Posttherapy scan in a 21-year-old woman with papillary carcinoma of thyroid with lymph node metastasis, who received 5.55 GBq of radioiodine (¹³¹I), revealed halo-like diffuse tracer uptake in the pericardial region. Echocardiography showed no abnormality except pericardial effusion, which subsided after reinstitution of levothyroxine therapy. Although rare, false-positive radioiodine uptake can occur in pericardial effusion secondary to thyroxine withdrawal-related hypothyroidism and needs close monitoring of the patient.