Food sources of iodine
The major food sources of iodine are of marine origin, including kelp and seafood, plus fortified salt, dairy products, eggs and some vegetables. The iodine content of some foods may be affected by iodine levels in the soil, irrigation practices and fertilisers used in the environment and deficiency occurs when not enough iodine is consumed in the diet. Both dietary and supplemental iodine are well absorbed, with about 90% of ingested iodine absorbed.
Public health measures to reduce iodine deficiency
Two public health measures were introduced in Australia between 2009 and 2010 to reduce iodine deficiency. Food Standards Australia and New Zealand (FSANZ) introduced mandatory fortification of iodised salt used in breadmaking and the National Health and Medical Research Council (NHMRC) released a public statement in 2010 recommending that all women who are pregnant, breastfeeding or considering pregnancy supplement with 150 micrograms iodine daily. More recently in 2019, the NHMRC have updated their guidelines to recommend that the daily intake of iodine increase from 150 micrograms daily to 220 micrograms daily during pregnancy.
In Australia, it is recommended that pregnant women take an iodine supplement of 150 micrograms daily to reduce the risk to the foetus associated with iodine deficiency. In individuals with low iodine status, periconceptional supplementation of between 150-290 micrograms daily is recommended.
Iodine is essential for the production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which influence growth and development, metabolism and thermogenesis. The thyroid gland stores 80% of the body’s iodine pool and iodine comprises 65% of T4 and 59% of T3. Thyroid hormones are involved in carbohydrate and fat metabolism, protein synthesis and regulating basal metabolic rate.
Thyroid function is regulated by thyroid-stimulating hormone (TSH), secreted by the pituitary gland in the brain. TSH secretion increases the uptake of iodine in the thyroid and stimulates the production and release of T4 and T3. Where there is insufficient iodine, TSH levels remain elevated, which can eventually lead to enlargement of the thyroid gland, known as a goitre, and symptoms of reduce thyroid function including:
- Weakness, tiredness and sleepiness
- Dry skin
- Cold intolerance
- Hair loss
- Poor memory and concentration
- Reduced appetite and weight gain
Iodine supports thyroid gland health and assists thyroid hormone production and due to its role in thyroid hormone production, iodine supports nervous system health and is essential for the proper functioning of the brain and the entire nervous system.
Goitrogens in food
Goitrogens are substances that interfere with iodine utilisation, uptake into the thyroid or thyroid hormone production. Goitrogens are found in raw cruciferous vegetables including cabbage, kale, cauliflower, broccoli, turnips and Brussels sprouts, and in linseed, cassava, millet and soy. Cooking these foods deactivates the goitrogens. Certain chemicals from industry or cosmetics can also be classed as goitrogens. Iodine supplementation should be separated from goitrogens in order to maximise absorption. Soy in particular may inhibit iodine absorption and high levels of soy isoflavones, genistein and daidzein, can inhibit the production of thyroid hormones T4 and T3 production.
Iodine in pregnancy and breastfeeding
Iodine is an important nutrient during pregnancy and adequate maternal iodine is essential for the development of the foetal brain and nervous system, as it is necessary for the growth of nerve cells, nerve junctions and the myelin sheath which insulates nerve fibres.
During early pregnancy, the foetal thyroid gland development is incomplete, so the foetus is entirely dependent on maternal T4, and therefore on maternal iodine intake. Thyroid hormone production increases by 50% during early pregnancy and renal excretion of iodine increases by 30-50%. In later pregnancy, iodine passes the placenta for foetal thyroid hormone production. The foetal thyroid gland develops by 10-12 weeks of gestation and is capable of thyroid hormone production by approximately 16-20 weeks.
Dietary iodine requirements increase by approximately 50% during pregnancy, driven by increased thyroid production demand, increased renal iodine clearance and transfer of iodine to the foetus. After birth, babies who are breastfed will exclusively get their iodine from mother’s milk. Therefore, it is essential that breastfeeding women maintain adequate iodine levels to support their health and that of their baby’s. Higher iodine levels are required during lactation, with recommendations between 250-290 micrograms daily. It is recommended iodine supplementation start before conception and to continue throughout pregnancy and breastfeeding in order to maintain adequate iodine levels.