In assessing whether effects of smoking on thyroid function may be confounded by body mass, it is important to consider the possible causal directions of the association between thyroid function and body mass. A Danish population-based study showed that body mass index (BMI) was positively associated with thyrotropin level but negatively associated with free thyroxine level, suggesting that thyroid function may influence BMI.1 With this causal direction, BMI does not fulfill the criteria for being a confounder or a mediator of the associations of smoking with thyroid function.2,3
Åsvold BO, Bjøro T, Nilsen TI, Vatten LJ. Tobacco Smoking and Thyroid Function: Is Weight Gain a Confounder?—Reply. Arch Intern Med. 2008;168(1):114. doi:10.1001/archinternmed.2007.20
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