Diagnosis of Cushingoid Features with Low ACTH

The patient presents with symptoms like obesity, moon facies, abdominal striae, and hypertension, which are characteristic of Cushing's syndrome (excess cortisol).

Analysis of Test Results

• Overnight Dexamethasone Suppression Test (ON DST): Serum cortisol is greater than 50 nmol/L. This indicates a failure to suppress cortisol production, confirming hypercortisolism.
• Serum ACTH Level: Serum ACTH is less than 1.1 pmol/L. This is a critical finding, indicating that the ACTH production from the pituitary gland is suppressed.
• Imaging Studies: Both adrenal imaging and brain CT/MRI are unremarkable. This rules out significant adrenal tumors (adenoma/carcinoma) or pituitary tumors (Cushing's disease) as the cause.

Reasoning for Diagnosis

• Low ACTH levels (< 1.1 pmol/L) exclude pituitary causes (Cushing's disease) and ectopic ACTH production, as these conditions typically result in normal or high ACTH levels.
• The suppression of ACTH suggests that the negative feedback mechanism of the HPA axis is intact but overridden, most commonly by exogenous glucocorticoid administration.
• The patient's known history of rheumatoid arthritis increases the likelihood of long-term treatment with glucocorticoids (like prednisone or dexamethasone), a common therapy for RA.
• Therefore, the combination of Cushingoid symptoms, cortisol non-suppression, suppressed ACTH, normal imaging, and a history suggestive of steroid use strongly points towards exogenous glucocorticoid use.

The diagnosis is concluded to be exogenous glucocorticoid use because it is the most common cause of ACTH suppression in the context of Cushingoid symptoms when pituitary and adrenal sources have been ruled out by imaging.