The importance of iodine for metabolism and overall health

We need iodine for metabolism, the nervous system, cognitive function, and the immune system. In addition, iodine is essential for a child’s growth and development. However, although table salt and bakery bread are fortified with iodine in Denmark and many other countries, iodine deficiency remains a widespread health problem and is becoming more common as the number of vegetarians and vegans increases. Iodine deficiency increases the risk of goiter, an underactive thyroid, reduced resistance to infection, impaired fertility, and breast cancer. Pregnant women who are deficient in iodine are at risk of fetal abnormalities, while children who are deficient in iodine are at risk of intellectual disability or stunted growth. On the other hand, we must also avoid getting too much iodine, as excessive intake can cause health problems as well, according to a new review article published in Nutrients.

Iodine is found primarily in marine sources such as fish, shellfish, fish sauce, and seaweed, as well as eggs and dairy products. In many parts of the world, the soil is very low in iodine, which is reflected in crops and throughout the food chain. For this reason, Denmark and more than 120 other countries have introduced mandatory iodine fortification of household salt and salt used in the food industry to prevent goiter. However, many people avoid refined table salt because it contains an aluminum-based anti-caking agent that is believed to damage the nervous system, and sea salt cannot provide sufficient iodine to meet the body’s needs.
The daily requirement for iodine is 150 micrograms for adults and approximately 250 micrograms for pregnant women. However, according to a large population study from 2024 published in The Lancet Global Health, it is estimated that around 68% of the world’s population is deficient in iodine to some degree. Women, pregnant women, children, vegetarians, and vegans are particularly vulnerable.
Widespread iodine deficiency is a serious health problem because iodine has many vital functions in the body throughout life. This is highlighted in the new review article, in which the authors also discuss the role of iodine deficiency in growth, development, and a number of chronic diseases. The main points are summarized below:

Iodine’s role in metabolism

The thyroid gland, located at the front of the neck, is responsible for producing thyroid hormones. This process takes place in close interaction with thyroid-stimulating hormone (TSH), which is produced by the pituitary gland.
The thyroid gland primarily produces T4 hormone, which contains four iodine atoms and functions as an inactive precursor. When cells need energy, one iodine atom is removed from T4 with the help of selenium-containing enzymes. This converts T4 into T3, which contains three iodine atoms. T3, the active thyroid hormone, can then enable oxygen to enter cells so they can convert calories from food into energy. If too little T3 is produced, metabolism becomes too slow, while excessive T3 production causes metabolism to become too fast.
Iodine deficiency can typically result in goiter, in which the thyroid gland enlarges because it lacks iodine. Long-term deficiency can also cause an underactive thyroid (hypothyroidism), resulting in fatigue, weight gain, feeling cold, constipation, dry skin, hair loss, reduced fertility, and many other symptoms due to cellular lack of energy. Hashimoto’s disease, which is caused by a chronic inflammatory condition of the thyroid gland, is the most common cause of an underactive thyroid.

Iodine’s role in the immune system, as an antioxidant, and in cancer prevention

Although immune system functions also depend on normal thyroid function, it is less well known that iodine also has a direct immune-stimulating effect. Iodine contributes to the ability of certain white blood cells (neutrophils and macrophages) to carry out phagocytosis, in which they engulf and destroy bacteria and other pathogens. Iodine also plays a role in the formation of various T cells, which are particularly important for fighting viruses, fungi, and abnormal cells. In addition, iodine plays a role in the formation and activation of various cytokines that regulate inflammatory processes.
Overall, iodine functions as an antioxidant that protects cells against damage from free radicals and oxidative stress. Iodine contributes, among other things, to regulating inflammatory processes that can otherwise increase oxidative stress in cells. It has also been shown that women who consume more iodine through their diet have a lower risk of developing breast cancer. This is partly because iodine plays a role in the immune system and partly because iodine helps regulate the estrogen balance and a specific gene (MCF7) in breast cancer cells. This was reported in an earlier study published in the International Journal of the Medical Sciences. Iodine may therefore be relevant as an adjunctive treatment for hormone-dependent breast cancer.

The importance of iodine during pregnancy and for a child’s growth

Iodine deficiency can result in reduced fertility. In addition, pregnant women’s need for iodine increases by approximately 65% (to around 250 micrograms per day) to accommodate increased production of thyroid hormones and the fetus’s needs during growth. The need for iodine is also greater during breastfeeding to meet the baby’s needs.
Iodine deficiency during pregnancy is associated with poorer development of the child’s nervous system, including poorer language and motor skills, lower IQ, and other signs of intellectual disability. There is also a risk of stunted growth.

Iodine requirements

As mentioned above, the recommended daily intake of iodine for adults is 150 micrograms and approximately 250 micrograms for pregnant women. According to the WHO, the safe upper intake level is 1,100 micrograms per day, while European health authorities set the limit at 500–600 micrograms per day.
Excessive iodine intake can occur particularly as a result of iodine-rich diets, such as excessive consumption of certain types of seaweed such as dulse, high-dose iodine supplements, and iodine-containing contrast agents used in X-rays and CT scans to make organs and tissues more visible.
Although most people tolerate relatively high amounts of iodine, excessive intake can cause overactivation of the thyroid gland, resulting in an overactive thyroid, high blood pressure, abnormal heart rhythms, and strain on the kidneys.
Large amounts of iodine can also expose the thyroid gland to an “iodine shock” and a biological effect known as the Wolff-Chaikoff effect, in which the thyroid gland suppresses the production of thyroid hormones to protect the body against dangerously high thyroid activity. This temporary blockage of hormone production, which lowers metabolism, generally lasts for up to a couple of days, after which the thyroid gland often adapts.
People with an underactive thyroid, such as those with Hashimoto’s disease, should also exercise caution with very high-dose iodine supplements, particularly if they are also deficient in selenium.
Although pregnant women have an increased need for iodine, they should also avoid excessive intake because it can harm the development of the child’s nervous system.

• Iodine intake follows a U-shaped curve, meaning that we should neither get too little nor too much
• 68% of the world’s population is iodine deficient, making iodine deficiency one of the most widespread mineral deficiencies
• Widespread iodine deficiency has serious health consequences.

References:

Tea Delic and Sandra Karanovic Stambuk. Iodine in Health and Disease: A Comprehensive Review. Nutrients. 2026

Laura Croce et al. Moderne challenges of iodine nutrition: vegan and vegetarian diets. Frontiers in Nutrition 2025.
Simone Passarelli et al. Global estimation of dietary micronutrient inadequacies: a modeling analysis. The Lancet Global Health 2024

Frederick R. Stoddard et al. Iodine Alters Gene expression in the MCF7 Breast Cancer Cell Line: Evidence for an Anti-Estrogen Effect of Iodine. International Journal of Medical Sciences. 2008

Francesca Coperchini et al. Thyroid Disrupting Effects of Old and New Generations PFAS. Frontiers in Endocrinology 2021