Which tests may be used to evaluate suspected Cushing syndrome in a child with obesity?

• A. Morning cortisol, ACTH, and dexamethasone suppression test or 24-hour urinary free cortisol.
• B. Fasting insulin levels and HbA1c.
• C. Growth hormone stimulation test.
• D. MRI brain scan alone.

Answer: (A)

Screening for Cushing syndrome relies on biochemical evidence of excess cortisol and the source of that excess. In a child who is obese but may have CS, you start with tests that directly assess cortisol production and the pituitary-adrenal axis. Measuring morning cortisol together with ACTH helps characterize baseline adrenal activity and whether ACTH is driving cortisol production. A dexamethasone suppression test—and/or 24-hour urinary free cortisol—shows whether cortisol remains high despite suppression attempts or is elevated over a day, respectively. If these tests indicate hypercortisolism, they also help distinguish ACTH-dependent from ACTH-independent causes, guiding subsequent localization efforts.

The other tests in the options don’t serve as initial screening for Cushing syndrome: fasting insulin and HbA1c evaluate glucose metabolism, not cortisol excess; a growth hormone stimulation test assesses the growth hormone axis rather than cortisol; MRI of the brain alone cannot establish the diagnosis of Cushing syndrome and is used to localize a pituitary lesion after biochemical confirmation.