The plasma sugar, 11-hydroxycorticosteroid, and growth hormone responses to insulin have been studied in patients with Cushing's disease. They showed an impaired or absent plasma 11-hydroxycorticosteroid and growth hormone rise during the test, as compared with control subjects, despite the injection of amounts of insulin which produced a similar degree of hypoglycaemia. This test proved of value in differentiating between these patients and those with 'simple ' obesity since the latter usually showed a normal growth hormone and adrenal response provided an adequate amount of insulin was administered.
The patients with Cushing's disease also had an impaired adrenal response to pyrogen and to dexamethasone administration and failed to show a normal plasma 11-hydroxycorticosteroid circadian rhythm. Their response to corticotrophin, lysine vasopressin, and metyrapone, however, was normal or enhanced. It is suggested that these findings imply an abnormality of hypothalamic or cerebral control and not a primary defect of pituitary function as proposed originally by Harvey Cushing.
The growth hormone response to insulin remained impaired in four out of six patients totally adrenalectomized for Cushing's disease but was normal in three patients adrenalectomized for other reasons. It is suggested that the defect which impairs the adrenal response to insulin may, on occasions, also impair the mechanism normally operative for growth hormone secretion.