Relationship of smoking and/or passive exposure to tobacco smoke on the association between serum thyrotropin and body mass index in large groups of adolescents and children.

BACKGROUND: Hypothyroidism may be a risk factor for obesity, because metabolic rates tend to be decreased in this disorder. Currently, it is under debate whether smoking may modify the association between hypothyroidism and obesity. Against this background, the aims of our study were to investigate whether there is an association between serum thyrotropin (TSH) and body mass index (BMI) in children and adolescents and whether smoking mediates this association.
METHODS: Data from 6435 children (ages 3-10) and 5918 adolescents (ages 11-17) from the "The German Health Interview and Examination Survey for Children and Adolescents" (KiGGS) were analyzed. Smoking status was classified into two categories: smokers and nonsmokers. A subject was classified as being exposed to environmental tobacco smoke (ETS) if someone in the parental home smoked more than 1 day a week. Serum TSH levels were measured with an ELISA method. Serum TSH levels were associated with BMI and obesity by multivariable linear regression stratified by smoking status and exposure to ETS.
RESULTS: In adolescents, there was an association between serum TSH levels and BMI, and it was stronger in smokers (β=0.62; 95% confidence interval [CI 0.38-0.85]) than in nonsmokers (β=0.18 [CI 0.09-0.28]). Likewise, it was stronger in adolescents exposed to ETS and adolescents not exposed to ETS. In children, who were either exposed or not exposed to ETS, there was an association between serum TSH levels and BMI. In them, however, the strength of this association was similar in those exposed and those not exposed to ETS.
CONCLUSIONS: Active and passive smoking may mediate the association between thyroid function and BMI in adolescents. In smoking adolescents, hypothyroidism may lead to an increase of the BMI, whereas this is not the case in nonsmoking adolescents.