An enhanced treatment effect can be expected from a higher serum thyroglobulin level after radioactive iodine therapy

The aim of this study was to investigate if increased serum thyroglobulin (Tg) levels after radioactive iodine (RAI) showed more therapeutic effects in patients with differentiated thyroid cancer (DTC).

Data of 65 patients with DTC who underwent RAI from June 2014 to September 2016 were reviewed. Serum thyroglobulin was measured immediately before (Tg1) and 48 h (Tg2) after RAI under TSH stimulation. Differences and ratios between serum Tg measurements (DeltaTg = Tg2 - Tg1 and RatioTg = Tg2/Tg1) were calculated. The treatment response of distant metastasis was assessed using the RECIST criteria.

There was no difference in the median values of Tg1 and Tg2 (2.6 [range, 0.7-1957.5] ng/mL vs. 7.4 [range, 0.7-5276.0] ng/mL, p = 0.240) in all patients (73 scans, 65 patients). In subgroup analysis, Tg levels increased slightly in patients with distant metastasis (8 scans, 7 patients) (Tg1 vs. Tg2; 48.9 [range, 2.4-1957.5] ng/mL vs. 63.2 [range, 4.4-5276.0] ng/mL, p = 0.408). Among patients with distant metastasis, one patient with a partial response to treatment had a more than 4000fold increase in Tg levels and one patient with stable disease showed a 20fold increase in Tg levels. In contrast, five patients with disease progression showed only two to eightfold increase or more than 100fold decrease in Tg levels at 48 h after RAI. However, there was a significant increase in serum Tg levels in patients without distant metastasis (65 scans, 58 patients) after RAI (Tg1 vs. Tg2; 2.0 [range, 0.7-141.9] ng/mL vs. 6.8 [range, 0.7-577.7] ng/mL, p = 0.026).

A higher elevation of Tg levels after RAI may be associated with a better treatment outcome in DTC patients with distant metastasis. An increase in Tg levels after RAI may reflect the destruction of cancer and thyroid cells.