This Metyrapone Test is a stimulation test to assess the hypothalamic pituitary adrenal axis. In essence, it is a practical dynamic test to assess adrenocorticotropic hormone reserve.
Preparation
This test can be done randomly at any time of the day.
Materials
Lavender top tube placed on ice will be required since samples should be transferred frozen. Also, plasma would need to be separated from the cellular component of the blood as soon as a sample is drawn. Analytes to be evaluated include ACTH, serum cortisol, and 11-deoxycortisol.
Precautions
Since metyrapone inhibits essential enzymes involved in adrenal steroidogenesis, patients are predisposed to acute adrenal insufficiency. Possible adverse events during the test include symptomatic orthostatic hypotension, emesis, colicky abdominal pain, musculoskeletal discomfort, severe fatigue, and allergic reaction. In patients with suspected primary adrenal insufficiency, baseline plasma ACTH and early morning cortisol levels may be more than appropriate to diagnose the test.
Pathophysiology principles
Metyrapone inhibits the action of 11 beta-hydroxylase (CYP 11 beta-1), an adrenal enzyme, critical in the conversion of 11-deoxycortisol to cortisol. A significant decline in serum cortisol releases negative feedback inhibition of cortisol on the pituitary gland and hypothalamus. This results in an increased synthesis and secretion of adrenocorticotropic hormone (ACTH) and corticotropin releasing hormone (CRH) by the anterior pituitary and hypothalamus respectively.
Test procedure
A single dose of metyrapone is administered at bedtime followed by an evaluation of early morning serum cortisol drawn at 8 AM.
The dose of metyrapone is based on the patient’s body weight. For patients who weigh less than 70 kg, the suggested dose is either 2 g or 30 mg/kg body weight (not to exceed 2 g). In patients weighing between 70 to 90 kg, 2.5 g of metyrapone is recommended. 3 g of metyrapone is required in those weighing greater than 90 kg. The patient is instructed to have a normal supper. They will then ingest an appropriate weight-based dose of metyrapone either at midnight or bedtime.
It is recommended to have a small snack at bedtime. A patient will then report to the lab the following day between 7:30 AM and 9:00 AM. The following plasma analytes would then be drawn – serum 11-deoxycortisol, serum cortisol, and serum ACTH.
Interpretation of the test
A metyrapone challenge test is the most sensitive method to detect defects in the secretion of pituitary ACTH. By blocking a critical negative feedback inhibiting factor involved in suppressing ACTH synthesis, the lack of an appropriate rise in serum cortisol (a surrogate marker of ACTH reserve), the integrity of the HPA axis can be adequately assessed with this test. This test is, therefore, useful in evaluating secondary adrenal insufficiency, especially in the early phase of the disease (up to the first three months), where an ACTH (cortrosyn) stimulation test can be normal even in the setting of adrenal insufficiency.
A normal response is an 8 AM 11 deoxycortisol level > 7 mcg/dL (or greater than 200 nmol/L SI units) in the setting of low early morning cortisol <5 mcg/dL (138 nmol/L in SI units). This suggests optimal 11 beta-hydroxylase blockade by metyrapone.
What is adrenal insufficiency ? serum 11-deoxycortisol level <7 mcg/dL in the setting of concomitant serum cortisol of < 5 mcg/dL
What about ACTH levels? Although ACTH levels tend to be higher amongst patients with primary AI compared to secondary AI, after a metyrapone challenge, there is significant overlap between both conditions and healthy patients. This makes the interpretation of ACTH levels as a means of distinguishing between primary and secondary adrenal insufficiency problematic.
Interpretation of relevant analytes post suppression with metyrapone
| Condition | 11-deoxycortisol | Cortisol | ACTH |
| Healthy patient | High | low | High |
| Primary AI | low | low | High |
| Secondary AI | low | low | High |
In patients with a serum 11-deoxycortisol level <7 mcg/dL in the setting of concomitant serum cortisol of >5 mcg/dL, it may suggest suboptimal inhibition of 11 beta-hydroxylase by metyrapone. In this scenario, a higher dose of metyrapone may be required (greater than the suggested weight-based dose).
Causes of false positive results
- Rapid metabolizers of metyrapone (up to 5% of normal people) or in the setting of CYP P450 enzyme inducers like anticonvulsants (phenobarbital, phenytoin, and rifampin).
- Recent exposure to exogenous glucocorticoids may transiently reduce ACTH production.
- Assessment of cortisol levels by the conventional immunoassay method can lead to falsely elevated levels of post-suppression cortisol, and this is because 11-DOC levels can be assayed in addition to serum cortisol by this method. In this scenario, Liquid Chromatography with Tandem Mass Spectrometry is the best method to assay serum cortisol levels.
Other applications of the test
It can be utilized in the evaluation of recurrent Cushing’s disease. An increase in serum 11-deoxycortisol > 5.2 mcg/dL (150 nmol/L) on the metyrapone challenge test two weeks after hypophysectomy is highly predictive of relapse.
References
- Fiad TM, Kirby JM, Cunningham SK, McKenna TJ. The overnight single-dose metyrapone test is a simple and reliable index of the hypothalamic-pituitary-adrenal axis. Clin Endocrinol (Oxf) 1994; 40:603.
- Dolman LI, Nolan G, Jubiz W. Metyrapone test with adrenocorticotrophic levels. Separating primary from secondary adrenal insufficiency. JAMA 1979; 241:1251.
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