Effects of Chronic Iodine Excess ....

....in a Cohort of Long-Term American Workers in West Africa. Wolf-Chaikoff effect debunked...

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http://jcem.endojournals.org/cgi/content/full/87/12/5499



A cross-sectional survey of 102 Peace Corps volunteers in Niger, West Africa, in 1998 had previously demonstrated a high rate of thyroid dysfunction and goiter attributable to excess Iodine from their water filters. The Peace Corps volunteers were followed-up a mean of 30 wk after they ceased using iodine-based water filtration systems. Goiter was present in 44% of subjects during excess Iodine ingestion and in 30% after removal of excess iodine. Mean serum Iodine decreased from 293 µg/liter during excess iodine ingestion to 84 µg/liter after cessation of excess iodine. Mean total serum T4 values increased from 100.4 to 113.3 nmol/liter (7.8 to 8.8 µg/dl). Mean serum free T4 increased from 32.2 to 34.7 pmol/liter (2.5 to 2.7 ng/dl). Mean serum TSH decreased from 4.9 to 1.8 mU/liter. Mean serum thyroid peroxidase antibody levels decreased from 33,000 to 22,000 IU/liter (33 to 22 IU/ml).

We found that during prolonged excess iodine exposure there were marked increases in serum total iodine concentrations, and the prevalence of goiter, elevated serum TSH values, and elevated serum thyroid peroxidase antibody values increased. The prevalence of all abnormalities decreased after removal of excess iodine from the drinking water system.

PEACE CORPS VOLUNTEERS in Niger, West Africa, were noted to have a high prevalence of goiter from 1995–1998. Initial investigation revealed 39 suspected cases of thyroid abnormality. Peace Corps volunteers in many areas of Africa drink iodine-enriched water to avoid microbial contamination, and it was subsequently found that the two-stage iodine-resin ceramic filters in use in Niger delivered a mean concentration of 10 mg iodine/liter to the drinking water (1). There was no evidence of a high concentration of iodine in the diet, salt, medications, or nutritional supplements.

As the arid climate in Niger results in the daily consumption of 5–9 liters water, the volunteers consumed at least 50 mg iodine daily, which is approximately 300 times the daily U.S. Recommended Dietary Allowance (2). Urinary iodine excretion in this iodine-enriched population ranged from 392–153,780 µg/liter (median, 5,048 µg/liter). Volunteers used the water purification devices described above for up to 32 months. After the discovery of this iodine overload, all iodine-based filters in Niger were discontinued, and other methods of purification were used (boiling the water or treating the water with microfiltration and chlorine purification).

In May 1998, a cross-sectional survey of volunteers then in Niger was conducted just before removal of the excess iodine from the drinking water and, on the average, 30 ± 11 wk later (range, 3–83 wk) in Niger or after return to the U.S. Goiter prevalence and levels of serum TSH and thyroid peroxidase (TPO) antibodies significantly decreased after removal of the water filters and correction of the iodine excess, strongly suggesting that the iodine-induced abnormalities were not permanent....

...Previously published reports have described both subclinical and overt thyroid dysfunction as a result of excess iodine ingestion. Goiter, hypothyroidism, and/or a rise in serum TSH values have been reported to result from ingestion of excess iodine in medications such as amiodarone (5), as a natural contaminant of drinking water (6), as a byproduct of iodine-containing water purification systems (7, 8, 9, 10), in iodine-containing mouth rinses (11), and in the diet (seaweed ingestion) (12). Individuals with underlying autoimmune thyroid disease, those with a previous history of postpartum thyroiditis or subacute thyroiditis, or patients who have undergone partial thyroidectomy have all been shown to be prone to iodine-induced goiter and hypothyroidism (13). Additionally, although the natural history of thyroid dysfunction related to acute excess iodine ingestion has been well characterized, the effects of chronic iodine excess remain poorly understood.

Acute excess iodine ingestion has long been known to result in a transient decrease in iodine organification, termed the acute Wolff-Chaikoff effect (14). With sustained excess iodine exposure, however, most individuals’ thyroid glands escape from this 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+/I- symporter protein, resulting in a decrease in thyroid Iodide content (15). In some individuals this escape phenomenon does not occur, and those patients develop iodine-induced hypothyroidism. Such hypothyroidism generally is reversible when the source of excess iodine exposure is removed....

...Although individuals with underlying autoimmune thyroid disease are more likely to develop complications of excess iodine ingestion, it is unclear whether excess iodine ingestion itself can lead to autoimmune disease. Animal studies have suggested that excess iodine exposure predisposes to the development of autoimmune thyroid disease (18). However, controversy exists about whether there is a relationship between excess iodine ingestion and the development of Hashimoto’s thyroiditis in humans (19). It has been observed that in areas of Japan where dietary iodine intake is high, the incidence of Hashimoto’s thyroiditis is higher than in areas of low to normal dietary iodine intake (20). An increase in lymphocytic infiltration often occurs after iodine repletion in iodine-deficient regions (21). However, other studies have failed to show a relationship between increased iodine intake and autoimmunity (22, 23). In this study some Peace Corps volunteers had anti-TPO antibodies during excess iodine exposure. These abnormal TPO antibody titers decreased in some, but not all, subjects when excess iodine was eliminated. This suggests that excess iodine ingestion may induce thyroid autoimmunity in an otherwise healthy young population......