Context: Recent studies have suggested that the association of low thyroid function with high body mass is restricted to nonsmokers.
Objective: The aim was to study the association of thyroid function with body mass separately for smokers and never-smokers.
Design and setting: We conducted a cross-sectional, population-based study.
Subjects: We studied 27,097 individuals older than 40 yr of age who were without previously known thyroid disease.
Main outcome measures: We measured mean body mass index (BMI) and odds ratio for obesity (BMI > or = 30.0 kg/m(2)) according to categories of thyroid function, in women and men, and separately for current smokers and never-smokers. We also studied the association with BMI within the reference range of TSH (0.50-3.5 mU/liter).
Results: TSH within the reference range was positively associated with BMI (P for trend < or = 0.001 in all groups) and with the prevalence of obesity (P for trend < 0.005 in all groups). Among women, the association did not differ between current smokers and never-smokers, but in men the association was stronger for current smokers. Hypothyroid function was associated with higher BMI and higher prevalence of obesity in women (subclinical and overt hypothyroidism) and men (subclinical hypothyroidism), both in current smokers and in never-smokers.
Conclusion: The association of low thyroid function with high body mass was as least as strong in current smokers as in never-smokers, and our results clearly show that the association is not limited to nonsmokers, as previously suggested.