Cigarette Smoking and Iodine as Hypothyroxinemic Stressors in U.S. Women of Childbearing Age: A NHANES III Analysis

Background: Exposure to environmental thiocyanate through smoking has been suggested to lead to hypothyroxinemia, which potentially impairs brain development in the fetuses of affected women, though studies are conflicting. It was hypothesized that iodine status might modulate the effects of thiocyanate exposure on the prevalence of hypothyroxinemia in women of childbearing age. Design: The study population comprised 6967 women (age range: 15– 44 years) from the National Health and Nutrition Examination Survey (NHANES) III database. Smoking status was stratified into nonsmokers and those who smoked 1–10, 11–20, 21–30, and 31+ cigarettes a day. Iodine status was stratified based on urinary iodine excretion as <50, 50–99, 100–199, 200–299, and 300+ μg/L. Hypothyroxinemia was defined as the lower fifth percentile of total thyroxine levels among nonsmokers, adjusted for age and race/ethnicity. Univariate, multivariate, and regression analyses were conducted to evaluate the impact of smoking and urinary iodine excretion on the prevalence of hypothyroxinemia. Results: Increasing levels of cigarette smoking are associated with increasing prevalence of hypothyroxinemia [χ²(4) = 14.15, p = 0.007]. When analyzed by urinary iodine level, the hypothyroxinemic effect of smoking was limited to the highest two urinary iodine strata [χ²(4) = 41.48, p < 0.001; and χ²(4) = 40.62, p < 0.001]. A significant interaction effect between smoking and urinary iodine was noted, underscoring the relationship between high levels of urinary iodine excretion and smoking with respect to hypothyroxinemia. Conclusions: Heavy smoking was associated with a higher prevalence of hypothyroxinemia. The impact of thiocyanate exposure from smoking on the prevalence of hypothyroxinemia is limited to those women of childbearing age with the highest urinary iodine excretion. Iodine supplementation should be cautiously considered in women of childbearing age who are smokers.