Thyroid hormones regulate basal metabolic rate, body temperature, heart rate, and the metabolism of carbohydrates, fats, and protein. Every molecule of thyroid hormone contains iodine atoms: thyroxine contains four iodine atoms and triiodothyronine contains three. Without adequate dietary iodine, the thyroid gland cannot produce sufficient thyroid hormone, leading to subclinical or overt hypothyroidism. Symptoms including fatigue, weight gain, cold intolerance, and cognitive slowing are common presentations of thyroid insufficiency and are frequently attributed to age rather than to a correctable nutritional deficiency.
The UK Iodine Situation
The UK has historically relied on dairy produce as the primary dietary iodine source, as dairy cows are supplemented with iodine. As plant-based milk alternatives have replaced dairy for many consumers, iodine intake has declined. UK surveys have found that teenage girls and young women are particularly likely to be iodine insufficient. Adults following dairy-free diets without replacing iodine from other sources are at meaningful risk of iodine deficiency.
Best Dietary Iodine Sources
- Seaweed: the most concentrated food source of iodine; however, intake should be limited to avoid excess iodine, which can also impair thyroid function; nori sheets used in sushi provide a moderate, safe amount
- White fish and shellfish: cod, haddock, prawns, and crab provide 100 to 150 micrograms per portion
- Dairy produce: milk, yoghurt, and cheese provide 50 to 100 micrograms per serving from iodine-supplemented feed
- Eggs: provide approximately 27 micrograms per egg from iodine in chicken feed
- Iodised salt: not widely used in the UK unlike the US; check the label if using as a supplementary source