Insulin Resistance Unveiled: Cushing's Disease in a Patient With Type 1 Diabetes Mellitus and Worsening Glycemic Control

Background/Objective: Type 1 diabetes mellitus is an autoimmune disease often characterized by endogenous insulin deficiency and often sensitivity to exogenous insulin administration. Cushing's disease, though rare, should be considered as a cause of insulin resistance and increased insulin requirements in individuals with type 1 diabetes mellitus. Case Presentation: A 21-year-old female with type 1 diabetes mellitus presented with steadily increasing insulin requirements via her hybrid closed-loop insulin pump. She subsequently developed hypertension, weight gain, violaceous striae, and cystic acne. Laboratory evaluation revealed unsuppressed cortisol of 16.6 μg/dL after a 1-mg dexamethasone suppression test, with a simultaneous adrenocorticotropin hormone level of 73.3 pg/mL. Pituitary MRI showed a 1.9 cm sellar mass with local invasion. She underwent transsphenoidal hypophysectomy. Postoperative cortisol was 8.9 μg/dL after intraoperative dexamethasone exposure. Residual hypercortisolism was confirmed, necessitating gamma knife radiation and pharmacologic treatment with a steroidogenesis inhibitor. Discussion: We present a case of Cushing's disease due to a corticotropin-secreting pituitary macroadenoma in a young woman with type 1 diabetes. Her initial presentation included rising insulin requirements, followed by overt hypercortisolism. Despite surgery, persistent hypercortisolism required further intervention with gamma knife radiation and osilodrostat. She experienced reductions in both weight and insulin needs, with normalization of cortisol levels on maintenance osilodrostat. Conclusion: Cushing's syndrome should be considered in the differential diagnosis of patients with type 1 diabetes and increasing insulin requirements. This case underscores the importance of regular review of automated insulin delivery data and consideration of endocrine causes of insulin resistance and increased insulin requirements in those with type 1 diabetes.