Urinary iodine and thyroid function in a population of healthy pregnant women in the North of Spain

Abstract

Background: Iodine is an essential trace element for the synthesis of thyroid hormones, which are keys in maternal metabolism during pregnancy as well as in neurological development during fetal and postnatal life. This was a prospective study on iodine status and thyroid function in women during pregnancy in the Basque country to assess whether there was any relationship among maternal urinary iodine, maternal thyroid function and thyrotropin (TSH) in newborns, and to explore any difference in women experiencing miscarriages.

Methods: We analyzed TSH, free T(4) (FT(4)), free T(3) (FT(3)), thyroid peroxidase antibody (TPO-Ab) titers in serum and urinary iodine concentrations (UIC) in 2104 women in the first trimester of pregnancy and in 1322 of them in their second trimester. We obtained neonatal TSH levels in 1868 cases.

Results: In the first (T1) and second trimesters (T2), the median UICs were 88.5 μg/L and 140 μg/L, respectively. No relationship was found between UIC and FT4, or maternal and neonatal TSH. In T1 and T2, 9.7% and 7.5% of women were TPO-Ab positive, respectively. The total miscarriage rate was 10%. The percentage of miscarriages in healthy women was 8.9%, lower than in women with overt hypothyroidism (21.2%; p < 0.001) and than in women with subclinical hypothyroidism (15.6%; p < 0.025). The miscarriage rate was not higher in TPO-Ab-positive women.

Conclusions: In this study most women had iodine deficiency during pregnancy. Neonatal TSH is not correlated with maternal UIC during pregnancy. Pregnant women with hypothyroidism have a higher rate of miscarriages.

Keywords: Iodine epidemiology; Iodine measurement; Pregnancy; TSH; Thyroid hormones.