MON-451 Contrast Induced Hyperthyroidism (Jod Basedow Phenomenon) in a Patient With a Thyroid Nodule

Abstract Disclosure: M.P. Reyes: None. P. Ginier: None. M. Koteles: None. Introduction: Iodine-induced hyperthyroidism (Jod-Basedow phenomenon) occurs when excessive iodine triggers thyrotoxicosis in individuals with nodular thyroid disease. Repeated iodinated contrast exposure can cause severe complications. Identifying high-risk patients and monitoring thyroid function post-contrast exposure is essential. Clinical Case: A 75-year-old male with no prior thyroid disease underwent two iodinated contrast-enhanced CT scans: an abdominal/pelvic CT on 11/29/24 for abdominal pain and vomiting, and a CTA chest on 12/3/24 to rule out pulmonary embolism due to new-onset atrial fibrillation. Combined, these delivered ∼80 g of iodine. The patient was asymptomatic prior to imaging but subsequently developed palpitations, tremor, weight loss, and heat intolerance.On presentation, he had atrial fibrillation with rapid ventricular response. Urinary iodine levels were significantly elevated, with a spot urine iodine of 1512 µg/L on 12/12/24 and 684 µg/L on 12/18/24, reflecting recent high iodine exposure from contrast-enhanced imaging. A follow-up 24-hour urinary iodine test on 12/20/24 showed a level of 972 µg, further confirming ongoing iodine excess. Laboratory tests showed TSH <0.01 µIU/mL, free T4 4.7 ng/dL, and free T3 7.4 ng/dL. Thyroid antibodies were negative. A 2.2 cm left-lobe thyroid nodule was found on ultrasound. Due to recent iodine exposure, radioactive iodine uptake was not performed, as iodine saturation would have rendered the test uninterpretable. RAIU typically becomes reliable four to six weeks post-exposure.A presumptive diagnosis of Jod-Basedow hyperthyroidism was made. Methimazole 40 mg/day and beta-blockade were started, leading to partial improvement. He was readmitted with recurrent atrial fibrillation after missing a beta-blocker dose. Follow-up thyroid function tests showed a shift to hypothyroidism (TSH 10.53 µIU/mL, free T4 0.38 ng/dL), necessitating methimazole discontinuation. Further evaluation of the thyroid nodule via fine-needle aspiration and assessment of abdominal lesions were deferred until stable euthyroidism. Clinical Lessons This case illustrates how high-dose iodinated contrast can unmask thyroid autonomy, causing severe hyperthyroidism. The patient’s course, marked by new-onset atrial fibrillation, readmission, and hypothyroidism, highlights the importance of thorough risk assessment and post-contrast thyroid function monitoring. Baseline thyroid testing and ultrasound should be considered in high-risk individuals. Monitoring with thyroid function tests at two and four weeks post-exposure is crucial for detecting iodine-induced dysfunction. Careful antithyroid medication titration is necessary to avoid overtreatment and iatrogenic hypothyroidism. Recognizing Jod-Basedow hyperthyroidism in high-risk patients is vital to prevent arrhythmias and optimize thyroid management. Presentation: Monday, July 14, 2025