Iodine is an essential trace element required for the proper functioning of the thyroid gland. It is mainly consumed as iodide salts obtained from sea salt, shellfish and seawater fish and vegetables, which are more bioavailable than the organic form of iodine. The iodine content of soil is considered to be one of the most variable of all mineral levels, influenced by local geography and the type and quantity of fertiliser used in agriculture. The amount of iodine present in local drinking water (0.1 -100 µg/L) is reported to be a good indication of soil levels. In iodine-deficient areas, the iodide concentration in drinking water is <2 µg/L (<1 5.8 nmol/L), whereas in areas close to the sea, the drinking water contains 4 to 10 µg/L (31.5-78.8 nmol/L).
Iodide is rapidly absorbed from the small intestine and distributed via the blood to a range of tissues, most notably the thyroid, which traps absorbed iodide through an ATP-dependent iodide pump. The thyroid contains 80% of the body’s iodine pool, which is approximately 15 mg in adults. Some is also found in the salivary, gastric and mammary glands (exclusively during pregnancy and lactation in the latter), as well as in the ovaries. As is the case with the thyroid, uptake into these tissues is regulated by thyroid-stimulating hormone (TSH).
Iodine is excreted via the kidneys and excretion occurs when the needs of the thyroid have been met and an excess remains. The amount of iodine excreted in the urine reflects plasma levels and has been used since the middle of the 20th century to assess iodine status. Interestingly there is no renal conservation mechanism for this mineral and the only evidence of iodine preservation comes from the scavenging and recycling of thyroid hormones by the selenium-dependent deiodinase DII. Of the total amount excreted, 20% occurs via faeces and additional losses can occur through sweat, which, although a minor eliminatory pathway under normal circumstances, can be a significant contributor for people living in hot climates with low dietary consumption.
Kohlmeier (2003) notes the Wolff-Chaikoff effect, which is the reduction in thyroid hormone production in response to acute large doses of iodine and was first described over 50 years ago. It is reported to occur through the downregulation of the active-transport mechanism present in the thyroid in response to high blood levels. Other evidence points to temporary inhibition of thyroid peroxidase and therefore reduced thyroglobulin iodination reactions.
Iodine can occur in foods as either an inorganic or organic salt, or as thyroxine in animal sources. Unlike many other essential nutrients, the organic form of iodine found in animal products has poor bioavailability, whereas the iodide salts found in the sea are almost completely absorbed (Jones 2002).
However, irrespective of whether it is animal or plant derived, food from the land has enormous variability in terms of iodine content, from 1 to 10µg/kg, due to iodine’s high solubility, which results in it leaching from the soil with heavy rain and weathering.
Additionally, chemicals known as goitrogens are naturally found in some foods (e.g. brassica [cabbage] family) and these interfere with iodine utilisation and thyroid hormone production.
Due to the high levels in the ocean of bioavailable iodide, all sea-dwelling creatures, animal or plant, are considered superior dietary sources.
• Seawater fish
• Sea vegetables such as seaweeds
• Sea salt
• Iodised salt (fortified form of table salt)
• Commercially manufactured breads due to the iodate dough oxidisers
• Dairy milk (variable)
In Australia milk no longer supplies a significant amount of iodine, whereas in the United Kingdom it is still an important dietary source because of the use of both supplemented feeds and iodine-based antiseptics in the dairy industry.