INTRODUCTION — The metyrapone stimulation test is based upon the principle that its administration results in reduction of serum cortisol concentrations, which should be followed by an increase in corticotropin (ACTH) secretion and the immediate precursor of cortisol, 11-deoxycortisol [1]. This test has been considered as potentially more physiological and less invasive way to assess the hypothalamic-pituitary-adrenal (HPA) axis than the insulin-induced hypoglycemia test.

However, in many countries, the metyrapone stimulation test has been used infrequently during the last 10 years. During that time, metyrapone was unavailable in many countries and measurement of 11-deoxycortisol in blood and urine became less available. At the same time, synthetic ACTH and cortisol assays were widely available [2,3].

With recent enhanced availability of metyrapone and a more specific measurement of serum 11-deoxycortisol performed by liquid chromatography with liquid chromatography-tandem mass spectrometry (LC-MS/MS), the test may begin to be performed more frequently [2,3]. This topic will focus on the physiological mechanisms that underlie its usefulness as an alternative test to evaluate ACTH secretory reserve, particularly when the insulin-induced hypoglycemia test may be contraindicated or inconvenient [4-6]. Other tests to evaluate the HPA axis are discussed separately. (See "Initial testing for adrenal insufficiency: Basal cortisol and the ACTH stimulation test" and "Insulin-induced hypoglycemia test".)

GENERAL PRINCIPLES — The metyrapone stimulation test is based upon the principle that decreasing serum cortisol concentrations normally produces an increase in corticotropin (ACTH) secretion due to a decrease in glucocorticoid negative feedback. The test is performed primarily to detect partial defects in pituitary ACTH secretion.

Because it is essentially devoid of glucocorticoid activity, 11-deoxycortisol does not inhibit ACTH secretion. Thus, in healthy individuals, the decrease in serum cortisol concentrations leads sequentially to decreased negative feedback at hypothalamic and anterior pituitary, which increases corticotropin-releasing hormone (CRH) and ACTH secretion and adrenal steroidogenesis; the resultant secretion of cortisol precursors, in particular, 11-deoxycortisol (the substrate of CYP11B1), can be measured by immunoassay, high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), or fast liquid chromatography-tandem mass spectrometry (LC-MS/MS) in blood or its metabolites in urine [7].

The increase in serum 11-deoxycortisol concentrations provides an index of the increase in ACTH release; a failure of these values to rise can indicate either ACTH deficiency or primary adrenal disease. Thus, if the metyrapone test is abnormal, additional tests are needed to distinguish between these disorders. (See "Initial testing for adrenal insufficiency: Basal cortisol and the ACTH stimulation test".)

PROCEDURE — The metyrapone test is performed as an overnight, single-dose test based on blood levels of 11-deoxycortisol. It cannot be performed in a patient who is taking any glucocorticoid. Metyrapone is currently available through its distributor HRA Pharma (Paris, France) in many countries worldwide. In the United States, it can be obtained via its specialty pharmacy Direct Success Inc with order forms available on the web (metopirone.com) or by phone at 1-855-674-7663; order forms can be faxed to 1-855-674-6767.

Metyrapone administration may result in hypotension, nausea, and vomiting in patients with adrenal insufficiency; as a result, it should not be utilized in patients suspected of having severe adrenal insufficiency. (See 'Metyrapone side effects' below.)

Overnight, single-dose metyrapone test — This test is safe for outpatient use [4-6,8-10]. Metyrapone is taken orally (30 mg/kg body weight, or 2 grams for <70 kg, 2.5 grams for 70 to 90 kg, and 3 grams for >90 kg body weight) at midnight with a glass of milk or a small snack [11]. Serum 11-deoxycortisol and cortisol are measured between 7:30 and 9:30 AM the next morning; plasma corticotropin (ACTH) can also be measured [1,4,8-12]. There are alternate approaches to metyrapone dosing regimens [2].

A normal response to the overnight, single-dose test consists of:

●An 8 AM serum 11-deoxycortisol concentration of 7 to 22 mcg/dL (200 to 635 nmol/L) [4,6,8,11,13].

