Maternal-fetal thyroid function at the time of birth and its relation with iodine intake.

Background: The effects of moderate variations in dietary iodine intake on maternal and fetal thyroid function are poorly understood. Recent studies question the usefulness of neonatal screening of congenital hypothyroidism as a tool for monitoring iodine deficiency. We proposed to test the hypothesis of an association between iodine intake during the last trimester of pregnancy and fetal thyroid function at the time of birth.

Methods: The study was undertaken at term in 233 healthy pregnant women (29.7±5.6 years) and in their newborn. Inclusion of women in the study was done within the 24 hours before delivery.

Results: The median maternal urinary iodine concentration was 126.5 μg/L. The maternal free triiodothyronine (FT3), although not thyrotropin (TSH) and free thyroxine (FT4), correlated significantly with the urinary iodine concentration (r=0.17, p=0.013). The cord blood TSH, FT4, and FT3 correlated positively with the maternal urinary iodine concentration at the time of delivery (r=0.24, p=0.001; r=0.16, p=0.032; and r=0.24, p=0.003, respectively). The cord blood and heel blood TSH correlated positively with the amniotic fluid iodine concentration (r=0.21, p=0.015 and r=0.15, p=0.036). The cord blood TSH correlated positively with the cord blood FT4 (r=0.21, p=0.022) and FT3 (r=0.32, p=0.017). The maternal TSH correlated significantly with the cord blood TSH (r=0.22, p=0.014) and with the heel blood TSH (r=0.13, p=0.050).

Conclusions: The results of this study show the presence of a positive association of TSH-FT4 at the time of delivery, which may be modulated by the amount of iodine consumed by the mother during pregnancy.