Excessive Iodine Intake During Lactation Is Not Related to the Incidence of Thyroid Disease: A 3-Year Follow-Up Study

<b>Objectives</b>: This study aimed to investigate the relationship between excessive postpartum iodine intake and the incidence of thyroid disease in mothers, as well as child growth and development. <b>Methods</b>: Of 1054 participants in the 2019 nationwide survey that assessed maternal postpartum iodine intake, 684 mothers participated in a follow-up study. Data on maternal thyroid disease incidence and child growth and development from infant or toddler health checkups were collected. Iodine and nutrient intake were assessed using three-day dietary records, and serum thyroid hormones (triiodothyronine (T3), thyroid-stimulating hormone (TSH), and free thyroxine (free T4)) were measured. Relative risks (RRs) were estimated using Poisson regression analysis. <b>Results:</b> Among the 684 participants, 23 (3.4%) were diagnosed with thyroid disease by a physician during the follow-up period. The incidence of maternal thyroid disease was not significantly associated with excessive iodine intake, even after adjusting for confounding factors. Additionally, excessive maternal iodine intake was not related to subclinical hypothyroidism in mothers or child growth and development. <b>Conclusions</b>: After a three-year follow-up, no relationship was observed between high postpartum iodine intake and the risk of thyroid disease. Large-scale longitudinal studies are required to evaluate the long-term effects of excessive postpartum iodine intake on maternal health and child growth and development.