We present a patient with severe Cushing's syndrome secondary to an invasive pituitary tumour. Cortisol secretion fluctuated between severe hypercortisolaemia associated with features of Cushing's syndrome to recurrent symptomatic hypocortisolaemia. Normal ACTH precursors to ACTH ratio during the various stages of cortisol secretion indicated that periodic decreased prohormone processing did not contribute to the unique presentation in the patient. Thus, the the extreme fluctuations of plasma cortisol levels are most probably due to unexplained erratic changes of ACTH secretion.
