It is abundantly clear that the body needs iodine to make thyroid hormone. About one-third of the world’s population, about two billion people, is iodine deficient. This is the leading cause of hypothyroidism worldwide. It is also the leading cause of mental retardation.
During pregnancy T4 production doubles and so too do daily iodine requirements.  Iodine deficient pregnant women cannot produce the thyroid hormones that are needed for fetal brain development. They are at high risk of giving birth to infants with brain problems. Even moderate iodine deficiency in a pregnant woman will lower her infant’s IQ 10 to 15 points. Congenital hypothyroidism due to iodine deficiency is called cretinism and is rare now in the West, about 1 case in 4,500 live births. But, “In areas with iodine deficiency,… its incidence varies from 1 to 5%!” 
Iodized salt has proven to be effective at preventing iodine deficiency. The Morton Salt Company has sold iodized salt in the US since 1924.  Europe is behind us. As of the late 1990s, thirty-two European countries were still affected by mild to severe iodine deficiency.  It costs just over one dollar to iodize one ton of salt. 
People who avoid iodized salt or avoid all salt because they have high blood pressure, can become iodine deficient.  Vegetarians, in particular, are at risk of developing iodine deficiency, especially if they eat food grown in low iodine soil. Vegans, who avoid fish and dairy products, are at higher risk. [7,8] Milk products contain iodine because dairy plant equipment is sanitized with iodine.
People require about 100 mcg of iodine per day to make thyroid hormone. Larger amounts or sudden increases can suppress thyroid hormone production.
A graph of the toxic effects on the thyroid of iodine is U-shaped. Low iodine and high iodine intake both cause hypothyroidism. Urinary iodine excretion below 120 mcg/ 24 hours causes problems; so do excretion rates greater than 220 mcg/24 hours. 
High intake of iodine increases the risk of Hashimoto’s disease. 
Over supplementation of iodine has lead to epidemics of hypothyroidism. Even excess iodine use in dairy plants has caused hypothyroidism. [11,12]
Iodine or foods high in iodine, such as seaweed, are often considered useful in treating hypothyroidism but this is probably only true for people who are iodine deficient. As few people in the United States are iodine deficient, we need to be cautious; treatment with iodine may backfire.
In a 2007 paper, Jane Teas reported that 5 grams/ day of seaweed (Alaria esculenta), providing 475 mcg of iodine/day, increased TSH by 1.69 mU/L.  In October 2010, Crawford et al reported a series of cases of thyroid dysfunction in adults who had consumed a brand of soymilk ‘enriched’ with Kombu seaweed and a case of a newborn baby with hypothyroidism whose mother had drunk the milk. Two other cases of newborns were reported whose mothers had made soup from it. The high iodine content of these foods was blamed. 
A 2008 placebo controlled trial measuring the effect of eating Kombu seaweed in Japanese adults found that eating 15 and 30 grams of Kombu (containing 35 and 70 mg of iodine) daily for about a week, significantly increased TSH exceeding the normal limits in some subjects. 
Iodine overdosing leading to hypothyroidism is common enough that a clinical trial of iodine restriction is worth trying for people who have diets high in iodine. A 2003 paper describes a clinical experiment done in Japan in which thirty-three hypothyroid patients were asked to stop consuming any iodine-containing foods including seaweed products for 1-2 months. Their median TSH level, initially 21.9 mU/L, was reduced to 5.3 mU/L after this iodine restriction. The TSH of twenty-one patients (63.6%) dropped by more than half to less than 10 mU/L. In a third of the patients TSH normalized at less than 3.9 mU/L. 
Mainstream medical journals now caution against iodine supplementation for treating hypothyroidism.
Besides iodine, other minerals, particularly selenium and zinc, can affect thyroid function. Deficiencies in either of these minerals can also increase the risk of hypothyroidism.  The effects are small in comparison to iodine.
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