The insulin tolerance test (ITT) is a reproducible test for assessment of growth hormone (GH) and cortisol secretion in hypopituitary patients

Abstract

Listen

Translate article

The Insulin Tolerance Test (ITT) is the gold-standard for assessment of GH and ACTH reserve. GH and cortisol responses of <9mU/l and <500nmol/l respectivly have been defined as evidence of deficiency. Using these criteria we studied the reproducibility of the ITT in patients with pituitary surgery±radiotherapy. Two ITTs were performed within 7 days in 15 patients (7 men, age 22–51, 6 patients with GHD and 5 patients with cortisol deficiency). Serum GH and cortisol were measured by immunoradiometric and immunofluorimetric assay, respectivly. The CV of the assay for cortisol and GH was 5.8% and 5.9%. All patients achieved a hypoglycemia <2.2mmol/l and were symptomatic. In our series the median GH response was 13.2±18.1 mU/l with a range of 0–84.3 mU/l. The mean difference in the peak GH response between two tests was 3.6±1.3ng/ml. In 3/15 cases GH all values were undetectable on both occasions. In 2/12 cases the detected values were within 1 CV of the assay, in 3/12 cases within 2 CVs of the assay and in 7/12 cases the observed value was ≥ than 2 CVs of the assay. There was 1 case of clinical importance where the observed values around 9ng/ml was discordant in terms of defining GHD and could have led to misclassification and different therapeutic decision. The median cortisol response was 544±210 nmol/l with a range of 262–1012 nmol/l. The mean difference in the peak cortisol response was 84.8±21.3 nmol/l. In 4/15 cases the detected values were within one CV of the assay and in 5/15 cases within 2 CVs of the assay. In 6/15 cases the observed difference was bigger than explicable by 2 CVs of the assay. 5/15 cases showed conflicting values in relation to the cut off point of 500 nmol/l and were discordant in terms of defining cortisol deficiency. In our series of patients with pituitary disease conflicting GH and cortisol results during ITT were observed in 7% and 30% of patients, respectively. This shows a good reproducibility for the GH response. There was only a low number of cases of misclassification by the arbitrary definition of GH and cortisol deficiency.