Context: Pregnant women are often iron deficient, and iron deficiency has adverse effects on thyroid metabolism. Impaired maternal thyroid function during pregnancy may cause neurodevelopmental delays in the offspring.

Objective: To investigate if maternal iron status is a determinant of TSH and/or TT4 concentrations during pregnancy

Design and Outcome Measures: In a representative national sample of Swiss pregnant women (n=365) in the 2^nd and 3^rd trimester, samples of urine and blood were collected, and data on maternal characteristics and supplement use was recorded. Concentrations of TSH, TT4, hemoglobin, mean corpuscular volume, serum ferritin (SF), transferrin receptor (TfR) and urinary iodine (UI) were measured. Body iron stores were calculated and stepwise regressions done to look for associations.

Results: The median UI (range) was 139 (30-433) µg/L. In the 3^rd trimester, nearly 40% of women had negative body iron stores, 16% of woman had a TT4 <100 nmol/L and 6% had a TSH >4.0 mU/L. Compared to the women with positive body iron stores, the relative risk of a TT4 <100 nmol/L in the group with negative body iron stores was 7.8 (95% CI: 4.1; 14.9). Of the 12 women with TSH >4.0 mU/L, 10 of them had negative body iron stores. SF, TfR and body iron stores were highly significant predictors of TSH (standardized : -0.506, 0.602, and -0.589, respectively, all p<0.0001) and TT4 (standardized : 0.679, -0.589, and 0.659, respectively, all p<0.0001).

Conclusions: Poor maternal iron status predicts both higher TSH and lower TT4 concentrations during pregnancy in an area of borderline iodine deficiency.