The metyrapone stimulation test is based on the principle that decreasing serum cortisol concentrations is expected to produce an increase in ACTH secretion.
Use
The utilization of the metyrapone test has become less frequent as a result of the larger availability of plasma ACTH assays. The limited accessibility to metyrapone in certain countries, as well as the limited number of clinical laboratories who have maintained the urinary 17-hydroxycorticosteroid (17-OHCS) and serum 11-deoxycortisol tests, have also further limited the use of the metyrapone test.
Metyrapone blocks the conversion of 11-deoxycortisol to cortisol by CYP11B1 (11-beta-hydroxylase, P-450c11), the last step in the synthesis of cortisol. It induces a rapid fall of cortisol and an increase of 11-deoxycortisol in serum.
The metyrapone test can be performed as an overnight single-dose test or as a 2- or 3-day test. It cannot be performed in a patient who is taking any glucocorticoid.
The 2-day test is used primarily in the differential diagnosis of hypercortisolism.
The 2-day test is a slight variation on the standard 3-day test: 24-hour urine and 8 am blood specimens are collected during and at the end of a baseline day and during and at the end of the day during which the patient takes 750 mg of metyrapone by mouth every 4 hours for six doses.
Urinary 17-OHCS excretion and serum 11-deoxycortisol are measured.
The 3-day test is used mainly for the evaluation of adrenal insufficiency.
The 3-day test is begun by obtaining a baseline 24-hour urine collection. Immediately after completing this collection, the patient begins taking metyrapone (750 mg orally every 4 hours for six doses) with a glass of milk or a small snack to minimize GI symptoms.
Subsequent 24-hour urine specimens are collected the day of and the day after metyrapone administration for measurement of urinary 17-OHCS and creatinine excretion. Serum 11-deoxycortisol, cortisol, and plasma ACTH can also be measured 4 hours after the last dose of metyrapone.
The single-dose overnight test can be used for both indications.
The single-dose test is performed by oral administration of metyrapone (30 mg/kg body weight, or 2 g for <70 kg, 2.5 g for 70 " “90 kg, and 3 g for >90 kg body weight) at midnight with a glass of milk or a small snack.
Serum 11-deoxycortisol and cortisol are measured between 7:30 and 9:30 am the next morning; plasma ACTH can also be measured.
Interpretation
Standard 3-Day Metyrapone Test
The increase in serum 11-deoxycortisol is used as a criterion of response as in the single-dose overnight test. Measuring serum cortisol and plasma ACTH is important, as a fall in serum cortisol confirms the metyrapone-induced biosynthetic blockade and an increase in plasma ACTH confirms that the changes in steroid levels are ACTH dependent.
A normal response is a two- to threefold increase above the baseline 24-hour urinary 17-OHCS excretion on either the day of or, more often, the day after metyrapone administration. The serum cortisol concentration should decrease to <5 Ž ¼g/dL. The plasma ACTH concentration should exceed 75 pg/mL, with a mean of about 200 pg/mL 4 hours after the last metyrapone dose. An increase in serum 11-deoxycortisol to 7 " “22 Ž ¼g/dL or more at 8 am, 4 hours after the last dose of metyrapone.
Two-Day Metyrapone Test
A normal response to the 2-day test has not been defined. In the differential diagnosis of ACTH-dependent Cushing syndrome, however, a clear rise in plasma ACTH concentration indicates that the ACTH-secreting tumor responds to falling serum cortisol concentrations. In one large study, as an example, a positive response was defined as a >70% increase in urinary 17-OHCS excretion and/or more than a fourfold increase in serum 11-deoxycortisol concentrations.
Overnight Single-Dose Metyrapone Test
A normal response is 8 am serum 11-deoxycortisol concentration of 7 " “22 Ž ¼g/dL. A serum cortisol concentration at 8 am of <5 Ž ¼g/dL confirms adequate metyrapone blockade and thereby documents compliance and normal metabolism of metyrapone. Serum 11-deoxycortisol concentrations <7 Ž ¼g/dL with concomitantly suppressed cortisol values indicate adrenal insufficiency.
The ACTH response to metyrapone can distinguish between primary and secondary insufficiency. In general, patients with secondary adrenal insufficiency have ACTH responses from 10 to 200 pg/mL, whereas patients with primary adrenal insufficiency have higher responses. However, healthy individuals have an ACTH response of 42 " “690 pg/mL. Because of this overlap, the ACTH response alone cannot be used to distinguish between healthy individuals and those with adrenal insufficiency.
Limitations
Adrenal tumor with excess cortisol production: no increase or fall in urinary 17-KS. The test is positive in 100% of patients with adrenal hyperplasias without tumor, 50% of those with adrenal adenomas, and 25% of those with adrenal carcinomas.
Ectopic ACTH syndrome: It may not be accurate in this condition.
Metyrapone administration may result in hypotension, nausea, and vomiting in patients with adrenal insufficiency; as a result, the 2- and 3-day tests should not be performed outside of the hospital in patients suspected of having this disorder.
Acute or chronic ingestion of synthetic glucocorticoids can result in a subnormal response as a result of suppression of the corticotropes.
One of the more common causes of a false-positive result is unusually rapid clearance of metyrapone from the plasma, resulting in inadequate blockade of cortisol biosynthesis. This is manifested by a serum cortisol concentration >7.5 Ž ¼g/dL in the sample drawn at 8 am in the overnight test, by a serum cortisol concentration >5 Ž ¼g/dL 4 hours after the last dose of metyrapone, or by urinary cortisol excretion >20 Ž ¼g/24 hours the day metyrapone was administered in the standard 2-day test.