Thyroglobulin and 131I uptake of remaining tissue in patients with differentiated carcinoma after thyroidectomy

In 158 thyroidectomized patients with well-differentiated non-metastatic thyroid cancer the results of serum thyroglobulin (Tg) determination and 131I uptake values established immediately before radioiodine treatment (19 days after surgery) were compared. In 113 patients (72%) Tg was above 6 ng ml-1 (lower limit of detection) and 131I uptake exceeded 2%. In 11 patients (7%) Tg was undetectable and 131I uptake less than 2%. In these cases a radioiodine treatment was not performed; the thyroid ablation was achieved by surgery only. In 34 patients (21%) with undetectable Tg, 131I uptake values, however, varied between 3 and 46% (mean value: 9.6%). Only in 4 of these 34 patients was TSH not maximally (50 microU ml-1) stimulated because of a shorter (9, 10, 11, 13 days) period from thyroidectomy. In conclusion, remaining thyroid tissue, highly stimulated by TSH, can trap a remarkable amount of radioiodine, but may be unable to produce detectable amounts of Tg. Therefore, in contrast to 131I, measuring Tg is insufficient to document the success of thyroid ablation. For the first follow-up study after ablative therapy, the uptake test with radioiodine is mandatory.