MON-446 Immunotherapy Induced Thyroiditis Secondary to Nivolumab

Abstract Disclosure: H. Marasandra Ramesh: None. R. Subramani: None. S. Yavuz: None. Introduction: Immune checkpoint inhibitors (ICIs) are monoclonal antibodies that enhance T-cell activation by targeting CTLA-4 and PD-1/PD-L1. Their interaction with PD-L1 on thyroid tissue can lead to painless thyroiditis. We present a case of immunotherapy-induced thyroiditis. Case Presentation: A 67-year-old woman with metastatic left clear cell renal carcinoma (post-left nephrectomy), right frontal encephalomalacia, craniotomy with aneurysm clipping, residual left-sided weakness, anxiety, and hypertension presented with malaise, chills, back pain, confusion, and seizure concerns. She was on palliative ipilimumab and nivolumab every three weeks, completing three of eight planned cycles three weeks prior to presentation. History included a 60-pound weight loss in four months. CT head showed stable encephalomalacia and gliosis. MRI was contraindicated due to aneurysmal clips. EKG revealed sinus tachycardia. Labs showed TSH 0.02 uIU/mL (normal: 0.27-4.2 uIu/ml), free T4 2.5 ng/dL (normal: 0.9-1.7 ng/dl), free T3 4.55 pg/mL (2-4.4 pg/ml), and serum cortisol 0.5 ug/dL (normal: 4.8-19.5 ug/dl). Prior thyroid tests were normal. TSH receptor antibodies were <1 IU/L, thyroid-stimulating immunoglobulin 89, and anti-thyroid peroxidase antibodies 15.6 IU/mL (normal: 0-34 IU/ml). Findings were consistent with immunotherapy-induced thyroiditis. Methimazole 25 mg (later increased to 30 mg), hydrocortisone and carvedilol 6.25 mg twice daily were initiated, and nivolumab was discontinued. As the patient became lethargic during evaluation for acute metabolic encephalopathy, intubation was considered. After a goals-of-care discussion, her healthcare proxy opted against intubation, and she transitioned to hospice care. Conclusion : Nivolumab-induced thyroiditis usually develops about four weeks after therapy begins. It presents as painless thyroiditis with a transient thyrotoxic phase, which may resolve or progress to hypothyroidism requiring levothyroxine. Management involves discontinuing nivolumab, supportive care, and regular thyroid function monitoring. Presentation: Monday, July 14, 2025