TNM status and posttherapy whole-body scan findings aid in risk stratification of patients with thyroid cancer during initial therapy. Recently, the thyroglobulin (Tg) value measured during hypothyroidism just before 131I therapy (preablation Tg) has proved to be effective for predicting persistent/recurrent disease. In this study, we assessed the changes in serum Tg 48 hours after radioiodine ablative therapy performed in the hypothyroid state in order to evaluate if this parameter could be used in recombinant human thyrotropin (rhTSH)-treated patients. Because rhTSH-stimulated TG is traditionally measured 72 hours after the second injection of rhTSH corresponding to 48 hours post-131I therapy, the time course of serum Tg after radioiodine administration is an important clinical issue. To address this issue, we performed a prospective evaluation of 26 consecutive patients with low-risk differentiated thyroid cancer hypothyroidism for radioiodine ablation (3.7 GBq of 131I). Baseline Tg values were compared to posttherapy Tg values (at 24 and 48 hours). We found that Tg increased after 131I therapy because of the acute radiation effects on residual thyroid cells. Median values at each of the three time points were 1.8 ng/mL (baseline), 3 ng/mL (Tg-24), and 11.3 ng/mL (Tg-48) (Brahms Tg Kryptor assay, Brahms AG, Berlin, Germany). Tg-48 values were not statistically correlated with initial Tg values. Tg-48 remained below 15 ng/mL in 14 of 26 patients. In conclusion, the increase in Tg during the early post-131I therapy period means that first rhTSH-stimulated Tg cannot be used as a corresponding value for preablative hypo-Tg. We discussed whether rhTSH-stimulated Tg value might be useful in a subset of patients. In our opinion, this drawback does not outweight the expected benefits of rhTSH-aided therapy on quality of life of patients and overall cost of the therapy.