Iodine for Women: Are There Any Special Considerations?
Iodine rarely features in conversations about sports nutrition, yet it is essential for the thyroid hormones that govern metabolism, energy and development. Women deserve particular attention, because pregnancy and breastfeeding raise requirements and the UK does not have a national salt iodisation programme. Our editors explain what women need, where iodine comes from and where the risks lie.
Why iodine matters
Iodine is a trace element that the thyroid gland uses to produce thyroxine (T4) and triiodothyronine (T3). These hormones regulate the rate of metabolism, body temperature, heart rate and the development of the brain and nervous system.
When iodine intake is low, the thyroid cannot make enough hormone. The pituitary responds by releasing more thyroid-stimulating hormone (TSH), which can enlarge the gland, causing a goitre. Long-term deficiency may lead to hypothyroidism, with tiredness, weight gain, feeling cold and low mood.
Thyroid disorders are considerably more common in women than in men. While autoimmune disease, not iodine deficiency, is the main cause of hypothyroidism in the UK, adequate iodine is still essential for normal thyroid function.
In the UK, the reference nutrient intake for adults is 140 micrograms per day. The World Health Organization recommends 250 micrograms per day during pregnancy and breastfeeding, a figure also used by many expert groups.
Iodine status in the UK
Unlike many European countries, the UK has no mandatory salt iodisation, and iodised salt is not widely sold. Most iodine in the British diet comes from milk and dairy products, with fish and eggs as other important sources.
The iodine content of milk is a result of farming practices, such as iodine-supplemented cattle feed and iodine-based teat disinfectants. Organic milk has been found in some analyses to contain less iodine than conventional milk, although the gap can vary by season.
A national survey of UK schoolgirls published by Vanderpump and colleagues in 2011 found mild iodine deficiency overall, which raised concerns about women of childbearing age. Later data from various studies have also suggested that many pregnant women in the UK may not reach recommended intakes.
Iodine status is usually assessed at population level through urinary iodine. A single urine test is not reliable for diagnosing deficiency in an individual, because levels vary considerably from day to day.
Pregnancy, breastfeeding and planning
During pregnancy, the mother's thyroid hormone production rises, and the baby relies on maternal iodine to make its own thyroid hormones. Iodine is also transferred into breast milk after birth. This is why requirements increase so markedly.
Bath and colleagues, using data from the Avon Longitudinal Study of Parents and Children, found that children of mothers with low iodine status in early pregnancy were more likely to have lower scores on verbal IQ and reading tests. Even mild deficiency appeared to matter.
In the UK, NHS advice focuses on a varied diet including dairy and fish. Many pregnancy multivitamins include iodine, typically around 140–150 micrograms. Women who avoid dairy and fish may need to discuss iodine with their midwife or GP.
Women with existing thyroid conditions should not change their iodine intake without advice, since both deficiency and excess can affect thyroid function and treatment. The American Thyroid Association's guidelines highlight the importance of individual assessment in pregnancy.
Active women and plant-based diets
For active women, iodine is not a performance supplement, but deficiency can contribute to fatigue and reduced tolerance to training. Women with restrictive diets, low energy intake or elimination of food groups are more at risk.
Plant-based diets are a key concern. Most plant milks are not fortified with iodine, though some brands now add it. Vegan women who do not use fortified products or supplements may have very low intakes.
Sweat contains some iodine, and research suggests that athletes training in hot conditions may lose meaningful amounts. Data are limited, but it is another reason to ensure a reliable intake.
The table below shows typical iodine sources in the UK diet.
| Food | Iodine content | Notes |
|---|---|---|
| Cow's milk | Moderate to good | Main source in the UK; varies by season and farming |
| Yoghurt and cheese | Moderate | Contributes to daily intake |
| White fish (e.g. cod, haddock) | High | One of the richest food sources |
| Eggs | Moderate | Useful for vegetarians |
| Plant milks | Low unless fortified | Check label for added iodine |
| Seaweed | Very variable, often extremely high | Risk of excess intake |
Too much iodine: supplements and seaweed
Excessive iodine can also disrupt thyroid function, causing either hypothyroidism or hyperthyroidism, especially in people with underlying thyroid disease. Leung and Braverman reviewed the consequences of iodine excess and noted that sensitive individuals can react even to moderate increases.
The European Food Safety Authority has set a tolerable upper intake level of 600 micrograms per day for adults. Some kelp and seaweed supplements contain amounts well above this, and the content can vary between batches.
Seaweed itself, particularly brown varieties such as kombu, can contain very large amounts of iodine. Regular consumption of large portions is not advised, especially during pregnancy.
Practical guidance for women considering a supplement:
- choose potassium iodide or potassium iodate in a known amount, usually 140–150 micrograms;
- avoid kelp or seaweed supplements with uncertain iodine content;
- do not combine several iodine-containing products without checking the total;
- seek medical advice if you have a thyroid condition or take thyroid medication.
Conclusions of the editorial team
Iodine is essential for thyroid hormone production, and women need particular attention because of higher requirements in pregnancy and breastfeeding.
In the UK, dairy and fish are the main sources, so women who avoid them, including many following plant-based diets, may need fortified foods or a supplement.
Too much iodine is also harmful. Seaweed-based products can deliver unpredictable amounts, and a measured supplement is safer when one is needed.
You might also be interested in our articles on thyroid function and training, on nutrition for vegan athletes, and on supplements during pregnancy.
References
- Zimmermann MB. Iodine deficiency. Endocr Rev. 2009;30(4):376–408.
- Vanderpump MPJ, Lazarus JH, Smyth PP, et al. Iodine status of UK schoolgirls: a cross-sectional survey. Lancet. 2011;377(9782):2007–2012.
- Bath SC, Steer CD, Golding J, et al. 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.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315–389.
- Leung AM, Braverman LE. Consequences of excess iodine. Nat Rev Endocrinol. 2014;10(3):136–142.
