A retrospective audit of the combined pituitary function test, using the insulin stress test, TRH and GnRH in a district laboratory

Abstract

Objective: To assess the value of the combined insulin stress test (IST), thyrotrophin-releasing hormone (TRH) and gonadotrophin hormone-releasing hormone (GnRH) tests.

Design: A retrospective audit of 232 such tests performed between 1980 and 1989 inclusive.

Patients: One hundred and ninety-seven patients with known or suspected pituitary disease.

Measurements: IST, TRH and GnRH responses were retrieved from laboratory records. Case notes were surveyed for clinical data and additional results.

Results: A basal serum cortisol level of less than 100 nmol/l (or less than 200 nmol/l in patients who had recently received glucocorticoid replacement therapy) accurately predicted a subnormal response to hypoglycaemia. All patients with a basal cortisol level of greater than 400 nmol/l, except those who had recently received steroids, showed a normal cortisol response. In retrospect, by consideration of such basal values, 55% of ISTs could have been avoided if the only aim was to assess cortisol reserve. A deficient growth hormone (GH) response to hypoglycaemia was, however, common in patients with a normal cortisol response. Two-thirds of patients with GH deficiency would have been missed if an IST had been avoided on the basis either of basal cortisol levels alone, or of cortisol responses to an alternative test which did not test GH reserve. There was poor agreement between the pituitary response to TRH and GnRH and basal levels of thyroxine and gonadotrophins respectively, suggesting that these releasing hormone tests are misleading.

Conclusions: The IST provides information regarding pituitary function not provided by other tests of the hypothalamic-pituitary-adrenal axis, so that the choice between the IST and alternative tests must depend on a critical assessment of what information is required. Routine TRH and GnRH testing appears to yield little information of practical clinical value.