to concentrate iodine by 20 to 50-fold above serum levels. The kidneys excrete 90% of iodine left in circulation with variable amounts eliminated via feces and sweat. Iodine is present in concentrated amounts in the salivary glands, breast tissue, the gastric mucosa, and the choroid plexus. With the exception of iodine concentrated via lactating breast tissue, the physiological significance of iodine at the other sites is not yet known.8
In times of iodine excess, the sodium-iodide transport pumps decrease in activity. When iodine excess is severe or persistent, there is inhibition of thyroid hormone production within the gland. This effect is called the Wolff-Chaikoff effect, and is named after the first researchers to observe it in an animal model. The possible mechanisms for this reduction include an inhibition of thyroxin secretion, inhibition of oxidization of iodine within the thyroid, and inhibition of the iodination of tyrosine within the thyroid follicles. The Wolff-Chaikoff effect usually reverses in 2-3 weeks after withdrawal of the excess iodine. This effect has been utilized therapeutically to suppress hyperthyroid states, mostly prior to the advent of thioamide thyrotoxic drugs.9
In a slightly deficient iodine state, the thyroid gland swells to compensate, often leading to goiter. Also counter logical is the fact that in the face of iodine excess, the thyroid commonly ends up being slowed into a hypothyroid state.
Metabolism of Iodine
Iodine is readily absorbed via the entire length of the intestinal tract. Malabsorption of iodine is not known, even by those with other malabsorption syndromes. At any given point, the body of an adult will contain 15-20 mg of iodine, mostly within the thyroid gland. The kidneys are unable to conserve iodine, thus most plasma iodine not taken up by the thyroid or other tissues is lost in the urine. Lesser variable amounts are also lost in feces, sweat, saliva, and breast milk. The proportions of iodine lost by different routes is variable between and within individual subjects.8
Although iodine deficiency is the most common cause for hypothyroidism globally, nearly all cases of hypothyroidism in the industrialized world are secondary to autoimmune disease.
Thyroid Hormones
The physiological effects of iodine at nutritional levels relate to the effects of thyroid hormones, these include:
1.Regulation of basal metabolic rate;
2.Regulation of macronutrient metabolism;
3.Regulation on ion transport – muscle contraction;
4.Regulation of development, growth and sexual maturation.10
Iodine is first taken from circulation into the thyroid via a sodium-iodide symporter pump. It is then concentrated in the colloid space inside follicles formed by thyrocytes. After entering the colloid, it is oxidized via thyroid peroxidase (TPO) to allow it to bind with tyrosyl residues from tyrosine within the protein thyroglobulin (Tg). These complexes form monoiodotyrosine (T1) and diiodotyrosine (T2). Within thyroglobulin, these condense together to form thyroxine (tetraiodotyrosine - T4) and triiodotyrosine (T3). In response to thyrotropin (or thyroid stimulating hormone – TSH), iodinated thyroglobulin is taken into the peripheral follicular cells in droplets via pinocytosis. These droplets are broken down by lysosomes and proteolytic enzymes, freeing the T3, T4, and T2 from the thyroglobulin and releasing them into the circulation.11
T4, the predominate hormone in circulation, is a prohormone requiring 5’ deiodination to be activated into T3. Type 1 deiodinase does this for systemic use in the liver and kidneys, while Type 2 deiodinase does this for local use in the brain, brown fat and pituitary gland.12 T4 is also deiodinated into reverse T3 (rT3), as a means of peripherally deactivating T4 and degrading it. Contrary to claims of advocates of ‘Wilson Temperature Syndrome’, the presence of rT3 does not stimulate more rT3 formation, rather it inhibits it.
Dietary Iodine
Variations in Dietary Iodine
Iodine is abundant in soil and sea water, but regional levels vary tremendously based on past snow, rain, and glaciation, all of which leach it to the ocean. Many regions of the world have been known as ‘goiter belts’ due to the low soil iodine and high prevalence of goiters, such as the Great Lakes Region, the Himalayas, the Andes, and the Alps. Goiters in these regions were especially common when nearly all food was locally grown.
This regional variation in goiters is due to the fact that the amount of iodine found in plants varies with the concentration of iodine in the soil. For example, plants grown in iodine-rich soil average 1 mg iodine per kg of dry weight, while those grown in iodine deficient soil may have levels as low as 0.01 mg/kg. Since iodine is abundant in seawater, nearly all foods from the sea are rich in it.13
Iodine fortification of salt, which began in 1924 in the U.S., has all but eliminated goiters here. For example, 4-years after iodine fortification of salt, the rate of goiters in Michigan schools reduced from 40% to 10%. A few years later the rate was down to 1.4%.14
Regions with poor soil iodine have been especially hard hit when selenium is also lacking. Selenium is a critical cofactor for type 1 deiodinase, which converts T4 to T3. It is also critical for formation of glutathione peroxidase which protects the thyroid from oxidative stress.15
Goitrogens
Goitrogens are plant chemicals that block the formation of thyroglobulin and/or inhibit iodine uptake by the thyroid. These are found in many plants, especially: cruciferous vegetables, cassava, and millet, and to a lesser extent in soy. Most goitrogens are heat labile and rendered inactive when cooked.
The clinical relevance of this effect in the industrialized world is small. Nonetheless, cases have been documented of patients on raw food diets becoming hypothyroid due low intake of iodine and a high intake of raw vegetables containing goitrogens. Most goitrogens are indole compounds or isoflavones, both of which have evidence of protective effects against hormonally driven cancers.16
Dietary Sources of Iodine
Along with plant food, the other large sources of iodine in our diet include ocean-derived seafood, dairy products, and bread. Dairy products can vary 10-fold in their iodine content. The source of iodine in dairy products is either supplemental iodine given to the cattle or providone iodine used to sanitize the cow's teats during the milking process. Both of these sources of exposure are highly variable.
Some bakers still use iodine compounds in dough conditioning. However, according to a 2004 analysis of bread in the Boston, Massachusetts area, the amount of iodine in bread can vary by as much as 10-300 mcg. per slice.17
The densest source of iodine in foods is seaweeds, which are also known by the less derogatory name sea vegetables. These are dietary staples in Asia as well has many coastal areas throughout the world. Most types of seaweed used for consumption contain a few hundred micrograms of iodine per gram and servings tend to be 1-3 grams by weight. Yet these levels can range from a low of 16 μg/g in nori (the green paper that forms sushi rolls), to as high as 8100 μg/g in kelp granules. Most people will benefit by including traditional dietary iodine such as nori, kombu, wakame or dulse to their diets but those taking kelp supplements can easily exceed the upper safe limits of iodine and these have been documented to induce hyperthyroidism.18
Iodized salt is the main dietary source of iodine in the industrialized world.19 Table salt
