To improve the efficacy of radioactive iodine (RAI) therapy for differentiated thyroid cancer patients, a low-iodine diet (LID) prior to the therapy is recommended. In iodine-rich areas such as Korea, however, a strict LID is very difficult to maintain. We experienced the cases of three patients showing low adherence to the LID before initial RAI therapy, and analyzed the main food source supplying iodine during the LID, and examined the influence of the poorly maintained LID on the efficacy of RAI therapy. The dietary intake during the LID periods were assessed using three-day dietary records and remnant thyroid activity after the second RAI administration was also evaluated. All patients' mean daily iodine intake during two-week LID periods exceeded the 100 µg guideline set by the Korean Thyroid Association (median 110.9 µg, ranges 100.4-117.0 µg). Although the typical food sources of iodine intake are seaweeds in Korea, salted vegetables were the main contributor to the patients' iodine intake during the LID periods. Remnant thyroid activity was shown on a follow-up scan in all of 3 patients suggesting low efficacy of RAI therapy. In summary, the patients with low adherence to the LID guideline showed unsuccessful remnant ablation, and the main food source of iodine was salted vegetables. Further studies are necessary to examine the relationship between adherence of the LID and RAI efficacy according to dietary iodine intake levels, as well as food sources that cause low adherence to the LID. These data can then be used to develop more practical LID guidelines.
Citations
In the past few decades, the incidence of thyroid cancer has rapidly increased worldwide. Thyroid cancer incidence is relatively high in regions where the population's daily iodine intake is insufficient. While low dietary iodine has been considered as a risk factor for thyroid cancer development, previous studies found controversial results across different food types. Among different ethnic groups, dietary factors are influenced by various dietary patterns, eating habits, life-styles, nutrition, and other environmental factors. This review reports the association between dietary factors and thyroid cancer risk among ethnic groups living in different geologic regions. Iodine-rich food such as fish and shellfish may provide a protective role in populations with insufficient daily iodine intake. The consumption of goitrogenic food, such as cruciferous vegetables, showed a positive association with risk. While considered to be a risk factor for other cancers, alcohol intake showed a protective role against thyroid cancer. High consumption of meat such as chicken, pork, and poultry showed a positive association with the risk, but dairy products showed no significant association. Regular use of multivitamins and dietary nitrate and nitrite also showed a positive association with thyroid cancer risk. However, the study results are inconsistent and investigations into the mechanism for how dietary factors change thyroid hormone levels and influence thyroid function are required.
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