Effects of low-iodide diet on postsurgical radioiodide ablation therapy in patients with differentiated thyroid carcinoma

OBJECTIVE: Most patients with differentiated thyroid carcinoma (DTC) undergo total thyroidectomy followed by routine radioiodide thyroid remnant ablation. Most centres that routinely perform radioiodide ablation prescribe a low-iodide diet (LID) to increase the radioiodide accumulation in thyroid remnants. The efficacy of an LID on thyroid remnant ablation, however, has never been demonstrated convincingly. DESIGN AND METHODS: In a retrospective study, we studied two groups of DTC patients without distant metastases, who had received either a standard diet or an LID during ablation (LID group, n = 59, and control group, n = 61). Both groups were compared for radioiodide uptake in thyroid remnants during ablation and efficacy parameters of remnant ablation, 6 months after ablation. A subgroup without extrathyroidal tumour growth was analysed separately (stages T1-3, N0). RESULTS: In the total group, the LID during ablation decreased the 24-h urinary iodide excretion to 26.6 micro g compared with 158.8 micro g in controls whereas radioiodide uptake in thyroid remnants was increased by 65% (P < 0.001). Six months after ablation, patients were investigated after thyroid hormone withdrawal. In the total group, no significant effects of the LID during ablation were observed on thyroglobulin (Tg) or the percentage of patients with persistent neck activity after 185 MBq 131I. However, in the LID group, 65% of patients without Tg antibodies had undergone successful ablation (defined by absent neck activity and Tg < 2 micro g/l) compared with 48% in the control group (P < 0.001). In the subgroup (T1-3, N0), 8% of the patients who had undergone the LID had Tg >/= 2 micro g/l vs. 32% in the control group (P = 0.012), whereas successful ablation was achieved in 71% of patients without Tg antibodies in the LID vs. 45% in the control group (P < 0.001). CONCLUSION: We conclude from this study that a low-iodide diet during thyroid remnant ablation improves the efficacy of this treatment.