In Australia, about half the population is thought to have an iodine deficiency. Iodine can be found in some foods we eat; seafood such as tinned salmon and fresh oysters, iodised salt, and bread that has been fortified with iodised salt, trace amounts in dairy and trace amounts in fruits and vegetables, depending on the soil they’re grown in and the types of fertilisers used. Before modern farming practices, soils were rested between plantings and had time to replenish valuable nutrients. Also, dairies were previously cleaned using iodine as a sanitiser and iodine was found in milk and other dairy products. Now, soils are depleted of nutrients, such as iodine, and other sanitisers have replaced iodine, and dairy products are no longer a reliable source of iodine. The WHO recommended the fortification of salt as far back as the 1920’s to reduce global iodine deficiency but more and more people are cutting back on their salt intake because of its link to high blood pressure. Since 2009, all breads, except organic bread, are produced using salt that has been fortified with iodine. What if you don’t eat bread? Optimise your uptake of iodine to increase hormone production by eating iodine-rich foods at different times to eating brassica vegetables (eg. cabbage, Brussels sprouts and broccoli), sweet potato or corn because these veggies can stop your body from absorbing all the available iodine. Adding small amounts of iodised salt (about ½ teaspoon per day) to your meals can help ensure you are having your recommended iodine intake of 150µg/day. Don’t confuse adding iodised salt on your meals at home with salt found in commercially produced food as non-iodised salt is more commonly used. Found out more about iodine from the Australian Government's Nutrient Reference Values for iodine.
Selenium is a trace element found in some foods and soils and it is not produced by the human body. While iodine is required to make our metabolism hormones, T3 and T4, selenium is required to manage those hormones by turning the inactive T4 thyroid hormone into the active T3 hormone. Selenium also supports the body’s immune function. The largest stores of selenium are found in the thyroid gland as well as the liver and kidneys, and its role in metabolism is so important, the body will drag selenium from other areas of the body to use in the thyroid glands if your selenium levels are too low. Natural food sources of selenium can be found in seafood such as cod, prawns and salmon, in eggs and in Brazil nuts. In fact, just two Brazil nuts contain about 150µg of selenium while a single hard-boiled egg contains around 15µg. This page from the National Institutes of Health provides a table with many more foods that contain selenium. The jury is still out on what an optimal intake per day of selenium should be, currently, in Australia, it is recommended adult women have 55µg/day with an upper limit of 400µg/day. Recent studies are revealing benefits of increasing selenium intake include an improved immune response, a reduction of total cholesterol against HDL cholesterol and even reducing the risks of developing some cancers. You can find out more about the role of selenium in thyroid function here.
Zinc, Iron, and Copper
Zinc is required for the manufacture of the thyroid hormones, T3 and T4, and the conversion of T3 to its active form. Also, these thyroid hormones are required for the absorption of zinc which can cause a cycle of deficiency of both zinc and T3 and T4 hormones. Zinc is also required for proper immune function, wound healing and helps us with our senses of taste and smell. We aren’t able to store zinc so we need to regularly consume foods containing zinc. The best food sources of zinc are oysters, beef, chicken and fortified breakfast cereals. Find more sources on this zinc factsheet.
Iron and copper also play a role in optimising the thyroid function. Animal-based (haem iron) is readily absorbed and good sources include liver, beef, kangaroo and lamb. Vegetarian (non-haem) sources aren’t well absorbed but the best sources include fortified breakfast cereals, legumes such as kidney beans and chickpeas, spinach and cashew nuts. You can increase the absorption of non-haem iron by including some vitamin C, such as from strawberries or citrus fruit, with the meal. Nutrition Australia provides a more comprehensive list. Great food sources of copper include shellfish, whole grains, beans, nuts and organ meats. You can find other food sources of copper from this document from The National Institutes of Health.
If you think you might not have adequate levels of the nutrients I've mentioned above, your GP should confirm this with simple lab tests before you take any over-the-counter supplements.
Andersson, M., Karumbunathan, V., Zimmermann, M. B. (2012). Global iodine status in 2011 and trends over the past decade. The Journal of Nutrition, p 1. doi: 0.3945/jn.111.149393
Australian Government. (2007). The prevalence and severity of iodine deficiency in Australia. Australian Health Ministers Advisory Committee.Retrieved from https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0CEIQFjAF&url=http%3A%2F%2Fwww.foodstandards.gov.au%2Fcode%2Fproposals%2Fdocuments%2FThe%2520prevalence%2520and%2520severit y%2520of%2520iodine%2520deficiency%2520in%2520Australia%252013%2520Dec%25202007.pdf&ei=MW1qVe_5O4W48gWG8oCoAw&usg=AFQjCNFYqxtewZQtjzPYFq9GPx5lDj4_w&sig2=_vl4UrvEHAzHJXOo_BT2Jg&bvm=bv.94455598,d.dGc
Australian Government. (2014). Feature Article: Iodine. Australian Bureau of Statistics. Retrieved from http://www.abs.gov.au/ausstats/abs@.nsf/Lookup/4364.0.55.006Chapter1202011-12
Australian Government. (n.d.). Iodine. National Health and Medical Research Council. Retrieved from https://www.nrv.gov.au/nutrients/iodine
Kapil, U. (2007). Health Consequences of Iodine Deficiency. Sultan Qaboos University Medical Journal, 7(3), 267-272. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3074887/
Li, M. & Eastman, C. J. (2012). The changing epidemiology of iodine deficiency. Nature Reviews Endocrinology 8. doi: 10.1038/nrendo.2012.43