Seaweed-Derived Iodine Intake During the Korean Postpartum Period: A 1-Year Follow-Up Study

Background: Seaweed consumption is a major source of dietary iodine in Korea, particularly among lactating women during the postpartum period. This practice raises concerns regarding short-term iodine excess and its potential effects on thyroid function. We examined the prevalence of thyroid disease and hormone abnormalities 1 year after childbirth among postpartum women with varying levels of seaweed-derived iodine intake. Methods: Between 17 July 2021 and 10 December 2021, 147 postpartum women were enrolled within two weeks after childbirth at postpartum care centers in Korea, which provide structured residential maternal and infant care, including standardized meals, during the early postpartum period. Participants provided informed consent and completed baseline questionnaires and dietary assessments. Iodine intake, including seaweed soup consumption during the 8-week postpartum period, and infant growth indicators were evaluated. A total of 81 participants completed the 1-year follow-up. At follow-up, dietary records, thyroid disease prevalence, hormone levels, urinary iodine concentration, and infant growth indicators were assessed. Results: At 1 year, none of the 81 participants had thyroid disease. The prevalence of abnormal triiodothyronine (T3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) was analyzed by iodine-intake quartiles, revealing no significant differences (T3: p = 0.4175; FT4: p = 0.1591; TSH: p = 0.9344). Conclusions: These findings suggest that the evidence regarding an association between short-term postpartum iodine intake and thyroid outcomes one year after childbirth remains inconclusive. Owing to the limited sample size, high attrition, and observational design, the study may have been underpowered to detect clinically meaningful differences, and potential effects cannot be excluded. Therefore, these results should be interpreted cautiously, and larger, well-designed longitudinal studies with repeated thyroid assessments are needed to better clarify the long-term implications of postpartum iodine exposure.