Efficacy of high therapeutic doses of iodine-131 in patients with differentiated thyroid cancer and detectable serum thyroglobulin

Serum thyroglobulin (Tg) is usually the best marker of residual or metastat ic disease after treatment of differentiated thyroid cancer. We evaluated t he effect of so-called blind therapeutic doses of iodine-131 in patients wi th detectable Tg during suppressive levothyroxine treatment (Tg-on), and in patients with a negative diagnostic scintigram but detectable Tg during th e hypothyroid phase (Tg-off). Twenty-two patients with differentiated thyro id carcinoma underwent total thyroidectomy and radioiodine ablation. During the follow-up, six patients with detectable Tg-on and 16 patients with det ectable Tg-off were identified. All patients were treated with a blind ther apeutic dose of 7,400 MBq iodine-131. Diagnostic scintigrams were compared with post-treatment scintigrams. Tg-off was measured in 16 cases, 1 year af ter the administration of the blind therapeutic dose, at the time of the fo llow-up diagnostic scintigram. Six patients were followed up by Tg-on only. Post-therapy scintigrams revealed previously undiagnosed local recurrence or distant metastases in 13/22 cases (59%); the remaining nine post-therapy scintigrams were negative. At the time of the blind therapeutic doses, Tg- off values ranged from 8 to 608 mug/l. After 1 year of followup, Tg-off dec reased in 14/16 (88%) patients. In all patients who were followed by Tg-on only (n=6), a decrease in Tg Values was measured. It is concluded that blin d therapeutic doses resulted in a decrease in Tg levels in the majority of patients with suspected recurrence or metastases. The post-treatment scinti grams revealed pathological uptake in 59% of patients.