TEST OF GROWTH-HORMONE SECRETION IN ADULTS - POOR REPRODUCIBILITY OF THE INSULIN TOLERANCE-TEST

The insulin tolerance test (ITT) is regarded as the most reliable prov ocative test in the diagnosis of growth hormone (GH) deficiency in adu lts. In the present study the test was evaluated by investigating the range of GH responses in normal adult males and females and the intra- individual reproducibility of the test. Sixteen healthy non-obese adul ts, eight males (median age 31.5 years) and eight females (median age 31.8 years) were tested twice with the ITT, with a minimum of 72 h bet ween each test. The females were tested between day 3 and day 10 of th eir menstrual cycles. Adequate hypoglycemia was achieved in all cases with a median nadir blood glucose of 1.3 mmol/l (range 0.8-2.0). Growt h hormone in serum was measured by immunoradiometric assay and low val ues were confirmed by a different assay. Median peak GK concentration responses to the ITT were: in males 27.9 mu g/l, range 5.0-71.1 (test 1) and 30.5 mu g/l, range 7.9-69.5 (test 2); and in females 9.0 mu g/l , range 4.1-17.9 (test 1) and 8.4 mu g/l, range 0.09-42.4 (test 2), Th e rise in GH concentration during the ITT was higher in males than in females, In the males, all stimulated GH values were greater than or e qual to 5.0 mu g/l. In the females, four out of 16 tests gave values b elow 5.0 mu g/l and in one test the GH value was around the detection limit of the assays, There was poor reproducibility during repeated te sting, with no correlation between the results of the two tests. The r esults did not correlate to the magnitude of the hypoglycemia. The res ults of this study illustrate the complexity of the regulation of GH s ecretion and indicate that the ITT is less useful for diagnosing GH de ficiency in adults than previously anticipated, The diagnosis of GH de ficiency in adults and especially in adult females should not be based on the results of a single ITT alone.