Iodine Excess as an Environmental Risk Factor for Autoimmune Thyroid Disease

Abstract

The global effort to prevent iodine deficiency disorders through iodine supplementation, such as universal salt iodization, has achieved impressive progress during the last few decades. However, iodine excess, due to extensive environmental iodine exposure in addition to poor monitoring, is currently a more frequent occurrence than iodine deficiency. Iodine excess is a precipitating environmental factor in the development of autoimmune thyroid disease. Excessive amounts of iodide have been linked to the development of autoimmune thyroiditis in humans and animals, while intrathyroidal depletion of iodine prevents disease in animal strains susceptible to severe thyroiditis. Although the mechanisms by which iodide induces thyroiditis are still unclear, several mechanisms have been proposed: (1) excess iodine induces the production of cytokines and chemokines that can recruit immunocompetent cells to the thyroid; (2) processing excess iodine in thyroid epithelial cells may result in elevated levels of oxidative stress, leading to harmful lipid oxidation and thyroid tissue injuries; and (3) iodine incorporation in the protein chain of thyroglobulin may augment the antigenicity of this molecule. This review will summarize the current knowledge regarding excess iodide as an environmental toxicant and relate it to the development of autoimmune thyroid disease.

Activation of the Nrf2-Keap 1 Pathway in Short-Term Iodide Excess in Thyroid in Rats

“…Paul et al demonstrated that when normal volunteers received 1500  μ g of supplemental iodine daily for 14 days, a small decrease in serum T3 and T4 concentrations with compensatory increase of TSH was detected, although all values remained within the normal ranges [34]. However, the presence of handicaps such as an increased autoimmune susceptibility, fetal period, extremes of age, pregnancy, lactation, or an active pathological entity significantly impair these mechanisms [1, 35–37]. …”

“…Iodine being a critical constituent of thyroid hormones is essential for normal growth and development in all vertebrates [1, 2]. During thyroid hormone synthesis, there is a constant production of reactive oxygen species (ROS), especially hydrogen peroxide (H 2 O 2 ), which is subsequently utilized for the oxidation of iodide [3].…”

Prospective study of seaweed consumption and thyroid cancer incidence in women

“…Iodine excess intake has been linked to the development of autoimmune thyroid diseases (Luo et al, 2014;Sun et al, 2014). However, to our knowledge, other studies did not find associations of dietary iodine intake from edible seaweed or supplements with thyroid cancer incidence (Kolonel et al, 1990;Horn-Ross et al, 2001;Zhang et al, 2013).…”

“…However, some studies have found high iodine intake to be associated with the development of thyroid cancer in rodents (Kanno et al, 1992;Takegawa et al, 2000). Other recent evidence also suggested that high iodine intake may lead to autoimmune thyroiditis (Teng et al, 2006;Leung and Braverman, 2014;Luo et al, 2014), which may indirectly induce thyroid cancer, including papillary carcinoma (Antonaci et al, 2009).…”

Endocrinology and the nudge hypothesis

“…For instance, one could argue that iodization of salt has been followed by an increase in autoimmune thyroid disease. [4] However, the evidence for this being a causative link remains tenuous. Moreover, the prevention of iodine-deficiency disorders, with its far-reaching effects on maternal health and neonatal neurocognition, may outweigh any such presumptuous risks.…”