To investigate whether there is an association between serum thyroid-stimulating hormone (TSH) within the normal range and body mass index (BMI). The study was performed in 6164 subjects (2813 males) who attended the fifth Tromsø study in 2001, and in 1867 subjects (873 males) that attended both the fourth Tromsø study in 1994/1995 as well as the fifth Tromsø study. Height, weight, and serum TSH were measured in all subjects, and smoking status was recorded. Smokers and nonsmokers were analyzed separately. In the fifth Tromsø study, serum TSH was positively and significantly associated with BMI in the nonsmokers. Within the normal TSH range (defined as the 2.5-97.5 percentile), nonsmoking males in the highest TSH quartile had a mean BMI 0.4 kg/m(2) higher compared to those in the lower quartile, whereas the difference for nonsmoking women was 1.4 kg/m(2). Similarly, in nonsmokers in the longitudinal study, there was a significant and positive association between delta serum TSH (serum TSH in 2001 minus serum TSH in 1994) and delta BMI in those with serum TSH within the normal range both in 1994 and 2001. In these subjects, the quartile with the highest delta serum TSH had a mean increase in BMI from 1994 to 2001 that was 0.3 kg/m(2) higher compared to those in the quartile with the lowest delta serum TSH. For the smokers, relations between serum TSH and BMI were not statistically significant. In nonsmokers there is a positive association between serum TSH within the normal range and BMI.