Endocrine Abstracts

The insulin tolerance test (ITT) is used to determine the integrity of the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-somatotroph (GH) axes in patients suspected of ACTH and GH deficiency. In adults a cortisol response <500nmol/L is considered pathological and a GH peak <9mU/L is consistent with severe GH deficiency. Exercise is an alternative stimulus to GH release which may be used to diagnose GH deficiency during childhood but is rarely used during adult life. Diagnostic thresholds may be arbitrary or based on 'historical' assays.

We have studied 12 healthy male subjects aged 21 - 36 (median 23.5) years, BMI 24.2 (22.3 - 26.0) kg/m². Each subject, studied on two occasions, underwent an ITT (Actrapid 0.15 U/kg) during which blood was drawn every 15 minutes for two hours and an exercise stress test, exercising at 60% of peak power in an ambient temperature of 35C for one hour, blood drawn every 20 minutes for 100 minutes. Serum GH and plasma ACTH were measured using immunometric assays (Immulite 2000, DPC, Llanberis).

During the ITT, the peak (median (range)) GH, ACTH and cortisol responses were 99.2 (48 - 356) mU/L, 208.5 (96.4 - 509) ng/L and 741 (581 - 1229) nmol/L. The increment changes were: GH 98 (35.1 - 356) mU/L, ACTH 197 (89 - 496) ng/L and cortisol 530 (346 - 1036) nmol/L. During the exercise test Peak responses were GH 58.2 (8.1 - 156) mU/L, ACTH 70.2 (16.5 - 200) ng/L and cortisol 708 (440 - 1125) nmol/L. Incremental changes during exercise were: GH 58.0 (7.8 - 156)mU/L, ACTH 46.3 (3.8 - 183) ng/L and cortisol 208 (107 - 507) nmol/L. There was a positive correlation between exercise induced ACTH and cortisol peaks (r = 0.82, p = 0.002). No other relationships were evident.

This study provides normal pituitary response data in young men undergoing insulin induced hypoglycaemia and exercise.