In this large cohort, including more than two million individuals, we explored the associations between BMI and height and the occurrence of thyroid cancer. The risk was found to be moderately elevated with increasing BMI and height in both sexes among the follicle-cell derived (papillary and follicular) carcinomas. In medullary carcinomas, an opposite trend was present for BMI, pointing to a different pathogenetic influence.
The associations between BMI and height with thyroid cancer have been difficult to explore in previous small studies owing to the low incidence of this disease. In addition to the large size of the present cohort, we also had an almost complete follow-up with regard to thyroid cancer incidence and deaths. This was made possible by linkages to population-based registries of high quality. Only 0.04% of the study cohort (71 persons) was lost to follow-up, and 0.6% was censored by emigration from Norway.
After a pooled analysis of 12 case–control studies, including 2473 cases, Dal Maso et al (2000
) previously found a moderately increased risk of thyroid cancer related to height and weight at diagnosis. Dal Maso et al
(2000) used self-reported values for height and weight during the late teens and at diagnosis. In the present study, we relied on height and weight measurements performed on average 15 years before the diagnosis of thyroid cancer. Hence, our estimates were not diluted by a possible influence of individuals weight on the disease.
Iribarren et al (2001
) evaluated several potential predictors of thyroid cancer in a large American cohort with long follow-up, including more than 200
000 persons. Despite the large size of the cohort and the long follow-up, only 196 thyroid cancers were observed. In this study, no association between BMI, height or weight gain and thyroid cancer was observed.
Dal Maso et al (2000
) hypothesised that an association between BMI and thyroid cancer could be owing to a relationship with steroid hormones or other endocrine factors. However, they observed an association between BMI and thyroid cancer of similar magnitude in older and younger women. Also in the present study, there was no difference in the association between BMI and thyroid cancer in older and younger women.
Like Dal Maso et al (2000
), we observed a moderately increasing risk of thyroid cancer with increasing height. This association may be owing to dietary influence during childhood or adolescence and possible interactions with growth factors.
In conclusion, the present study showed that the risk of thyroid cancer increased moderately with increasing BMI and height in both males and females.