Insulin Tolerance Test
A diagnostic test for growth hormone and cortisol deficiency where insulin is injected to induce hypoglycaemia, then GH and cortisol responses are measured. It is considered the gold standard for diagnosing adult GH deficiency but carries risks and requires careful medical supervision.
Technical Context
ITT protocol: 0.1-0.15 U/kg regular insulin IV → adequate hypoglycaemia must be achieved (glucose <40 mg/dL or <2.2 mmol/L with symptoms) → GH and cortisol measured at 0, 30, 60, 90, 120 minutes. Normal responses: GH ≥5 μg/L (some use ≥3 μg/L cutoff), cortisol ≥18-20 μg/dL. ITT advantages: tests both GH and cortisol axes simultaneously, gold standard for adult GHD diagnosis. Contraindications: epilepsy, ischaemic heart disease, adrenal insufficiency (known — could be dangerous), and elderly patients. Safety requirements: physician supervision throughout, IV glucose immediately available, minimum glucose of 40 mg/dL confirmed with point-of-care testing. The ITT has been partially replaced by safer alternatives: GHRH-arginine test for GH axis (but depends on GHRH availability), glucagon stimulation test (for both GH and cortisol — cosyntropin tests cortisol axis separately), and macimorelin test (oral GHS-R agonist specifically for GHD diagnosis, FDA-approved).