What is the role of dexamethasone suppression testing in the diagnosis of McCune-Albright syndrome (MAS)?

Updated: Jan 05, 2021
  • Author: Gabriel I Uwaifo, MD; Chief Editor: George T Griffing, MD  more...
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Normally, cortisol levels are suppressed by overnight administration of dexamethasone (0.050 mg/kg; not to exceed 1 mg). Elevated cortisol levels at 8:00 AM suggest Cushing syndrome but do not distinguish between ACTH-dependent and ACTH-independent excess cortisol production.

Low-dose/high-dose dexamethasone suppression test

Low-dose (2 mg/1.7 m2/day)/high-dose (8 mg/1.7 m2/day) dexamethasone suppression testing can help distinguish ACTH-dependent Cushing syndrome from pituitary and ectopic sources and confirm the ACTH-independent nature of excessive cortisol secretion in MAS. Because the recommended treatment of ACTH-independent Cushing syndrome is bilateral adrenalectomy, such testing should be performed preoperatively.

Low-dose/high-dose dexamethasone suppression tests in infants are performed in a hospital setting. A Foley catheter is placed, and urine is collected in 24-hour increments for free cortisol determinations. ACTH and cortisol levels are obtained at 8:00 AM each day. After baseline measurements are collected, low-dose dexamethasone is administered for 2 days, followed by high dose dexamethasone for 2 days.

Lack of suppression of cortisol production with low-dose dexamethasone but suppression with high-dose dexamethasone suggests ACTH-dependent Cushing syndrome. Lack of suppression with high-dose dexamethasone suggests either ectopic ACTH production or ACTH-independent Cushing syndrome. Diagnosis of ACTH-independent Cushing syndrome consistent with MAS is confirmed in this situation if ACTH levels are also suppressed.

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