Thyrotropin variations may explain some positive radioiodine therapy scans in patients with negative diagnostic scans

Thyroglobulin (Tg) is a specific marker of residual thyroid cancer or tumor recurrence. In patients with elevated Tg levels and negative diagnostic radioiodine (¹³¹I) whole-body scans (dWBS), administration of a therapy dose may reveal foci that were not initially apparent. The aim of this study was to identify factors, other than ¹³¹I activity, which might explain why a post-therapy ¹³¹I whole-body scan is sometimes positive despite a negative dWBS. Patients and methods: We reviewed data on all patients with elevated Tg levels and negative dWBS with 185 MBq ¹³¹I off-T₄ at follow-up, who subsequently received an empiric therapy dose of 3700 MBq of ¹³¹I. During a 5-yr period, 22 patients met these criteria. ¹³¹I therapy could be given immediately after negative dWBS in 9 patients, with an average of 8 extra days of hypothyroidism. In the other 13 patients, therapy was given an average of 8 months later. Results: The therapy scan was negative in 16 patients, while it showed uptake in the thyroid bed in 5 patients and distant metastases in two. In the latter two patients, the TSH level was suboptimal at the time of dWBS (9 and 25 μIU/ml), and had risen to 34 and 70 μIU/ml respectively at the time of therapy. Overall, a positive scan following therapy occurred in 7 patients (6/9 patients treated immediately and 1/13 patients treated in a separate setting; p<0.01). In patients with positive therapy scans, the mean TSH level was 73 μIU/ml at the time of dWBS and 103.5 μIU/ml at the time of therapy (41% increase; p<0.05). In patients with negative therapy scans the mean TSH level was 84 μIU/ml at dWBS and 86 μIU/ml at the time of the therapy scan (2% increase). Conclusions: Our study suggests that interval increase in TSH level with a longer period of stimulation may have contributed to making the whole-body scan positive at the time of therapy. Nowadays, patients with elevated Tg are directly given a therapy dose of ¹³¹I. Special care should be taken when preparing patients who have been on suppressive levothyroxine therapy for a long time, in order to avoid misclassifying the tumor as non-functioning.