The impact of smoking on thyroid volume and function in relation to a shift towards iodine sufficiency

The aim of the study was to investigate whether the influence of smoking on thyroid volume and function changes in relation to a higher iodine intake in the population. The study comprised a total of 8,219 individuals each examined in one of two separate cross-sectional studies performed before (n = 4,649) and after (n = 3,570) a mandatory iodization of salt in year 2000 in two areas with established mild and moderate iodine deficiency. Participants answered questionnaires regarding life style factors and a thyroid ultrasonography was performed. Blood samples were analysed for serum thyroid stimulating hormone, free thyroxin and free tri-iodothyronine. All procedures used were similar in the two cross-sectional studies. The overall difference in thyroid volume between heavy smokers and non-smokers across the age groups was reduced after iodization of salt from 24% (19-29%) to 12% (6-18%). After the iodization the odds ratio for having thyroid enlargement was still increased for smokers (OR: 1.9 (CI: 1.5-2.6)) compared to non-smokers. The aetiological fraction of thyroid enlargement due to smoking was 39.3% before the iodization and 24.2% after the iodization. Like before iodization smokers had a lower mean thyroid stimulating hormone and a higher free thyroxin in serum than non-smokers. Conclusively, in areas approaching iodine sufficiency a decline in the impact of smoking on thyroid volume was seen. The effect of smoking on hormonal level was unchanged after the iodization. Thus the effect of smoking on thyroid volume seems to be dependent on iodine intake, whereas the effect on function seems mainly to depend on other factors.