Thymus uptake of131I in patients with differentiated thyroid carcinoma

The importance of recognizing thymic radioiodine uptake as the cause of a false-positive mediastinal focus in the whole-body 131 I scan, done for the evaluation of post-thyroidectomy cases of differentiated thyroid carcinoma, is illustrated with the corresponding clinicoradiorological correlation. The pattern of mediastinal uptake could vary based upon the pattern of thymic hyperplasia in an individual case. Three different patterns of mediastinal uptake were observed in the cases described in the present report. Recognizing the patterns and the clinical settings (where this was to be suspected by the treating physician) was important to obviate unnecessary aggressive treatment, such as, surgery or radioiodine therapy. In a review of the literature, we found that a majority of the cases were reported in the young population (related to the thymus reaching its peak size during adolescence and gradual atrophy in the following decades) and in patients undergoing a six-month follow-up whole body diagnostic scan after thyroid remnant ablation treatment, and also in patients receiving a second course of 131-iodine treatment for a persistently elevated thyroglobulin (Tg) level. The indicators that should raise the suspicion of false-positive radioiodine uptake to the attending physician include: (a) undetectable/low serum thyroglobulin level (although this may not be always the case, depending on the clinical setting), (b) a well-controlled disease with no other abnormal focus in the rest of the body, (c) typical butterfly-shaped uptake (although this may not be the case always), (d) young age of the patient, and (e) computed tomography (CT) documentation of the enlarged thymus without any lymphadenopathy (frequently a non-contrast CT is quite efficient in clinching the diagnosis)