From Seaweed to Salt: A Brief History and the Importance of the Mineral Iodine for Thyroid Function

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Increasing rates of deficiency shed light on the importance of routine daily iodine intake

The importance of iodine as a trace mineral, and appropriate use as a supplement has been a topic of debate in medical circles since the discovery of this mineral in the 1800’s. The earliest use of iodine-containing substances was prior to the actual discovery of the mineral itself. As early as 3600 B.C., organic materials containing high levels of iodine such as seaweed and burnt sea sponge were used for the reduction of thyroid goiters in Chinese medicine.[1]

In the early to mid-1800’s, the first medical uses of the newly coined substance iodine (its name owing to the Greek word ioeides meaning violet-colored) were documented.

Around the 19th century the actual element iodine was discovered incidentally in the process of gunpowder production after a purple vapor was observed as the by-product of the reaction of seaweed ash treated with sulphuric acid. In the early to mid-1800’s, the first medical uses of the newly coined substance iodine (its name owing to the Greek word ioeides meaning violet-colored) were documented, as iodine grains dissolved in distilled alcohol continued to be used for the treatment of thyroid goiters.[2] By the end of the 19th century it was determined that the thyroid gland contained a high level of iodine, giving further understanding to the medical usage of the substance iodine.[3]

Iodine and thyroid hormone metabolism

The human body in iodine-sufficient areas contains up to 20 mg of iodine, of which 15 mg or roughly 75% is in found in the thyroid gland.[4] Iodine is essential for normal thyroid function, as it is required for the synthesis of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). The thyroid hormones T4 and T3 contain four and three atoms of iodine per molecule respectfully.

The human body in iodine-sufficient areas contains up to 20 mg of iodine, of which 15 mg or roughly 75% is in found in the thyroid gland.

Thyroglobulin is the precursor protein, produced by the follicular cells in the thyroid, that is necessary for the formation of the thyroid hormones T3 and T4. Iodination (the addition of further iodine) of thyroglobulin is necessary for the subsequent production of T3 and T4. In iodine deficiency states, the pituitary gland secretes more thyroid stimulating hormone (TSH) in attempts to stimulate the thyroid gland to produce more active hormone (T3). This leads to increased thyroglobulin production as well as growth and division of the follicular cells in the gland as they work hard, trying to do their job. The thyroid gland size increases, producing what is known as a goiter – the common physical sign of iodine deficiency. An increased amount of thyroglobulin is released into the blood, however T3 and T4 are unable to be produced as there is inadequate iodine.

Under iodine sufficient conditions, the adult thyroid gland takes up approximately 60 mcg (about the 10 to 40%) of the iodine circulating in blood, however in the setting of chronic deficiency this percentage can be up to 85%.[5],[6],[7] Under most states of iodine sufficiency, approximately 90% of dietary iodine eventually is excreted in the urine, with exception being the lactating female due to iodine excretion in the breast milk.[8] Iodine is essential for thyroid and brain development in the infant, also increasing the need for intake in both pregnant and nursing mothers.[9]

Who might be iodine deficient?

Iodine deficiency is not uncommon, as soils are deficient in mountainous regions, locations with frequent flooding, and inland regions of central Asia, Africa, central and eastern Europe, and the Midwest region of the US.[10] Iodide in seawater is oxidized and volatilizes back into the soil through rain, offering some protection for coastal regions. Gaseous iodine released from seaweed also may contribute to improved iodine levels in coastal regions.[11]

While iodine deficiency is decreasing in the majority of global regions it is increasing in the Americas.

The United Kingdom (UK) is one of the top 10 countries having iodine deficient school-aged children.[12] In 2011, the greatest proportions of children with inadequate iodine intake were in European regions. While iodine deficiency is decreasing in the majority of global regions it is increasing in the Americas. This is attributed to decreased intake of iodized salt due to health concerns, or selections of non-iodized salt products such as kosher or sea salt.[13]

How can I test to see if I am deficient?

Because iodine is primarily excreted in the urine, with the exception of pregnant and lactating females, the urine is the best biological fluid to use for assessment of deficiency. As the kidneys excrete approximately 90% of ingested iodine this number primarily reflects adequacy of intake and is relative consumption in recent days. Thus, the most accurate measurement is a median 24-hour urinary iodine concentration (UIC) as levels vary with intake of both iodine and fluid. Iodine deficiency is classified according to the median UIC as mild (UIC 50-99 mcg/l), moderate (UIC 20-49 mcg/l) or severe (UIC <20 mcg/l).[14]

If a spot measurement is made rather than a 24-hour collection due to difficulty with the longer collection period, expressing the UIC as a function of urinary creatinine is necessary to correct for the variable influence of fluid intake.[15] Other clinical findings used to assess for iodine deficiency include the presence of a goiter as well as measurement of thyroglobulin and newborn TSH. Thyroglobulin levels will be elevated with iodine deficiency, as iodination is necessary to produce T4 and T3. Iodine deficiency will effectively block this conversion, increasing thyroglobulin because it is not able to convert.

