Thyroid strain in pregnancy: a nationwide study of iodine nutrition and thyroglobulin among Faroese pregnant women

OBJECTIVE: Abnormal thyroid function is particularly problematic in pregnant women. Iodine is important to maintain normal thyroid function, and the World Health Organization (WHO) recommends a raised iodine intake in pregnant compared with non-pregnant adults. The raised iodine intake level from 100 to 150 µg/L includes a safety margin, and we hypothesized that the thyroid is not strained until urinary iodine concentration (UIC) is below 100 μg/L. DESIGN: Nationwide cross-sectional study. SETTING: Routine prenatal care at the National Hospital System of the Faroe Islands, 2020-2022. PARTICIPANTS: A total of 623 pregnant women, representing 63% of all pregnancies in the Faroe Islands during the study period, with no known thyroid disease. EXPOSURE: Iodine-containining dietary intake was assessed indirectly through urinary iodine concentration (UIC) measured on spot urine using the Sandell-Kolthoff reaction. MAIN OUTCOME: The primary outcomes were serum Thyroglobulin (s-Tg) and thyrotropin (TSH) concentration serving as indicators of potential thyroid strain and thyroid function. MEASURES: UIC and TSH was measured in all participants. sS-Tg was measured in a randomly selected subset of 236 participants. RESULTS: Women were seen in median gestational week 20. None had elevated TSH; the median UIC was 108 µg/L, and the median s-Tg was 10.3 µg/L. Serum Tg differed only for the group with UIC below 50 µg/L (P = .02), but not when UIC was above 50 µg/L. TSH increased with higher UIC (P < .001). CONCLUSIONS AND RELEVANCE: Thyroglobulin levels increased only in the group of Faroese pregnant women with UIC below 50 µg/L, indicating that strain on the thyroid gland was seen with low UIC levels parallel to that of non-pregnant adults. Our results suggest that the UIC limit recommended in pregnancy may be overly strict and warrant reconsideration to balance health efficacy.