One hundred-forty healthy pregnant women hospitalized for delivery and their newborn infants were studied. The presence of cotinine in urine and serum was used to classify mothers as smokers (n = 50) or nonsmokers (n = 90). Smoking and nonsmoking mothers had identical urinary iodine levels on day five after delivery, but smoking was associated with reduced iodine content in breast milk (smokers, 26.0 mcg/L; nonsmokers, 53.8 mcg/L; p < 0.001) and in the infants' urine (smokers, 33.3 mcg/L; nonsmokers, 50.4 mcg/L; p = 0.005). In smokers, iodine transfer into breast milk correlated negatively with urinary cotinine concentration. Smoking mothers had significantly higher serum levels of thiocyanate, which may competitively inhibit iodide transport into breast milk.

Comment: Iodine deficiency during the fetal stage or the first years of life may lead to hypothyroidism, resulting in impaired brain development. During the period of breastfeeding, thyroid function of the infant depends on the presence of adequate amounts of iodine in maternal milk. In addition to any other adverse effects smoking may have on infant development, the results of the present study suggest that smoking while breastfeeding increases the risk of iodine deficiency-induced brain damage in the child. Women who breastfeed should not smoke, but if they do, an iodine supplement should be considered.

Laurberg P, et al. Iodine nutrition in breast-fed infants is impaired by maternal smoking. J Clin Endocrinol Metab 2004;89:181-187.