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Excess iodine can exacerbate autoimmune thyroiditis. Assessing status should be completed prior to additional iodine supplementation. Selenium status tends to be insufficient with autoimmune thyroiditis and moderate supplementation may be indicated.
Although iodine deficiency is the major cause of hypothyroidism worldwide, autoimmune thyroiditis (e.g., Hashimoto’s) is the primary cause in the United States and westernized countries. Some concern has been raised about supplementing iodine in individuals with Hashimoto’s as it can stimulate thyroid peroxidase production, which in turn triggers an increase in TPO antibodies and potentially an autoimmune flare-up (Raymond 2021).
However, a 2021 review of the literature suggests that the increase in TPO and TPO antibodies that may occur with the recommended iodine intake of 150 ug/day is likely transient and may not be clinically significant, especially in those with stable iodine status (urinary iodine concentration between 100-300 ug/L). Researchers emphasize that environmental and genetic factors are likely contributors to the increasing trend in autoimmunity rather than iodine supplementation being a major cause (Ruggeri 2021).
It is important to note that excess iodine exposure may increase the risk of hypothyroidism. Past research on 3,018 Chinese subjects indicates that excess iodine exposure can promote thyroid disfunction. Overt hypothyroidism, subclinical hypothyroidism, and autoimmune thyroiditis increased as iodine intake from drinking water and iodized salt increased. Iodine exposure was determined to be insufficient, sufficient, or excess with mean urinary iodine concentrations of 84-88 ug/L, 214-243 ug/L, and 634-651 ug/L respectively. Upon follow-up, the highest iodine exposure appeared to accelerate conversion to autoimmune thyroiditis for those with baseline elevations in TPO and thyroglobulin antibodies (Teng 2006).
Iodine status should be assessed prior to providing additional supplemental iodine, especially in those at risk for autoimmune thyroiditis. Ideally, urinary iodine concentration should be between 100-300 ug/L (Ruggeri 2021) and serum iodine between 36-79.3 ug/L (284-625 ug/L) (Yu 2020).
The thyroid gland has the highest concentration of selenium in the body. Selenium is crucial to thyroid hormone production and function and is often insufficient in those with autoimmune thyroiditis. Serum selenium should be maintained above 80 ug/L (1.02 umol/L) while levels above 120 ug/L (1.52 umol/L) have been associated with remission and better outcomes in Hashimoto’s. Supplementation with 50-100 ug/day, preferably in the organic form such as selenomethionine, may be supportive for autoimmune thyroiditis (Duntas 2015).
Duntas, L H. “The Role of Iodine and Selenium in Autoimmune Thyroiditis.” Hormone and metabolic research = Hormon- und Stoffwechselforschung = Hormones et metabolisme vol. 47,10 (2015): 721-6. doi:10.1055/s-0035-1559631
Raymond, Janice L., et al. Krause and Mahan's Food & the Nutrition Care Process. Elsevier, 2021.
Ruggeri, R M, and F Trimarchi. “Iodine nutrition optimization: are there risks for thyroid autoimmunity?.” Journal of endocrinological investigation vol. 44,9 (2021): 1827-1835. doi:10.1007/s40618-021-01548-x
Teng, Weiping et al. “Effect of iodine intake on thyroid diseases in China.” The New England journal of medicine vol. 354,26 (2006): 2783-93. doi:10.1056/NEJMoa054022
Yu, Songlin et al. “Establishing reference intervals for urine and serum iodine levels: A nationwide multicenter study of a euthyroid Chinese population.” Clinica chimica acta; international journal of clinical chemistry vol. 502 (2020): 34-40. doi:10.1016/j.cca.2019.11.038