Endocrinological tests can often confirm the clinical diagnosis of pituitary adenoma. Serum GH and IGF-1 levels screen for acromegaly. Failure to suppress GH levels after an oral glucose load (oral glucose tolerance test [OGTT]) confirms the diagnosis. Although any macroadenoma may cause moderate increases in serum PRL, levels greater than 200 ng/ml (3600 mU/l) are highly suggestive of a prolactin secreting adenoma.

Endocrinologic studies that suggest Cushing's disease include an elevated 24-hour urine free cortisol (UFC), loss of the normal daily variation in cortisol levels, and suppression of serum cortisol levels after high dose dexamethasone but failure to suppress after low dose dexamethasone. Petrosal vein sampling after corticotropin-releasing hormone (CRH) stimulation may be required to confirm and localize the pituitary source. At times, prior to diagnosing Cushing's disease, other ectopic sources of excess ACTH, such as bronchogenic or pancreatic carcinoma and pulmonary carcinoid tumors, must be excluded. This can usually be accomplished with a CT scan of the chest and abdomen. Obesity, alcoholism, and depression also elevate serum cortisol levels and the diagnosis of Cushing's disease should be made with caution in these settings.