Coexisting mild autonomous cortisol secretion and isolated adrenocorticotropic hormone deficiency in a patient with an adrenal incidentaloma.

Low adrenocorticotropic hormone (ACTH) in a patient with an adrenal incidentaloma is usually attributed to ACTH-independent cortisol secretion. A 71-year-old woman with a 20-mm left adrenal mass had no cushingoid features. After a 1-mg dexamethasone suppression test, serum cortisol was 5.59 μg/dL (SI: 154 nmol/L) (cutoff, <1.8 μg/dL [SI: <50 nmol/L]), and ACTH was below 1.5 pg/mL (SI: below 0.33 pmol/L) (reference range, 7-63 pg/mL [SI: 1.5-13.9 pmol/L]). Late-night serum cortisol was 7.29 μg/dL (SI: 201 nmol/L) (cutoff, <5.0 μg/dL [SI: <138 nmol/L]), supporting mild autonomous cortisol secretion. Combined pituitary stimulation testing showed a blunted, delayed ACTH increase from 4.2 (SI: 0.92 pmol/L) to 11.8 pg/mL (SI: 60 pmol/L) without a cortisol response, whereas other anterior pituitary axes were preserved. A short Synacthen test showed an inadequate peak cortisol of 10.8 μg/dL (SI: 298 nmol/L) (adequacy cutoff, ≥18 μg/dL [SI: approximately 500 nmol/L]), and dehydroepiandrosterone sulfate was 37 ng/mL (SI: 0.10 μmol/L) (reference range, 70-495 ng/mL [SI: 0.19-1.34 μmol/L]). Possible isolated ACTH deficiency was diagnosed. Annual testing remained unchanged over 4 years.