Relationship between body mass index and thyrotropin in euthyroid women: differences by smoking, race and menopausal...

To evaluate potential effect modifications on the association between BMI and thyrotropin (TSH) by smoking, race, and menopausal status among euthyroid women.

A cross-sectional population-based study was carried out in the city of Rio de Janeiro, Brazil, in 2004-2005. Households (1,500) were selected using three-stage probability sampling. A sample of 1,084 women aged 35 years or older and with TSH values within the reference range (0.3-4.0 mIU/l) was investigated. Weight and height were measured at household and blood collected for serum TSH and anti-thyroperoxidase (anti-TPO) antibody analysis.

Overall, BMI was positively and significantly associated with serum TSH (β = 0.87; p = 0.001). This association was modified by smoking, race, and menopausal status (p < 0.05). Adjusted regression coefficients were 1.78 versus 0.58 comparing smokers with non-smokers, 1.39 for Blacks compared to 0.79 for Non-Blacks, and 0.70 for women in menopause compared to 1.04 for premenopausal women. The percentage of high anti-TPO (greater than 35 UI/ml) was 8.8%, and the association between TSH and BMI was no longer significant in this group.

BMI was positively associated with serum TSH specifically in its normal range, but only for those women who tested negative for anti-TPO. Smoking and race are negatively associated with anti-TPO, possibly explaining the effect modification observed.