What test is most definitive for confirming his diagnosis?

The presence of bitemporal hemianopia, coupled with facial changes, suggests the presence of a mass lesion, likely a pituitary adenoma, which can affect adjacent structures such as the optic chiasm. In this context, the most definitive test to confirm the diagnosis of an acromegaly caused by a growth hormone-secreting tumor is the oral glucose tolerance test (OGTT) with serial growth hormone measurements.

During an OGTT, the patient consumes a glucose-rich drink, and in a healthy individual, growth hormone levels should decrease as a response to the hyperglycemia. However, in cases of growth hormone excess, such as with a pituitary adenoma, growth hormone levels will remain elevated despite the glucose challenge. This characteristic response is crucial for diagnosing acromegaly linked to a functional pituitary adenoma, as it provides direct evidence of growth hormone dysregulation.

Other tests mentioned, such as measuring early morning growth hormone or random IGF-1 levels, can provide supportive evidence but are not as definitive. While elevated IGF-1 levels can indicate increased growth hormone secretion over time, they do not show the failure of suppression, which is specifically tested by the OGTT. Prolactin levels, although