Excess iodine exposure, thyroid

Acute excess iodine ingestion has long been known to result in a transient decrease in iodine organification, termed the Wolff—Chaikoff effect (Wolff and Chaikoff, 1948). With sustained excess iodine exposure, however, most individuals thyroid glands escape from acute Wolff— Chaikoff effect, despite continued excess iodine exposure, and resume synthesis of normal amounts of T4 and T3. The mechanism responsible for this escape or adaptation to the iodine load probably involves a decrease in the Na /H symporter protein, resulting in a decrease in thyroid iodide content (Eng et ai, 1999). In some individuals... 

Therefore, based on the above Hmited evidence, we conclude that excess iodine exposure may influence maternal-fetal TH metabohsm by three mechanisms (1) excess iodine affects the maternal hormone thyroid level by inhibition of thyroid function and/or Dl activity (2) excess iodine has an effect on maternal—fetal TH transfer by affecting placental D2 and D3 activity and (3) excess iodine transferred by the placenta has a direct inhibition on the development and function of the fetal thyroid gland (Figure 88.2). 

Table 97.2 Thyroid parameters during and after excess iodine exposure...

Reinwein D, Durrer HA, Meinhold H. 1981. Iodine, thyroxine (T-), triiodothyronine(T ), 3, 3,5 -triiodothyronine(rT ), 3,3 -diiodothyronine (T ) in normal human thyroids. Effect of excessive iodine exposure. Horm.Metab.Res. 13 456. 

It is difficult to define the normal range of iodine intake in humans, and despite efforts to provide iodine supplementation in many geographic areas of the world, endemic iodine deficiency and attendant goiter remain a world health problem (147). Exposure to excess iodine may sometimes lead to the development of thyroid disease. This unusual type of iodide-induced goiter has been found, for example, in 10% of the population of a Japanese island where fishermen and their families consume large quantities of an iodine-rich seaweed and have an iodine intake as high as 200 mg/d (148). 

The use of K1 serves as an important remedy to protect from radioiodine exposure under nuclear accident conditions. In principle, under normal circumstances, excess iodine decreases sodium—iodide symporter (NIS) on the thyroid cell surface, thereby inhibiting further access for iodine into the thyroid. Excess iodide administration at the appropriate time decreases thyroid radioactive iodine uptake (RAIU) by increasing the amount of nonradioactive... 

Consistent with other studies (Bagchi et al., 1985, 1995 Li et al, 1993), excessive iodine not only triggered the onset of autoimmune thyroiditis, but also enhanced the development of inflammation. Our study found that all these effects were in positive correlation with the amount of iodine and exposure duration. The larger the amount and the longer the duration, the higher the incidence of autoimmune thyroiditis and the more acute the inflammation (Figure 91.3). 

Compared with normal baseline, iodine supplementation at deficient iodine baseline aggravated the development of autoimmune thyroiditis. For example, the incidence of autoimmune thyroiditis was 66% when normal baseline NOD.H-2 mice were fed on five-fold-high iodine water for 6 months, while that of iodine deficient baseline was 60% after exposure to five-fold-high iodine water for only 4 months. Many et al. (1995) reported that with exposure to 100-fold-high iodine supplementation, lymphocyte infiltration appeared after 96 days in normal NOD mice compared with 4 days in the iodine deficient baseline group. It indicated that iodine deficiency of the thyroid was more vulnerable to the threat of excessive iodine. 

Iodine-induced thyrotoxicosis in apparently normal thyroid glands has been reported in two patients after exposure to 2—10 mg iodine daily for 2—12 months before thyrotoxicosis was diagnosed. Although the pathogenesis of this kind of IIH remains obscure, the positive family history for thyroid disease in both patients may indicate a latent abnormality of their thyroid glands (Skare and Frey, 1980). Moreover, 10 further cases of iodine-induced thyrotoxicosis in apparently normal thyroid glands have been reported (Savoie et al, 1975). In contrast, a more-than-adequate or excessive iodine intake may lead to hypothyroidism and autoimmune thyroiditis, as shown in China in areas with more than adequate and excessive iodine consumption (Teng et al., 2006). 

X-ray fluorescence, used by expert hands, is a highly interesting tool to follow changes of intrathyroidal iodine stores in time, subsequent e.g. to the exposure of the thyroid gland to excess iodine... 

To prevent radioactive iodides from lodging in the thyroid gland during exposure to excessive radiation, a potential appHcation of iodine acting as a thyroid-blocker has arisen. Eor this purpose potassium iodide was recommended (66). 

A number of dietary deficiencies may increase the risk of deleterious cyanide effects. Iodine deficiency is involved in the etiology of such thyroid disorders as goiter and cretinism. These disorders may be exacerbated by excess exposure to cyanide (Delange and Ermans 1971 Ermans et al. 1972). Protein deficiencies and vitamin B12, riboflavin and other vitamins and elemental deficiencies may subject people... 

There are several causes for exposure to an iodine excess, caused by iodine-containing antiseptics (10000 pg of iodine/ ml) and radiologic contrast media (250-370 mg of iodine/ ml).The aim is to bring to attention the evidence that even a minor iodine overload may impair thyroid function during a period of development when thyroid hormones are very important for the brain (Ares et aL, 1995, 2007). The precocious diagnosis and treatment of changes in thyroid function associated with iodine excess could have beneficial effects in the prevention of developmental abnormalities. We surest a protocol to evaluate thyroid function when iodine... 

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