Iodine intake as a risk factor for thyroid cancer: a comprehensive review of animal and human studies

Abstract

Thyroid cancer (TC) is the most common endocrine malignancy and in most countries, incidence rates are increasing. Although differences in population iodine intake are a determinant of benign thyroid disorders, the role of iodine intake in TC remains uncertain. We review the evidence linking iodine intake and TC from animal studies, ecological studies of iodine intake and differentiated and undifferentiated TC, iodine intake and mortality from TC and occult TC at autopsy, as well as the case–control and cohort studies of TC and intake of seafood and milk products. We perform a new meta-analysis of pooled measures of effect from case–control studies of total iodine intake and TC. Finally, we examine the post-Chernobyl studies linking iodine status and risk of TC after radiation exposure. The available evidence suggests iodine deficiency is a risk factor for TC, particularly for follicular TC and possibly, for anaplastic TC. This conclusion is based on: a) consistent data showing an increase in TC (mainly follicular) in iodine deficient animals; b) a plausible mechanism (chronic TSH stimulation induced by iodine deficiency); c) consistent data from before and after studies of iodine prophylaxis showing a decrease in follicular TC and anaplastic TC; d) the indirect association between changes in iodine intake and TC mortality in the decade from 2000 to 2010; e) the autopsy studies of occult TC showing higher microcarcinoma rates with lower iodine intakes; and f) the case control studies suggesting lower risk of TC with higher total iodine intakes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13044-015-0020-8) contains supplementary material, which is available to authorized users.

Thyroid cancer incidence in Canada: a national cancer registry analysis

“…Dietary iodine deficiency or excess has never been proven to have a causal role in thyroid cancer. 19 Some speculate that obesity and diabetes may be associated with increasing incidence of thyroid cancer, but this association is still controversial and is confounded by increased use of diagnostic imaging leading to more detection. 20,21 Within Canada, it is unlikely there is enough difference among provinces for any of these risk factors or genetic mutations to cause such a large and differential increase in thyroid cancer incidence.…”

Ocena skuteczności profilaktyki jodowej w ciąży — analiza przeprowadzona w jednym z referencyjnych ośrodków ginekologiczno-położniczych w Polsce

“…Thyroid hormones not only ensure the proper functioning of the body, but during foetal life and childhood they are also responsible for the proper development of the whole organism, the nervous system included [1,2].…”

Dietary iodine intake, thyroid diseases and the prevalence of papillary carcinoma (PTC)

“…External radiation to the thyroid increases risk of thyroid cancer, particularly when the radiation occurs in children or adolescents [12][13][14][15][16][17][18][19][20].The Chernobyl nuclear accident in 1986 exposed population of Belarus, Ukraine, and the Russian Federation to internal radiation from radioactive iodines deposited in the thyroid, resulting in sharp increase in pediatric and adolescent thyroid cancer [18]. About 60% of the Belarusian thyroid cancer and 30% of the Ukrainian cases, mainly PTC, were identified about 20 years after the accident [21].…”