Iodine

Iodine is an essential nutrient required for the production of thyroid hormones. These hormones are important for normal growth and development, particularly of the brain. The major dietary sources of iodine include seafood, especially seaweed, baked bread and dairy milk. Inadequate amounts of iodine may lead to a range of conditions, including goiter, hypothyroidism, and in severe cases, intellectual disability.¹

In October 2009, the mandatory fortification of baked bread with iodised salt was introduced in Australia in response to the re-emergence of mild levels of iodine deficiency in the population², particularly in Tasmania. The 2011–12 National Health Measures Survey (NHMS), which began 18 months after mandatory fortification was introduced, included a measure of urinary iodine excretion and is a key data source for measuring how effective mandatory fortification has been in increasing population iodine levels.

This article looks more closely at how iodine levels vary across the Australian population and how they may have changed in response to mandatory fortification.

The World Health Organization (WHO) considers a population iodine deficient if the median urinary iodine concentration (UIC) is less than 100 μg/L. They also recommend that no more than 20% of the population have iodine concentrations below 50 μg/L.¹

The NHMS results showed that Australian adults had a population median of 124.0 μg/L, which was well within the WHO range for iodine sufficiency. Likewise, 12.8% had a UIC less than 50 μg/L.

Iodine levels between men and women were significantly different, with men having a higher median UIC than women (131.0 μg/L compared with 118.0 μg/L). Women were also more likely to be iodine deficient, with a higher proportion having a UIC less than 50 μg/L (15.8% compared with 9.6%).

In general, children had higher UIC than adults, with those aged 5–11 years having the highest median UIC, followed by those aged 12–17 years. Adults aged 45–64 years had the lowest median UIC.

Rates of iodine deficiency were also lowest among young children, with only 5.9% having iodine levels under 50 μg/L compared with around 15% of those aged 35–54 years.

Adults living in Western Australia had the highest median UIC in 2011–12 (157.4 μg/L) while Tasmanians had the lowest (108.0 ug/L). Tasmania also had the highest proportion of people with iodine levels under 50 μg/L, at 14.9%. Iodine deficiency is generally thought to be higher in Tasmania because of the nature of the state's soil³ and the subsequent low levels of iodine in food products in the state.⁴ Consequently, they were the first state to introduce a voluntary iodine fortification program in 2001³, which resulted in a significant improvement in their population's iodine levels.⁵ The success of this program was instrumental in the decision to implement the national fortification program in 2009.⁶

The NHMS showed that median iodine levels were lower in Inner Regional areas of Australia (114.0 μg/L), but higher in Major Cities (128.0 μg/L). People living in Inner Regional Australia were also more likely to be iodine deficient (15.4% had UIC less than 50 μg/L) than those living in Major Cities (11.9%). This may be due to the varying access to and affordability of certain foods, such as fresh seafood, or the different diets typically consumed by people living in various parts of Australia.

There was no clear relationship between the level of disadvantage and rates of iodine deficiency in 2011–12.

Results from the NHMS showed that iodine levels tended to increase with increasing Body Mass Index. The median UIC for normal weight or underweight adults was 114.0 μg/L compared with 124.0 μg/L for overweight adults and 135.0 μg/L for obese adults. Normal weight and underweight adults were also more likely to be iodine deficient (16.5%) than those who were overweight or obese (10.5%).

Given that iodine is important for early growth and development, a particular group of interest for fortification monitoring is school age children.²

The 2003–04 Australian National Iodine Nutrition Study, published in the Medical Journal of Australia, measured the iodine status of Australian school children aged 8–10 years in New South Wales, Victoria, Queensland, Western Australia and South Australia.⁷ The results showed that Australian school children as a group were mildly iodine deficient, particularly those living in south eastern Australia. This outcome was influential in the decision to implement a national fortification program. The 2011–12 NHMS showed that median iodine levels for children aged 8–10 years had significantly increased in each of these states. The highest iodine levels were in Western Australia, with a UIC of 261.3 μg/L. This indicates that West Australian children generally have iodine levels well above the recommended dietary requirements.

Another group of interest for fortification monitoring are women of childbearing years. This is due to the high susceptibility of the developing fetus to iodine deficiency if these women were to become pregnant.²

The NHMS showed that iodine levels were relatively low among women of childbearing years. Although women aged 16–44 years had sufficient iodine levels overall (a median UIC of 121.0 μg/L), around 18.3% had iodine levels less than 50 μg/L, compared with the national average of 12.8%. Likewise, nearly two thirds (62.2%) had a UIC less than 150 μg/L, which is the iodine level recommended by WHO for pregnant and breastfeeding women.1 These results indicate that mandatory fortification may not be enough to meet the additional iodine requirements of these women.

For more information on iodine, see the Data downloads section in the Key findings of this publication.