A comparison between short ACTH and insulin stress tests for assessing hypothalamo-pituitary-adrenal function.

OBJECTIVE: Insulin-induced hypoglycaemia is the standard method for assessment of the hypothalamo-pituitary-adrenal (HPA) axis of patients with pituitary or hypothalamic disease. It has been claimed that a normal cortisol response to the 30-minute ACTH stimulation test (AST) obviates the need to perform the insulin stress test (IST) in these patients. The objective of our study was to compare both tests in a group of consecutive patients with pituitary disease. SUBJECTS AND METHODS: Thirty patients with pituitary disease were evaluated by standard IST (0.1 U of soluble insulin/kg body weight, i.v.) after fasting from midnight and AST (250 micrograms synacthen, i.v.). In the IST, a plasma glucose of < 2.2 mmol/l was taken as the hypoglycaemic threshold and blood was collected at 0, 30, 60, 90 and 120 minutes. In the AST blood was collected at 0 and 30 minutes. Serum cortisol was measured by standard radioimmunoassay and glucose by the glucose oxidase method. Cortisol responses to the stimuli were compared at cut-off levels of 550, 500, 450 and 400 nmol/l. RESULTS: At 550 nmol/l, out of 30 patients, 17 showed an abnormal IST of whom 9 had normal responses to AST (53%). At 500 nmol/l, 12 patients had an abnormal IST of whom 6 had normal AST (50%). At 450 nmol/l, of 9 patients with an abnormal IST, 5 had a normal AST (56%). At 400 nmol/l, 5 patients had an abnormal IST all of whom (100%) showed a normal AST. CONCLUSION: There is a clear discrepancy between the results of the two tests at different cortisol cut-off levels. The ACTH stimulation test is not reliable for assessing the HPA axis in patients with pituitary disease and the insulin stress test remains the standard method.

Development and validation of a two-site immunochemiluminometric assay for rat growth hormone-releasing hormone.

Abstract We describe the development and validation of a two-site immunochemiluminometric assay for rat growth hormone-releasing hormone (GHRH) based on the affinity purification of polyclonal rabbit antisera to rat GHRH using a human 1-29 GHRH affinity column. Assay sensitivity is 3.2 pg/ml using 100 mul of unextracted sample and the working range for the assay within 15% confidence limits is 64 to 5,000 pg/ml. Rat hypothalamic extract and secreted material demonstrated a single large peak of immunoreactive material coeluting with synthetic rat GHRH on high-performance liquid chromatography with a smaller, earlier peak which probably represents methionine sulphoxide [Met(O)(27)] GHRH. Extracted material diluted in parallel to the standard curve. Incubated rat hypothalami readily released measurable amounts of rat GHRH which responded appropriately to depolarization with 60 mM K(+) in a Ca(2+)-dependent manner.