Factors determining inadequate hypoglycaemia during insulin tolerance testing (ITT) after pituitary surgery

Background  Some patients fail to achieve adequate hypoglycaemia following a standard dose of intravenous insulin during the insulin tolerance test (ITT). Persistent acromegaly or Cushing's disease may contribute to inadequate hypoglycaemia.
Aim  To identify factors that predict failure to achieve adequate hypoglycaemia during an ITT after pituitary surgery.
Methods  We reviewed consecutive ITTs performed over a 10-year period in 76 patients following pituitary surgery. Analyses were performed to determine if body mass index (BMI), fasting blood glucose (FBG), cortisol, GH status or underlying diagnosis influenced the outcome.
Results  Adequate hypoglycaemia (blood glucose < 2·2 mmol/l) was not achieved in 33 patients (Group 1) following a standard dose of neutral insulin (0·1 units/kg); 43 patients (Group 2) achieved adequate hypoglycaemia. Group 1 had significantly higher BMI, FBG, baseline cortisol and peak cortisol concentrations than Group 2. Peak GH response was not different. Multiple regression analysis showed that FBG was the only independent predictor of adequate hypoglycaemia. An insulin dose of 0·2 units/kg achieved adequate hypoglycaemia in 80% of patients with FBG ≥ 5·5 mmol/l. In patients with acromegaly or Cushing's disease, failure to achieve adequate hypoglycaemia was associated with persistent disease.
Conclusion  FBG is an important determinant of the dose of insulin required to achieve adequate hypoglycaemia during an ITT in patients after pituitary surgery. A standard insulin dose of 0·1 U/kg is insufficient for adequate hypoglycaemia in patients with FBG > 5·5 mmol/l. Adequate response to a standard dose of insulin suggests a likelihood of cure of acromegaly or Cushing's disease after pituitary surgery.