Results of a new study show that cigarette smoking during pregnancy is associated with potentially harmful changes in both maternal and fetal thyroid function. Adverse outcomes associated with thyroid dysfunction during pregnancy include increased risk of miscarriage, premature birth, low birth weight and impaired neuropsychological development of the baby.

“We studied the influence of cigarette smoking on thyroid function of two groups of women at different stages of pregnancy – one in the first trimester and the other in the third trimester,” says coauthor Bijay Vaidya, PhD, of Peninsula Medical School at Royal Devon and Exeter Hospital in the United Kingdom. “In both groups we found that smoking during pregnancy is associated with changes in the mothers’ thyroid hormone levels.”

Vaidya and colleagues also measured thyroid hormone levels in the umbilical cord of babies born to smoking mothers. Results found that smoking-related changes in thyroid function extend to the fetus, which the Vaidya believes could have potentially harmful biological consequences. Additionally, mothers who stopped smoking during pregnancy were found to have thyroid hormone levels comparable to levels found in non-smokers—suggesting that changes in thyroid function are rapidly reversible.

The study is published in February 2009 issue of the Journal of Clinical Endocrinology & Metabolism.