6917 Multiple Red Herrings Complicating the Diagnosis of Cyclic Cushing’s Syndrome

Abstract Disclosure: A. Grover: None. R. McGlotten: None. L.K. Nieman: None. Introduction: Cyclic Cushing’s syndrome (CCS) is a rare disorder characterized by intermittent hypercortisolemia. It may be caused by pituitary tumor (Cushing disease), ectopic ACTH tumor secretion (EAS), or primary adrenal disease. We present a case of CCS with multiple red herrings complicating the diagnosis. Case: A 57-year-old Cushingoid appearing woman presented elsewhere with weight gain, brain fog, weakness, hypertension, worsening diabetes, and OSA. Screening tests showed more than three episodes of hypercortisolism with interval eucortisolism: salivary and 24-urine cortisol (UFC) ranged from 0.06-0.34 µg/dL (reference range, RR, ≤0.09 µg/dL) and 59.7-137.1 µg/24 h (RR 4-50 µg/24 h) respectively. The patient was symptomatic throughout. Differential diagnosis was CCS vs a physiologic non-neoplastic condition. As she had no clear etiology for the latter, further testing was performed. ACTH was indeterminate (23-27 pg/mL, RR <46). Pituitary MRI showed a left 7 mm lesion. Inferior petrosal sinus sampling (IPSS) with Corticotropin Releasing Hormone (CRH) administration showed peripheral baseline ACTH levels of 9.0 and 10.0 pg/mL, suggesting that she had cycled out of a hypercortisolemic phase. The left petrosal baseline values were low (9 pg/mL,16 pg/mL), suggesting poor catheterization, which was confirmed by a low prolactin-normalized ACTH ratio. The petrosal levels were lower than expected (maximum 767 pg/ml), due to suppression of normal corticotropes vs poor tumor response. However, a central:peripheral step-up was consistent with Cushing's disease (CD) or pseudo-CS. She was referred to us for re-evaluation. A low FSH, LH, low normal TSH, and UFC of 160 µg/24h supported a diagnosis of CS. ACTH was 14.6 pg/mL. Pituitary MRI showed a 1.3 cm x 0.8 cm mass extending into the left cavernous sinus. She did not suppress cortisol during a dexamethasone suppression test (DST). After CRH administration ACTH levels doubled. Based on the macroadenoma and CRH response, CD was established; IPSS was not repeated. After transsphenoidal hypophysectomy, pathology revealed ACTH positive adenoma. Post-operative adrenal insufficiency required hydrocortisone replacement. Conclusion: CCS can pose a diagnostic challenge due to unpredictable duration and frequency of hypercortisolism. Intermittently elevated cortisol levels with CS symptoms should raise suspicion for CCS. In our case, low peripheral ACTH levels further suggested this possibility. DST may give spurious results in CCS owing to spontaneous falls or rises in cortisol at the time of testing. Prolonged hypercortisolism is required in all patients to ensure suppression of normal corticotropes, which if not complete, may confound results in patients with EAS. Finally, inadequate catheterization may affect accuracy of lateralization in any patient with CS and is ideally evaluated both by venography and prolactin measurement. Presentation: 6/1/2024