●A serum cortisol concentration at 8 AM of less than 5 mcg/dL (138 nmol/L) confirms adequate metyrapone blockade and thereby documents compliance and normal metabolism of metyrapone.

●Because of potential cross-reactivity of 11-deoxycortisol in cortisol immunoassays, it is important to exclude this as a confounding factor if cortisol levels exceed 5 mcg/dL, or to measure cortisol by tandem mass spectrometry [14].

Metyrapone side effects — By reducing cortisol production, metyrapone can result in hypotension, nausea, vomiting, abdominal discomfort or cramping, and muscle and joint pain in patients with adrenal insufficiency. Metyrapone can also cause dizziness, sedation, allergic rash, or, rarely, decreased white blood cell count or bone marrow suppression. In one study, hydrocortisone 10 mg was administered once samples were collected, before discharging the patient [2].

INTERPRETATION — The metyrapone test is a sensitive test of pituitary corticotropin (ACTH) secretory reserve found to be more sensitive than insulin tolerance test in certain studies [6] or slightly less in other studies [15]. It depends upon the release of pituitary ACTH secretion from negative feedback inhibition by cortisol; hypocortisolemia is a less potent stimulus of ACTH release than hypoglycemia or other stresses. Thus, a patient with partial hypopituitarism may maintain normal daily ACTH and cortisol secretion and respond to insulin-induced hypoglycemia with an appropriate increase in ACTH and cortisol secretion, yet be unable to increase ACTH secretion appropriately when cortisol biosynthesis is blocked by metyrapone [8]. Conversely, a patient who responds normally to metyrapone almost always responds normally to hypoglycemia or other stresses.

Normal response — When used to diagnose hypoadrenalism, a normal response to the metyrapone tests indicates an intact hypothalamic-pituitary-adrenal (HPA) axis; such a patient does not have any form of adrenal insufficiency and requires no further investigation. An abnormal test could reflect either primary or secondary adrenal insufficiency. Distinction between the two can be determined by finding a high basal or stimulated plasma ACTH concentration, which would indicate primary adrenal insufficiency [4,5,16].

Partial defects in ACTH secretion — The metyrapone test is a sensitive method to detect partial defects in pituitary ACTH secretion [4-6,17]. Thus, morning basal plasma ACTH and serum cortisol concentrations, basal urinary excretion of cortisol, the responses to insulin-induced hypoglycemia, and both the 1 and 250 mcg ACTH stimulation tests may all be normal, but the metyrapone test may be subnormal [10,18]. An example would be a patient with a suspected pituitary mass who has no clinical manifestations of ACTH deficiency, an intermediate 8 AM serum cortisol concentration, but an abnormal metyrapone test. (See "Diagnostic testing for hypopituitarism" and "Initial testing for adrenal insufficiency: Basal cortisol and the ACTH stimulation test".)

Serum 11-deoxycortisol concentrations less than 7 mcg/dL (202 nmol/L) with concomitantly suppressed cortisol values indicate adrenal insufficiency.

However, in one study, the sum of 11-deoxycortisol and of cortisol >15 mcg/dL (450 nmol/L) following a single-dose, overnight metyrapone test yielded better diagnostic accuracy than using 11-deoxycortisol levels alone [5]. In a study of 31 patients with various HPA axis abnormalities comparing insulin tolerance test with overnight metyrapone test, a cutoff of 144 nmol/L (5 mcg/dL) for 11-deoxycortisol yielded the highest sensitivity of 82.4 percent to detect patients responding normally to insulin tolerance test, but only 64.3 percent of those with subnormal response to insulin tolerance test [15].

