Excess iodine supplementation, effects

Experimental Studies on the Effect of Excessive Iodine Supplementation on the Thyroid Tissue... 

Effects of chronic excessive iodine supplementation on the morphology and structure of the thyroid... 

Lack ofiodine leads to iodine deficiency disorders (IDD), while excessive iodine dietary intake can result in pathological problems, namely goitrogenic effect (Underwood, 1971 WHO, 1996). Low iodine intake resulting in IDD is recognized as a global concern, while excessive iodine intake is not so frequent. For most people it is unlikely that they will exceed the upper level of iodine intake, which is given in Table 2.1 (FAO/WHO, 2006), from normal foods and supplements. 

Despite controversy over the relatively low German UL, iodine supplementation, like selenium, has a high application risk (BfR, 2006). This is demonstrated by the slight difference between the RDA of iodine (200 pg iodine/adult/day) and the UL or upper tolerable level (500 pg -iodine/adult/day D-A-CH, 2000). The UL differs from the RDA by a factor of 2.5. Thus suppiementation with more than the 2.5 times the RDA to prevent or treat deficiency and hypothyroidism increases the risk of producing the adverse effects of excess iodine (hyperthyroidism). 

The risk of excessive iodine intake in sheep is high if mineral blocks are used for supplementation. When the effects of the major and trace elements in the amounts administered via the mineral blocks were assessed separately, only iodine reduced plasma immunoglobulin (Ig)G concentration (Boland etaL, 2004). Rose etal. (2007) fedlactating ewes 5.5, 9.9, 14.8 and 21 mg-iodine/kg-diet dry matter. The lambs of the mothers given the two highest iodine doses had only half the IgG in the plasma of those born to mothers fed the lowest dose (Rose et al., 2007). Absorption of sufficient Ig from the mother is required to maintain the health of the mammalian neonate. A deficiency of Ig in the neonate increases the incidence of pneumonia, septicemia and navel infections, increasing neonatal mortality. 

The beneficial effects of iodine supplementation in the prevention and control of developed thyroid abnormalities due to iodine deficiency have been discussed so far in this chapter. However, supplementation with excess iodine, including the improvement of a previous iodine-deficient state, may cause thyroid dysfunctions, viz., iodine-induced hypothyroidism/iodide goiter in susceptible subjects (Roti and Vagenakis, 2000) and iodine-induced hyperthyroidism (IIH) especially in individuals over 40 years of age and who have been iodine deficient for a long period in the past (Vidor et ai, 1973). It may also increase the ratio of papillary/follicular carcinomas (Slowinska-Klencka et ai, 2002). In other words, both low and excess intake of iodine is related to further risk of thyroid disease. Although a daily intake of up to 1000 pg/day by a normal adult individual is quite safe (WHO, 1994), the upper limit is much lower in a population that has been exposed to iodine deficiency in the past. Therefore, to prevent IDD, the recommended iodine requirement in an adult individual is fixed within a narrow range of 150 rg/day (Knudsen et ai, 2000). Iodine supplementation under certain conditions in certain populations causes adverse effects, e.g., iodide goiter and iodine-induced hypothyroidism, IIH, iodine-induced thyroiditis and thyroid cancer. 

Effect of chronic excessive iodine and fluorine supplementation on morphology and function of thyroid follicular epithelial cells... 

Data from filled questionnaires in the above study showed that iodized salt had been used in the families of all inspected children (100%) over the last 10 years. Elevated median urinary iodine in children living in both areas is most likely due to high levels of iodine added to salt. Hyperthyroidism was reported from other authors in the last years as a side-effect of excessive salt iodization (Delange etal., 2002 Isidro San Juan et al., 2004 Volzke et al., 2003 Zois et al., 2003 Markou et al., 2003). Their findings confirm the necessity for the correction of iodine supplementation. 

For some trace elements, continued suboptimal dietary intake— in the presence of physiological, nutritional, or other metabolic stress— may eventually have a detrimental effect. Then additional dietary supplementation may have a health restorative effect. Such effects are most clearly demonstrated in experimental animals. Examples include the effects of boron in the presence of vitamin D depletion, or the need for increased vanadium when there is either an experimentally induced deficient or excess supply of dietary iodine. ... 

Teng et al. (2006) explored the effect of iodine intake on thyroid diseases in China. Baseline characteristics of three populations were estabfished in three communities in 1999 and then again 5 years later. The communities had different levels of iodine nutrition mild deficiency more than adequate and excessive intake. Salt iodization had been implemented in China in 1996. In the general population, median UI increased from 165 pg/1 in 1995 to approximately 300 pg/1 in 1999. The concern was with oversupplementation of iodine to a level that is more than adequate, in a region in which iodine intake was previously mildly deficient, which in turn may accelerate the development of subclinical hypothyroidism to overt hypothyroidism. High levels of iodine intake may increase the incidence and prevalence of autoimmune thyroiditis, making it imperative to tailor supplementation needs to each region. 

SPECIALTY FOODS AND HIGHLY POTENT NUTRITIONAL SUPPLEMENTS. It is fortunate that most consumers are not able to purchase pure mineral salts and to mix them into their foods, because without special measuring equipment and experience in preparing such mixtures, it is very easy to add toxic excesses of trace elements like iodine and selenium to foods. However, even some of the products that are sold by health food stores may have undesirable, or even toxic effects, because they are very rich in certain minerals. The potentially hazardous products may be considered as highly potent nutritionai suppiements, in order to distinguish them from the less potent specialty foods, which are usually safe in the amounts consumed. Table M-27 shows the approximate amounts of these minerals that are provided by various specialty foods and nutritional supplements. 

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