Serum iodine concentration in pregnant women and its association with thyroid function.

Objective: This study aims to investigate the association of serum iodine concentration (SIC) with thyroid function-associated parameters in pregnant women in mild iodine deficient area, and explore its potential to predict individual iodine nutrition status in pregnant women.

Methods: A total of 741 pregnant women undergoing prenatal examinations in their second trimester at the Women's Hospital, Zhejiang University School of Medicine, from March 2021 to May 2022 were finally recruited into the study. Venous blood and morning urine were collected. Serum free triiodothyronine (FT3), free thyroxine (FT4), thyro-stimulating hormone (TSH), total thyroxine (TT4), total triiodothyronine (TT3), thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), UIC (urinary iodine concentration) and SIC were measured.

Results: The median SIC was 54.44 μg/L. SIC was linearly and positively associated with concentrations of FT4 [ β = 0.10, 95 % CI: (0.07, 0.12), TT4 [ β = 0.29, 95 % CI: (0.26, 0.33), TT3 [ β = 0.14, 95 % CI: (0.09, 0.18). There was an inverted "U" shaped relationship between SIC and FT3. The associations between SIC and thyroid hormones remained robust when participants taking specific drug were excluded.

Conclusions: SIC is associated with thyroid function-related parameters in pregnant women in their second trimester, indicating that the SIC may have the potential to predict individual iodine nutrition status in pregnant women.