Primary versus secondary adrenal insufficiency — The ACTH response to metyrapone may theoretically distinguish between primary and secondary insufficiency, but it is neither used nor recommended for this purpose. In general, patients with partial secondary adrenal insufficiency have ACTH responses from 10 to 200 pg/mL (2 to 44 pmol/L), while patients with primary adrenal insufficiency have higher responses [4,5,16]. However, healthy individuals have an ACTH response of 42 to 690 pg/mL (9 to 210 pmol/L) [19]. Because of this overlap, the ACTH response alone cannot be used to distinguish between healthy individuals and those with adrenal insufficiency. As noted, the modern, high-sensitivity immunometric ACTH assay has rendered the metyrapone test unnecessary in most patients for differentiating primary and secondary adrenal insufficiency. (See "Diagnosis of adrenal insufficiency in adults".)

The increase in serum 11-deoxycortisol concentrations may be exaggerated in women taking an oral contraceptive and in patients with hypothyroidism, hypoglycemia, diabetes mellitus, congestive heart failure, obesity, and chronic renal failure [20,21].

False-positive results — There are, however, settings in which false-positive results can be obtained:

●Unappreciated recent exposure to synthetic glucocorticoids by any route 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, which occurs in approximately 4 percent of normal subjects [20,22]. This results in inadequate blockade of cortisol biosynthesis and an 8 AM serum cortisol concentration to greater than 7.5 mcg/dL (210 nmol/L) in the overnight test. Metyrapone is metabolized by hepatic cytochrome P450 enzymes that are induced by many of the same drugs that increase steroid metabolism (eg, phenobarbital, phenytoin, rifampin, and mitotane). Therefore, these drugs should be stopped well before the metyrapone test is performed.

●Cortisol levels measured by conventional immunoassays can theoretically be falsely elevated by the interference of increased 11-deoxycortisol levels induced by metyrapone. Since the cross-reactivity of 11-deoxycortisol in the typical cortisol immunoassay is <5 percent and often <2 percent [2], this is usually a minor issue in most patients. Furthermore, the wider availability of liquid chromatography-tandem mass spectrometry (LC-MS/MS) steroid assays eliminates this concern [23]. However, obtaining a serum cortisol concentration by LC-MS/MS may require a special request to the local clinical laboratory; most laboratories routinely use a platform immunoassay.

Postoperative assessment of patients with Cushing's disease — Recurrence of Cushing's disease after pituitary surgery is common, and early identification of relapse is important [24]. It has been suggested that an increase in 11-deoxycortisol to greater than 5.2 mcg/dL (150 nmol/L) with a metyrapone test administered 14 days after pituitary surgery is predictive of relapse [25]. However, other approaches are likely to be more useful. (See "Primary therapy of Cushing's disease: Transsphenoidal surgery and pituitary irradiation".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Adrenal insufficiency".)

SUMMARY

●Metyrapone blocks the conversion of 11-deoxycortisol to cortisol by CYP11B1 (11-beta-hydroxylase, P450c11), the last step in the synthesis of cortisol, and induces a rapid fall of cortisol (figure 1). (See 'General principles' above.)

●The metyrapone stimulation test is based upon the principle that decreasing serum cortisol concentrations normally produces an increase in corticotropin (ACTH) secretion due to a decrease in glucocorticoid negative feedback. The test is performed primarily to detect partial defects in pituitary ACTH secretion and may be more physiological and less invasive than insulin-induced hypoglycemia. In many countries the metyrapone stimulation test has been used infrequently during the last 10 years. However, this may change with the enhanced availability of metyrapone and a more specific measurement of serum 11-deoxycortisol performed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). (See 'Introduction' above.)

●In healthy individuals, the decrease in serum cortisol concentrations leads sequentially to increases in ACTH secretion, adrenal steroidogenesis, and the secretion of cortisol precursors; in particular, 11-deoxycortisol, which can be measured by immunoassay, high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), or LC-MS/MS. (See 'Interpretation' above.)

●The increase in serum 11-deoxycortisol concentrations provides an index of the increase in ACTH release; a failure of these values to rise can indicate either ACTH deficiency or primary adrenal disease. Distinction between the two can be determined by finding a high basal or stimulated plasma ACTH concentration, which would indicate primary adrenal insufficiency. (See 'Primary versus secondary adrenal insufficiency' above.)

DISCLOSURE — The views expressed in this topic are those of the author(s) and do not reflect the official views or policy of the United States Government or its components.