Serum
thyroglobulin (Tg) is usually the best marker of residual or metastatic disease
after treatment of differentiated
thyroid cancer. We evaluated the effect of so-called blind therapeutic doses of
iodine-131 in patients with detectable Tg during suppressive
levothyroxine treatment (Tg-on), and in patients with a negative diagnostic scintigram but detectable Tg during the hypothyroid phase (Tg-off). Twenty-two patients with differentiated
thyroid carcinoma underwent total
thyroidectomy and radioiodine ablation. During the follow-up, six patients with detectable Tg-on and 16 patients with detectable Tg-off were identified. All patients were treated with a blind therapeutic dose of 7,400 MBq
iodine-131. Diagnostic scintigrams were compared with post-treatment scintigrams. Tg-off was measured in 16 cases, 1 year after the administration of the blind therapeutic dose, at the time of the follow-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 microg/l. After 1 year of follow-up, Tg-off decreased 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 blind therapeutic doses resulted in a decrease in Tg levels in the majority of patients with suspected recurrence or
metastases. The post-treatment scintigrams revealed pathological uptake in 59% of patients.