[Pericardial effusion as a possible cause of false positives in I-131 radionuclide scans]

The case of a patient who had undergone surgery for papillary carcinoma of the thyroid is reported. After administering a therapeutic 131I capsule, the radionuclide scan showed a diffuse, "halo-like" thoracic uptake in the precordial region. The patient's normal thyroglobulin levels and the characteristics of radionuclide uptake did not suggest metastasis. Chest radiography and ultrasonography identified pericardial effusion, which was attributed to hypothyroidism.