Why iodine is important

Iodine is an essential nutrient which means that the human body can’t make it on its own and we must get it from our diet. Although only required in very small amounts, iodine is an important component of thyroid hormones. Inadequate iodine intake leads to inadequate thyroid hormone production. These hormones maintain the body’s metabolic rate and support normal growth and development in children. As iodine is essential for normal brain development, it is particularly important that unborn babies (foetuses) and young children get enough of it.

The term iodine deficiency disorders is used to describe the wide range of effects low iodine status can have on health. One of the serious health effects of iodine deficiency disorders is goitre (enlargement of the thyroid gland leading to a swelling of the neck). In very severe iodine deficiency, stunted growth and intellectual disability can occur in children. A number of studies have reported adverse effects on hearing capacity, motor and cognitive function in children associated with moderate and severe iodine deficiency.

Foods that are good sources of iodine

Good sources of iodine include seafood (fish, shellfish and seaweed), commercially prepared bread and iodised salt (if salt is used). Milk, milk products and eggs are sources of iodine. The Eating and Activity Guidelines for New Zealand Adults (Ministry of Health, 2020) recommend choosing iodised salt when using salt, but do not recommend increasing overall salt intake. Although it comes from the ocean, sea salt is not a good source of iodine.

Iodine levels of meat, chicken, eggs and dairy products are reflective of the iodine content of the animal feed used. The iodine content of vegetables, fruits and grains generally reflect the iodine level of the soil in which they were grown. The iodine content of New Zealand soils is low and therefore locally produced foods are also low in iodine. It is difficult for most people to obtain adequate iodine by eating foods that are natural sources of iodine, which is why commercially prepared bread must be fortified with iodine.

Iodine fortification of bread

Iodine fortification of bread became mandatory in 2009 for most bread except for organic bread, non-yeast-leavened bread and bread mixes. Fortifying bread with iodine helps increase the amount of iodine in New Zealanders’ diet.

International guidance and experience have shown that using iodised salt is one of the best ways to prevent and correct iodine deficiency. In Aotearoa New Zealand, table salt was iodised from 1924 and the level increased in 1938. This significantly reduced the incidence of iodine deficiency. However, by the early 21st century, the problem had re-emerged. This was thought to be due to:

  • The move away from using iodophors (a type of disinfectant) as sanitizers in the dairy industry (contamination from iodophors used to increase iodine levels in milk).
  • Reduced use of discretionary salt and use of non-iodised salt.
  • Increase in processed foods and meals away from home (contain mostly non-iodised salt).

Consequently, from September 2009, New Zealand bread bakers were required to use iodised salt in most breads to help address the re-emergence of iodine deficiency in New Zealand.

Iodine status of New Zealand adults

The results of the 2014/15 New Zealand Health Survey (NZHS) found:

  • Urinary iodine levels almost doubled between 2008/09 and 2014/15 for adults overall and in all age, gender and ethnic groups.
  • In 2014/15 iodine status was adequate for men in all age and ethnic groups. Iodine status was adequate for women overall and women of Māori, Pacific and Asian ethnicity. However, women of European/Other ethnicity still had mild iodine deficiency.

View the biomedical results from the 2014/15 New Zealand Health Survey.

Iodine and pregnancy

Requirements for iodine increase during pregnancy and even with a well balanced diet, it is difficult for women to get enough iodine from food alone. The Ministry of Health recommends that pregnant and breastfeeding women take a registered 150 micrograms (mcg) daily iodine-only tablet daily available at pharmacies, as well as eating foods which are important sources of iodine. 

While consumption of other iodine containing supplements and kelp tablets will increase iodine intakes, care needs to be taken when considering this type of supplementation as this may lead to an intake of iodine beyond the safe upper level of intake. The margin between too much and too little is very narrow and the iodine content in seaweed products and kelp tablets is extremely variable.

For more information, refer to the Health Education publications Eating for Healthy Pregnant Women (HE1805) and Folic Acid and Iodine.

Diets with a risk of iodine deficiency

Dietary patterns that exclude foods that are good sources of iodine in the diet can have a negative impact on iodine status. For example, vegan diets and vegetarian diets with no fish or seafood products and little dairy products. Also, people who consume very little bread may be at risk of iodine deficiency given that bread is a major source of iodine in Aotearoa New Zealand.

Government monitoring of iodine status

Monitoring is done by regularly checking the iodine content of foods and by measuring the iodine status of the population, in particular the most vulnerable groups within the population, such as children.

Further reading