Glucagon Stimulation Test

The glucagon stimulation test (GST) is used to assess growth hormone (GH) reserve and the hypothalamic-pituitary-adrenal axis in adults with suspected hypopituitarism. Intramuscular glucagon causes a rise in plasma glucose followed by a fall, and indirectly stimulates the release of GH and ACTH and cortisol, though the exact mechanism is not fully known.

It is safer and better tolerated than the insulin tolerance test, and can be done in patients for whom hypoglycaemia is hazardous (older adults, heart disease, epilepsy). Diagnostic performance is slightly lower than for the insulin tolerance test, especially in obesity and diabetes.

Adult growth hormone deficiency affects 2 to 3 in every 10 000 adults and is usually acquired from pituitary tumours or their treatment, cranial irradiation, head trauma or infiltrative disease. Secondary adrenal insufficiency may accompany it. The GST is widely used where macimorelin is not available, and the test requires 3 to 4 hours.

Indications. Suspected adult GH deficiency (pituitary disease, other pituitary hormone deficiencies, previous cranial radiotherapy, childhood onset GH deficiency retesting at transition). Evaluation of the hypothalamic-pituitary-adrenal axis when the insulin tolerance test is contraindicated. Assessment of secondary adrenal insufficiency in patients on chronic glucocorticoids in selected cases.

Contraindications and cautions. Prolonged fasting over 48 hours, severe malnutrition, phaeochromocytoma, insulinoma and adrenal insufficiency with low baseline cortisol (a safe dose of hydrocortisone is arranged). Nausea and vomiting are common. In diabetes, glucose fluctuations are monitored. Pregnancy is a relative contraindication.

Preparation. Overnight fast of 8 to 10 hours (not longer than 12 hours because of glucose stores). Water allowed. Patient rests. Baseline glucose measured. Hydrocortisone and thyroid replacement are scheduled so as not to interfere (hydrocortisone is held for 24 hours when testing the cortisol axis, when safe). Written consent.

Procedure.

• Insert a cannula. Baseline samples for glucose, GH and cortisol at 0 minutes.
• Give glucagon 1 mg intramuscularly (1.5 mg if body weight is over 90 kg).
• Take samples for GH, cortisol and glucose at 30, 60, 90, 120, 150, 180, 210 and 240 minutes (every 30 minutes for 3 to 4 hours).
• Monitor symptoms (nausea, vomiting, headache, late hypoglycaemia) and capillary glucose every 30 minutes.
• At the end, the patient is given a meal and observed, because late hypoglycaemia may follow.

Growth hormone response.

• Peak GH under 3 microg/L (9 mU/L) in a patient with a body mass index of 25 to 30 kg/m², or under 1 microg/L in obesity (body mass index over 30 kg/m²): severe GH deficiency.
• Normal response: peak GH over 3 microg/L (or over 5 microg/L in some cut offs for lean people). Higher cut offs raise sensitivity but lower specificity.
• Interpretation depends on body mass index, age, sex steroid status and the assay.
• Prior probability matters: the test is done where there is a high pre-test probability (structural pituitary lesion, multiple pituitary hormone deficiencies, previous cranial irradiation).

Cortisol response.

• Peak cortisol 450 to 500 nmol/L (assay dependent) or more: normal adrenal reserve.
• Peak cortisol under 450 nmol/L (under 300 nmol/L is severe): secondary adrenal insufficiency, and replacement is considered.
• Sensitivity is about 90% and specificity 85% compared with the insulin tolerance test.

Glucose. Glucose rises in the first 60 to 90 minutes and falls at 120 to 180 minutes, and symptomatic hypoglycaemia may occur. The test does not require hypoglycaemia for validity.

Other pituitary axes. Basal TSH, free T4, prolactin, LH, FSH, sex steroids and IGF-1 are assessed in the same session.

Critical values and findings needing immediate action.

• Hypoglycaemia under 3 mmol/L with symptoms: oral glucose, or intravenous dextrose if drowsy.
• Severe vomiting or dehydration: antiemetic and fluids, and the test is stopped.
• Hypotension or symptoms of adrenal insufficiency with low baseline cortisol: intravenous hydrocortisone 100 mg.
• Peak cortisol under 300 nmol/L: start glucocorticoid replacement and give stress dose instructions and a steroid emergency card.
• Peak GH under cut off in a patient with multiple pituitary deficits: GH replacement is discussed.

Interferences and pitfalls.

• Obesity, diabetes mellitus and impaired glucose tolerance blunt GH responses, and false positives occur. A lower cut off for obese patients is used.
• Oral oestrogen raises cortisol binding globulin, causing falsely normal total cortisol, and is stopped 6 weeks before.
• Cortisol assays differ in specificity, and cut offs are set locally for the assay.
• Hypothyroidism, hypogonadism and depression blunt GH responses and are corrected first.
• Prolonged fasting (over 12 hours) or poor glycogen stores lower the glucose response and reduce stimulation.
• Nausea and vomiting occur in 10 to 30% and may limit absorption and interpretation.
• Recent pituitary surgery or radiotherapy: ACTH deficiency may develop later, so the test is repeated if the clinical picture suggests it.
• The test has been validated less well than the insulin tolerance test, and borderline results are confirmed by another test.

Special groups.

• Obese adults: lower GH peak cut offs apply, and sensitivity is lower.
• Older adults: GH peaks fall with age, and age adjusted cut offs are used.
• Children and transition patients: the test is less validated and used by paediatric endocrinologists with sex steroid priming.
• Diabetes: glucose response and interpretation are altered.
• Regions: availability of GH assays and confirmatory tests limits use.

Follow up tests. IGF-1, insulin tolerance test or macimorelin test for confirmation, morning cortisol and ACTH, synacthen test, TSH and free T4, LH, FSH, prolactin and sex steroids, pituitary MRI, glucose and HbA1c, lipid profile, body composition and bone density.