Iodoprophylaxis and thyroid autoimmunity: an update

Abstract

Adequate iodine intake is necessary for normal thyroid function. Iodine deficiency is associated with serious complications, but also iodine excess can lead to thyroid dysfunction, and iodine supplementation aimed to prevent iodine deficiency disorders has been associated with development of thyroid autoimmunity. The epidemiology of thyroid diseases has undergone profound changes since the implementation of iodoprophylaxis, notably by means of iodine-enriched salt, specifically resulting in decreased prevalence of goiter and neonatal hypothyroidism, improved cognitive function development in infancy, and reduced incidence of more aggressive forms of thyroid cancer. The main question we address with this review is the clinical relevance of the possible effect on autoimmunity exerted by the use of iodine-enriched salt to correct iodine deficiency. In animal models, exogenous iodine is able to trigger or exacerbate thyroid autoimmunity, but it is still not clear whether the observed immunological changes are due to a direct effect of iodine on immune response, or whether they represent a secondary response to a toxic effect of iodine on thyroid tissue. Previous iodine status of a population seems to influence the functional thyroid response to increased iodine intake and possibly the development of thyroid autoimmunity. Moreover, the prevalence of thyroid antibodies, regarded as hallmark of autoimmune thyroid disease, varies between populations under the influence of genetic and environmental factors, and the presence of thyroid antibodies does not always coincide with the presence of thyroid disease or its future development. In addition, the incidence of autoimmune diseases shows a general increasing trend in the last decades. For all these reasons, available data are quite heterogeneous and difficult to analyze and compare. In conclusion, available data from long-term population surveys show that a higher than adequate population iodine intake due to a poorly controlled program of iodine prophylaxis could induce thyroid dysfunction, including thyroid autoimmunity mostly represented by euthyroid or subclinical hypothyroid autoimmune thyroiditis. Close monitoring iodine prophylaxis is therefore advised to ensure that effects of both iodine deficiency and iodine excess are avoided.

The Role of Iodine for Thyroid Function in Lactating Women and Infants

“…The incidence of postpartum thyroiditis has not directly been demonstrated to be associated with iodine status ( 61 ). However, some studies, but not all, observed an increased risk for TPO antibody positivity in iodine excess ( 60 , 62 , 63 ), which in turn is associated with postpartum thyroiditis. Three small randomized studies have examined the effects of prenatal iodine supplementation on postpartum thyroiditis, but none of the studies observed a difference in frequency or severity of the condition ( 61 , 64 , 65 ).…”

The effect of sodium restriction on iodine prophylaxis: a review

“…Due to the primary relevance of optimal sodium intake, as well as iodine supply, further updated prospective studies on the effects of salt restriction on iodine status are required. Anyhow, there is no doubt that a proper monitoring of iodoprophylaxis is mandatory, to ensure a sustained adequate iodine supply, and to avoid iodine excess, a possible risk of uncontrolled iodine prophylaxis, that may result in an increased incidence of thyroid autoimmunity [71][72][73].…”

Autoimmune Hashimoto’s Thyroiditis and Hypothyroidism: Novel Aspects

“…An increased prevalence of antithyroid antibodies has been observed in residents in areas with iodine excess [38]. Salt iodination may induce the development of thyroid autoimmunity [39]. The presence of autoimmune thyroiditis has been associated with both iodine deficiency and iodine excess suggesting a U-shaped relationship between iodine status and thyroid autoimmunity risk in adults [40].…”