Plasma ACTH in diagnosis and control of adrenal disorders

Unstimulated plasma ACTH concentrations remain at or below the detection limit of conventional immunoassays. Grossly elevated ACTH concentrations are diagnostic in suspected adrenal insufficiency, remain elevated well above 200 ng/l during substitution therapy and obviate the need of further tests. For the diagnosis of secondary adrenal failure, plasma ACTH, cortisol and 11-desoxycortisol response to a single midnight dose of metyrapone (1.2 g/m2 = 30 mg/kg) discriminates between a normal (morning ACTH above 100 ng/l), diminished (morning ACTH detectable, but below 100 ng/l), and an absent (ACTH below 20 - 40 ng/l) ACTH reserve. In congenital adrenal hyperplasia, plasma ACTH concentrations mirror, together with 17-alpha-hydroxyprogesterone, the extent of ACTH suppression. Elevated ACTH concentrations were suppressed by prednisolone (25%), dexamethasone (2% of the hydrocortisone dose) or by addition of cyproterone acetate (100 mg/m2/day). Using selective venous catheterisation in clinically and biochemically proven Cushing's syndromes, a pituitary adenoma could be identified and localized in 6 of 8 patients by measuring ACTH concentrations in the left and right petrosal sinus before and after stimulation with corticotrophin releasing hormone.