Abstract Background: Our aim was to study the relationship between thyroid function and body weight in a large Finnish adult population, taking into account the effect of gender, because the results of previous studies are conflicting. Methods: A random sample of 4500 Finnish subjects aged 45–74 years old was selected from the National Population Register. The participation rate was 64%. Height, weight, waist circumference, and blood pressure were measured. Medications used, current smoking, the use of alcohol, and leisure-time physical activity (LTPA) wereas queried. Thyroid values (free T4, free T3, and TSH) were measured in 1307 men and 1434 women. Subjects receiving thyroid hormone (N=92) were excluded. Results: The mean age of the subjects was 59.7 years and their mean body mass index (BMI) was 27.4 kg/m². After adjustment for age, LTPA, and current smoking, TSH showed no linearity (p=0.09) across increasing BMI; in women TSH ranged from 1.85 ± 1.15 to 2.02 ± 1.29 IU/L and in men, from 1.79 ± 1.19 to 2.04 ± 1.33 IU/L (p=0.13). FT3 -values increased from 3.85 ± 0.67 to 3.97 ± 0.59 pmol/L in women (p=0.004), but not in men, with increasing BMI. FT4 -values decreased from 13.78 ± 2.07 to 13.31 ± 1.91 pmol/L with increasing BMI in men (p<0.001 for linearity), but not in women. Conclusions: TSH values did not increase along with BMI in men and women with BMI, but FfT3 levels increased in women, and FfT4 levels decreased in men along with increasing BMI. The reasons for these gender differences need further research.