Iodine: What It Is and How It Works

Iodine is a trace element needed in tiny amounts, yet without it the thyroid gland cannot make the hormones that set the pace of metabolism. The UK is unusual among European countries in having no mandatory salt iodisation, and surveys have shown mild deficiency in some groups. Our editorial team explains how iodine works and what it means for active people.

Iodine and the thyroid

The thyroid gland in the neck captures iodine from the bloodstream and incorporates it into two hormones: thyroxine (T4), which contains four iodine atoms, and triiodothyronine (T3), which contains three. T4 is largely a precursor that is converted to the more active T3 in tissues.

Thyroid hormones influence nearly every cell. They regulate basal metabolic rate, heat production, heart rate, protein synthesis and the development of the brain and nervous system in the foetus and young children.

Production is controlled by a feedback loop. The pituitary gland releases thyroid-stimulating hormone (TSH) when hormone levels fall, prompting the thyroid to take up more iodine. If iodine is scarce, persistent stimulation can cause the gland to enlarge, forming a goitre.

A comprehensive review by Zimmermann describes iodine deficiency as the world’s leading preventable cause of impaired brain development, which is why public health bodies pay close attention to population iodine status.

How much iodine and where it comes from

The UK reference nutrient intake for adults is 140 micrograms per day. EFSA has set an adequate intake of 150 micrograms for adults and 200 micrograms during pregnancy and breastfeeding, and a tolerable upper intake level of 600 micrograms per day for adults.

In the UK, milk and dairy products are the main sources, largely because iodine is added to cattle feed and used in dairy hygiene. White fish and shellfish are also rich sources, while eggs contribute smaller amounts. Plant foods contain little, depending on soil.

Iodised salt is available in the UK but is not widely used. Many plant-based milks are not fortified, which makes vegans and people avoiding dairy a group at particular risk.

A survey of UK schoolgirls by Vanderpump and colleagues found evidence of mild iodine deficiency, and the ALSPAC analysis by Bath and colleagues linked lower iodine status in pregnancy with lower verbal IQ and reading scores in children.

Iodine and athletes

Iodine is lost in sweat, and athletes who train for long periods in hot conditions may lose meaningful amounts. Studies in this area are limited, but they suggest that heavy sweating could contribute to lower iodine status in people with already marginal intake.

Athletes following plant-based diets or avoiding dairy for digestive reasons are more likely to fall short. Combined with sweat losses, this can create a risk that is easy to overlook.

Symptoms of mild deficiency are vague: fatigue, feeling cold, weight changes and reduced concentration. These overlap with overtraining and low energy availability, so blood tests of thyroid function may be useful when symptoms persist.

It is important not to confuse iodine status with thyroid disease. Many thyroid conditions in the UK, such as Hashimoto’s thyroiditis and Graves’ disease, are autoimmune and are not caused by iodine deficiency.

When iodine becomes too much

Excess iodine can be as problematic as too little. A review by Leung and Braverman described how high intakes can trigger both underactive and overactive thyroid, particularly in people with underlying thyroid disease or previous deficiency.

• Kelp and seaweed supplements can contain highly variable and sometimes very large amounts of iodine.
• Combined products such as a multivitamin plus a "thyroid support" supplement can push intake above the upper limit.
• Medicines such as amiodarone and iodinated contrast agents contain large amounts of iodine.
• Brown seaweed in cooking can add significant iodine; the NHS advises caution with seaweed during pregnancy.

Thyroid "boosting" supplements marketed for weight loss or energy should be avoided. Some have been found to contain actual thyroid hormone, which can cause palpitations, anxiety, bone loss and heart rhythm problems.

People with known thyroid conditions should never take iodine supplements without medical guidance.

Practical guidance

For most adults eating dairy and fish, iodine intake is likely adequate, and no supplement is needed. Those who avoid these foods should consider fortified plant milks or a supplement providing around the reference amount, preferably as potassium iodide.

Women planning a pregnancy, pregnant or breastfeeding should pay particular attention, as requirements increase and deficiency affects the baby’s development. The British Dietetic Association has published resources on iodine for these groups.

Athletes training in heat or for long durations who avoid dairy may benefit from discussing their intake with a sports dietitian.

Iodine is not a performance enhancer. Correcting a deficiency supports normal thyroid function, but extra iodine will not increase metabolism or aid fat loss in someone with adequate status.

The editorial team's conclusions

Iodine is essential for producing thyroid hormones that regulate metabolism and development.

In the UK, dairy and fish are the main sources, so vegans, people avoiding dairy and some heavy-sweating athletes may be at risk of low intake.

Both deficiency and excess can disturb thyroid function; kelp supplements and "thyroid boosters" are best avoided.

Our related articles cover thyroid function tests for athletes, micronutrients for vegan sportspeople, and electrolyte and mineral losses in sweat.

References

1. Zimmermann MB. Iodine deficiency. Endocr Rev. 2009;30(4):376–408.
2. Zimmermann MB, Boelaert K. Iodine deficiency and thyroid disorders. Lancet Diabetes Endocrinol. 2015;3(4):286–295.
3. Vanderpump MP, Lazarus JH, Smyth PP, Laurberg P, Holder RL, Boelaert K, et al. Iodine status of UK schoolgirls: a cross-sectional survey. Lancet. 2011;377(9782):2007–2012.
4. Bath SC, Steer CD, Golding J, Emmett P, Rayman MP. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). Lancet. 2013;382(9889):331–337.
5. Leung AM, Braverman LE. Consequences of excess iodine. Nat Rev Endocrinol. 2014;10(3):136–142.
6. EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific opinion on dietary reference values for iodine. EFSA J. 2014;12(5):3660.