Iodine is a trace element occurring naturally in the soil and ground. Its assets differ based on geographic area. Therefore, the availability of iodine varies in different countries and parts of the world. Iodine is found in drinking water and several common food products such as fish, seafood, eggs, and dairy products.
Iodine is necessary for the thyroid and is needed for the production of thyroid hormones. These hormones have effects on most organs in the body. In this way, thyroid hormones control metabolism. Iodine deficiency may result in low thyroid function, as well as in the larger thyroid gland (goitre), and increase the risk for overproductive thyroid.
Thyroid hormones are also important during pregnancy as they are needed for the development of the fetal nervous system. Severe iodine deficiency in the mother during pregnancy may therefore cause brain damage in the child and fetal goitre. To prevent iodine deficiency worldwide, the World Health Organization (WHO) recommends the introduction of iodinated salt in so call iodine fortification programs in many countries.
The levels of iodine in the Swedish ground are low, and therefore, goitre and low thyroid function were common during the 19th and early 20th century. In 1936, the Swedish iodine fortification program was started. After iodine was added to table salt, hypothyroidism and goitre (as a result of iodine deficiency) have disappeared. Monitoring of the iodine fortification program was done by us in 2007, and the measurement of iodine concentration in the urine (UIC) shows that the iodine level in the general population is sufficient in Sweden. Previously, the iodine intake varied within the country, as the availability of iodine-rich foods, such as fish and seafood, varied by the distance from the salty sea. We no longer see any geographical differences.
Even if we generally have a good iodine intake, the iodine fortification program is voluntary. This means that the food industry does not always use salt with iodine, and therefore, the iodine level in Swedish whole and semi-produced food may vary. Moreover, the use of non-iodinated salt and flake sea salt has become more popular in Swedish households. The intake of iodine in the Swedish population hence depends on trends in food consumption, and the use of iodinated salt in households, restaurants, and within the food industry.
The need for iodine increases during pregnancy. Therefore, the risk of iodine deficiency is larger among pregnant women. Severe and moderate iodine deficiency affects thyroid hormone levels, but in mild iodine deficiency, the hormone production in the woman is normal. More pronounced forms of iodine deficiency are unusual worldwide, but mild iodine deficiency is common. Mild iodine deficiency occurs in Sweden, as well as in Norway, Denmark, and Finland. Whether mild iodine deficiency has consequences for the health of the child is unknown. The only large study published was from India and Thailand and did not show that iodine supplementation resulted in any advantages compared to the regimen that is advocated today. They studied 800 pregnant women and examined the children at the age of 5-6 years. But there have been objections to that study and it is not considered to give the ultimate answer to the question about the effects of mild iodine deficiency.
The purpose of the SWIDDICH study is to evaluate if there is an advantage in increasing the iodine intake among pregnant women. Both too little and too much iodine may harm the thyroid and therefore it is important to evaluate the effects of a changed iodine intake thoroughly. It should benefit both pregnant women and their children. If our study, together with the study that is already published, does not show any difference between groups, neither we in Sweden nor people in other countries with similar iodine intake need to take action. The world is waiting for a definite answer and we hope that this will be given by this study.
Mild iodine deficiency has no definitive consequences for the individual. But, on a societal level, it is important. In the long run, it may increase the frequency of thyroid diseases in adults, and even if a few IQ point changes will not be noted in the individual, it is important for society overall. The cost of one IQ point loss in a population is estimated to large sums if worse production capacity and less competitiveness are considered. Withholding the iodine levels contributes to a sustainable society.
In the SWIDDICH study, we use two multivitamins where one contains 150 µg iodine. The other tablet is not supplemented with iodine. Both are harmless to use during pregnancy and lactation and can be combined with a normal diet and iodinated salt.