SUN-063 Diagnostic Pitfalls in Hypercortisolism: Unmasking Multifragment ACTH Interference in a Complex Case of Ectopic Cushing’s Syndrome

Abstract Disclosure: D. Deyar: None. D. Nandiraju: None. Introduction: Hypercortisolism presents diagnostic challenges, particularly when assay interference complicates the evaluation of cortisol and ACTH (adrenocorticotropic hormone) levels. We present a case of a 78-year-old female with hypercortisolism and discordantly low ACTH levels, ultimately attributed to multifragment ACTH interference. This case underscores the importance of considering assay limitations in the diagnostic workup of endocrine disorders. Case Presentation: A 78-year-old female presented with generalized weakness, fatigue, nausea, vomiting and hypotension over two weeks. Six months prior, she was diagnosed with bullous pemphigoid and treated with a prednisone taper. Subsequent fatigue and weight loss led to evaluation for adrenal insufficiency, though cortisol remained elevated. Laboratory Findings: - K: 3.3 mmol/L (n 3.5-5.0) - Glucose 98 mg/dl (n 70-99) - AM cortisol: 49.5 µg/dL (n 5-25) - Cortrosyn stimulation test: Cortisol increased to 69.8 µg/dL - 1 mg dexamethasone suppression test: Cortisol 44.2 µg/dL (unsuppressed) - ACTH: <9 pg/mL (n 10-60)-Urinary free cortisol: 306-680 µg/24h (n 20-90) - DHEA-Sulfate: 17 µg/dL, (n 35-430) - FSH, LH, prolactin, TSH, free T4: Normal - Imaging studies were non-diagnostic. The patient’s lab findings suggested hypercortisolism, likely due to ectopic ACTH secretion. However, the ACTH assay revealed potential interference from multifragment ACTH, as the sample demonstrated non-linear dilution properties when used as a diluent for another ACTH-containing sample. This interference was attributed to unconnected ACTH fragments, which disrupted the sandwich immunoassay, leading to falsely low ACTH results. Clinical lesson: Assay interference can arise from various causes, including heterophilic antibodies, cross-reactivity with hormone fragments. The hypothesis of multi-fragment ACTH interference was supported by the discordance between clinical findings and biochemical results. Multi-fragment ACTH can arise from the degradation of ACTH precursors or post-translational modifications, leading to fragments that cross-react with certain assays but lack biological activity. This phenomenon can result in falsely low ACTH measurements. The patient’s clinical and biochemical findings are most consistent with ectopic Cushing’s syndrome, likely secondary to an occult malignancy. In this case, the interference was due to fragmented ACTH peptides. Multifragments form from improper POMC cleavage, degradation of pro-ACTH intermediates, or post-translational modifications in chronic inflammation (bullous pemphigoid) or malignancy. Discordant biochemical and clinical findings should raise suspicion for interference, and switching to mass spectrometry-based assays can help. Laboratory specialists can investigate heterophilic antibodies or hormone fragments. Presentation: Sunday, July 13, 2025