What can I eat to ensure I get enough iodine?

Dietary sources that are higher in iodine include iodized salt, seaweed, milk, breads or grain products, seafood (fish and shellfish), eggs, and poultry. Iodized salt is the primary global intervention for promoting adequate iodine intake; however it is increasingly common that people are using non-iodized salts such as sea salt. Although sea salt contains some iodine, unless it has been iodized, levels are likely much lower than iodized salts which contains about 70 – 80 mcg per gram or half teaspoon. Iodine is significantly lost with storage of salt at high humidity. Salt used for the production of processed foods is non-iodized due to the lower cost, and thus it is not a source of iodine despite the high sodium content which is seen.

Iodized salt is the primary global intervention for promoting adequate iodine intake; however it is increasingly common that people are using non-iodized salts such as sea salt.

Organic milk also contains roughly 30-40% less iodine than conventional milk, owing to alternative processing methods.[16] Iodine in cow milk varies with use of iodophor cleansers for milk cans and teats, as well as due to variable iodine levels in soil and groundwater used in irrigation, crop fertilizers, and livestock feed.[17]

Salt iodization

Similar to the battles which exist surrounding mandatory fluoridation of water, struggles between the states and national committees concerning mandatory iodized salt in the mid 1900’s was a social issue.[18] Although it was not made mandatory, approximately 75% of US households started to utilize iodized salt around this time. In the US, salt was initially fortified with iodine at levels of 100 mg/kg, leading to estimated average intakes of 500 mcg of iodine per day. The Swiss also introduced iodine via salt in the mid-1900’s in a much more gradual and flexible manner: 7.5 mg/kg in 1962, 15 mg/kg in 1980, and 20 mg/kg in 1998.

Currently all iodized salt in the US contains 45 mg/kg according to labels, although actual amounts have been found to vary greatly.[19] Iodized salt is not widely available in the UK, with less than half supermarket chains having it available – being a major factor contributing to high levels of deficiency in this region.[20] Iodine levels of iodized salt in the UK are on average 11.5 mg/kg. The level and availability of iodized salt in other countries varies, as well as the iodizing technique. In the US, iodized salt is made using potassium iodide with other stabilizers such as dextrose, while the majority of countries that iodize salt use potassium iodate which is more stable.[21]

Recommended intake

The recommended dietary allowance of iodine varies depending on the source of guidelines, but generally ranges from 100 – 150 mcg for infants, children, and adults with the exception of pregnant or lactating women.[22] Requirements are greater in pregnant (220 mcg/day) and breast-feeding women (290 mcg/day) because of increased thyroid hormone production in early pregnancy, increased urinary iodine excretion, and iodine transfer to the fetus or nursing infant.[23],[24] The recommended upper daily limits in the absence of documented iodine deficiency or thyroid disease are 200 mcg for children 1 to 3 years, 300 mcg for children 4 to 8 years, 600 mcg for children 9 to 13 years, 900 mcg for adolescents, and 1100 mcg for adults.[25]

Iodine requirements are greater in pregnant (220 mcg/day) and breast-feeding women (290 mcg/day) because of increased thyroid hormone production in early pregnancy, increased urinary iodine excretion, and iodine transfer to the fetus or nursing infant.

Iodine repletion programs however use far higher dosages of iodine, with dosages as high as 8.7 mg/day being used safely in children between 7 – 13 years of age in a region where iodine deficiency is endemic.[26] A single oral dose of 570 mg iodine has been shown to resolve the condition of iodine deficiency for up to 6 months without any undesirable effects or alteration of thyroid function tests.[27] That said, with high doses of iodine such as these, a risk for iodine-induced hypo- or hyperthyroidism exists, particularly in the setting of autoimmunity.[28],[29],[30] Iodine-induced hypothyroidism often is transient, and may be a protective response of the body to prevent the production of too much thyroid hormone, which can be far more dangerous. The incidence of hyperthyroidism with iodine repletion in endemic goiter regions is low and is more common in individuals with nodular goiters.[31]

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