Iodine is a trace element that is naturally present in foods and is added to some types of salt. Iodine is an essential component of thyroid hormone which plays a key role in digestion, heart and muscle function, brain development and maintenance of bones. Getting enough iodine is important for everyone, especially infants and women who are pregnant. In this podcast, I’ll look at the roles of iodine, where it is found in food and what are the health issues with not getting enough of it.
Iodine is a key mineral and one of the first trace elements to be identified as essential. A core function of iodine is as an integral part of the thyroid hormones that regulate body temperature, metabolic rate, reproduction, growth, blood cell production, nerve and muscle function, and more. The thyroid gland releases a molecule called tetraiodothyronine or T4 for short or commonly known as thyroxine. This molecule has 4 iodine molecules in it hence the need for iodine. Upon reaching the cells, thyroxine has one of its iodine molecules removed to become the active form of thyroid hormone called triiodothyronine or T3. This all explains why three-quarters of the iodine in your body is found in the thyroid gland.
The hypothalamus regulates thyroid hormone production by controlling the release of thyroid-stimulating hormone or TSH from the pituitary gland. For someone with iodine deficiency, thyroid hormone production declines and the body responds by producing more TSH in an attempt to accelerate iodide uptake by the thyroid gland. If the iodine deficiency persists, the cells of the thyroid gland enlarge to trap as much iodide as possible. Sometimes the gland enlarges so much that it forms a visible lump in the neck which is what we call goitre. So, that explains the classic clinical presentation of someone with iodine deficiency.
Goitre isn’t the only side effect of not having enough iodine in your diet though. People with long-term iodine deficiency can develop hypothyroidism. This is a condition in which not enough thyroid hormone is produced. Symptoms of hypothyroidism include dry skin, hair loss, fatigue and slowed reflexes.
Goitre may be the earliest and most obvious sign of iodine deficiency, but the most tragic and prevalent damage occurs in the brain, especially in children. Children with even a mild iodine deficiency typically have goitres and perform poorly in school.
Mild iodine deficiency has re-emerged in Australia as a major public health issue in the last two decades. Even a mild form of iodine deficiency can give rise to hypothyroidism with symptoms of which include fatigue, goitre, mental impairment, depression, weight gain, and low basal body temperatures. The National Iodine Nutrition Study which is a survey of iodine levels of 1709 children from 88 schools in Australia indicated that, overall, Australian children were borderline iodine deficient.
A severe iodine deficiency during pregnancy causes extreme and irreversible mental and physical retardation known as cretinism. Cretinism affects approximately 6 million people worldwide and can be averted by the early diagnosis and treatment of maternal iodine deficiency. A worldwide effort to provide iodised salt to people living in iodine-deficient areas is ongoing.
If you are pregnant you need higher levels of iodine than usual for your baby’s healthy development. Mild to moderate iodine deficiency in pregnant women can affect their baby’s physical development and hearing, and cause learning difficulties. Severe iodine deficiency can also cause major physical and neurological abnormalities, miscarriage or stillbirth.
Because of the re-emergence of iodine deficiency in Australia, from September 2009, Food Standards Australia New Zealand (FSANZ) introduced mandatory iodine fortification into the food supply using iodised salt in bread as the vehicle. Iodised salt is now added to all commercially sold bread in Australia and New Zealand with the exception of organic and unleavened bread. A 2016 study looked at the impact of the mandatory iodine fortification program in Australia and I’ll link to this study in the show notes. On a positive note, people who were regular eaters of bread were 5-times more likely to have an adequate intake of iodine. But there were differences with people from a lower SES background less likely to get sufficient iodine in their diet. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5133088
Although fortification of bread-making flour with iodine has improved the general population’s iodine status, pregnant women in Australia are still not getting enough iodine to meet their needs. So, if you are pregnant, breastfeeding or considering becoming pregnant, ask your doctor for advice about your individual daily iodine needs. And this is where advice to take an iodine-containing supplement may be given. Importantly though, if you have a pre-existing thyroid condition, do not take iodine supplements without checking with your doctor
Iodine recommendations and sources
So where do you find iodine in your diet? Well, the ocean is the world’s major source of iodine so seaweed (such as kelp, nori, kombu, and wakame) is one of the best food sources of iodine. Other good sources include fish and other seafood. Dairy products contain iodine. However, the amount of iodine in dairy products varies by whether the cows received iodine feed supplements and whether iodine-based sanitising agents were used to sterilise dairy-processing equipment.
The amount of iodine in fruits, vegetables and grains is variable and generally reflects the amount of iodine present in the soil in which plants are grown or on which animals graze. Landmasses that were once under the ocean have soils rich in iodine; those in flood-prone areas where water leaches iodine from the soil are poor in iodine.
Iodine can also come from iodised salt added during or after cooking and of course, it is found in foods that have iodised salt added to them under mandatory fortification programs.
Consumption of foods that contain goitrogens which are substances that interfere with the uptake of iodine into the thyroid can exacerbate iodine deficiency. Foods high in goitrogens include soy, cassava, and cruciferous vegetables such as cabbage, broccoli, and cauliflower. For people who have adequate iodine intakes and eat a variety of foods, the consumption of reasonable amounts of foods containing goitrogens is not a concern. Although, science is yet to clearly define what a reasonable amount here is so it is more an issue if you have iodine deficiency, or at risk of it, and have a diet low in iodine that you may want to not go overboard with these type of goitrogen foods.
In Australia and New Zealand, labels indicate whether salt is iodised or not so if you are buying salt, you should opt for this form of salt. On average, half a teaspoon of iodised salt provides the RDI for iodine.
Iodine toxicity
As with any nutrient, too much iodine can be harmful. Excessive intakes of iodine can interfere with thyroid function and enlarge the glands, just the same as a deficiency can. During pregnancy, exposure to excessive iodine from foods, prenatal supplements or medications is especially damaging to the developing infant.
More than 120 countries worldwide have introduced programs of salt fortification with iodine in order to correct iodine deficiency in populations so you can see what a major public health issue deficiency can be. Iodine is a nutrient you certainly should be aware of to make sure you’re eating enough of the foods that contain iodine, especially if you are pregnant or breastfeeding.
So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.
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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.