Several methods are available to carry out the differential diagnosis between an adenoma and hyperplasia. The simplest test is to take blood at 0800, with the patient recumbent, in order to determine aldosterone levels, plasma renin activity, and cortisol levels, and then to repeat the blood sampling at 1200 after the patient has been standing upright for four hours. In patients with aldosterone-secreting adenomas, plasma renin activity is suppressed and plasma aldosterone levels decrease during the course of the morning as ACTH levels fall; the adenoma is sensitive to ACTH. Conversely, in patients with idiopathic bilateral hyperplasia, the hyperplastic adrenals are very sensitive to angiotensin II and hence the small amount of renin that is released on standing is sufficient to elevate aldosterone levels.
