Reevaluation of the impact of a stringent low-iodine diet on ablation rates in radioiodine treatment of thyroid carcinoma

Prior analyses of the impact of stringent, preablative low-iodine diets (LIDs) on ablation in patients with differentiated thyroid cancer postthyroidectomy are dated. We retrospectively reviewed first-time, short-term ablation rates for 44 LID patients and 50 patients following a regular diet (RD) who were verbally instructed to avoid salt, seafood, and multivitamins containing iodine. Patients who had undergone ablation were given between 100 and 200 mCi Of I-131, depending on the presence of metastases. We found a 68.2% ablation rate for LID patients, compared to a 62.0% rate for RD patients, a nonsignificant difference (p = 0.53). We observed a dose-response relationship for both patient groups, with higher ablation rates corresponding to higher doses of radioiodine administered. We also measured iodine levels in spot urine samples from 7 matched LID patients and 7 matched RD adherents (healthy volunteers) prediet and postdiet as well as 39 healthy volunteers. LID patients had a lower mean urinary iodine level postdiet (173.9 mug/L; range, 45-1,217 mug/L; standard deviation [SD] = 127.7) than the RD patients (mean, 381.4 mug/L; range, 140-630 mug/L; SD = 196.3) or the 39 normal controls (444.0 mug/L; range, 50-1,690 mug/L; SD = 413.4). Whereas the LID lowered urinary iodine levels by 69.4% from prediet values, the RD reduced urinary iodine by 23.6%. Although differences in the reduction of urinary iodine levels between the LID and the RD were substantial, both groups experienced equivalent outcomes. The level of iodine in the American diet has progressively decreased, and may be much lower now than when prior LID studies were conducted. We suggest that prescribing a refined, less stringent diet that avoids high-iodine-containing foods would offer equivalent outcomes with increased patient convenience.