Iodine is a small element with a large job: it is the essential building block of thyroid hormones, which regulate metabolism, body temperature, heart rate and, critically, the development of the brain in the fetus and young child. Iodine deficiency was once widespread, causing visible goitres (swelling of the neck) and intellectual impairment. Salt iodisation programmes have transformed global iodine status, but the problem is not entirely solved, and some groups remain at risk. This article explains iodine’s role, food sources and how to get the right amount.
Iodine and the Thyroid Gland
The thyroid gland, located at the front of the neck, actively takes up iodide from the blood. It uses iodine to synthesise two main hormones: thyroxine (T4), containing four iodine atoms, and triiodothyronine (T3), containing three. These hormones travel through the blood and influence nearly every cell:
- Setting the basal metabolic rate
- Regulating heart rate and body temperature
- Supporting normal growth and bone development
- Guiding brain and nervous system development in the fetus and infant
- Influencing cholesterol metabolism, gut motility, skin and hair health
The pituitary gland senses circulating thyroid hormone and releases thyroid-stimulating hormone (TSH) accordingly. When iodine is lacking, TSH rises, driving the thyroid to enlarge in an effort to trap more iodine. This enlargement is a goitre.
How Much Iodine Do You Need?
| Group | RDA or AI (µg per day) |
|---|---|
| Infants 0 to 6 months (AI) | 110 |
| Infants 7 to 12 months (AI) | 130 |
| Children 1 to 8 years | 90 |
| Children 9 to 13 years | 120 |
| Teens and adults (14+ years) | 150 |
| Pregnancy | 220 |
| Breastfeeding | 290 |
The WHO recommends 250 µg per day for pregnant and breastfeeding women. The tolerable upper intake level for adults is 1,100 µg per day. Pregnancy and breastfeeding raise needs because the fetus relies on maternal thyroid hormone in the first trimester, and iodine is secreted into breast milk.
Food Sources of Iodine
Iodine content in foods varies widely because it depends on soil and sea content, animal feed, and processing.
| Food | Serving | Approx. iodine (µg) |
|---|---|---|
| Kelp (dried seaweed) | 1 g | 16 to 2,984 (extremely variable) |
| Nori | 1 sheet | About 12 to 40 |
| Cod, baked | 3 oz | About 158 |
| Iodised salt | ¼ teaspoon (1.5 g) | About 71 (in the US; India’s iodisation standard is higher per gram) |
| Plain Greek yoghurt | ¾ cup | About 87 |
| Milk, cow | 1 cup | About 56 to 112 |
| Egg | 1 large | About 26 |
| Shrimp | 3 oz | About 35 |
| Enriched bread (iodate dough conditioners) | 1 slice | Varies, up to about 45 |
Values are approximate, derived from NIH data. Fish and seafood are naturally rich. Dairy contains iodine from cattle feed supplements and iodine-based sanitisers used in milking. Plant foods vary with soil iodine. Sea salt, Himalayan pink salt and rock salt, however, are typically not iodised and provide little iodine, a point often overlooked by people switching to “natural” salts.
Iodised Salt: A Public Health Success
Universal salt iodisation (USI) has been the main global strategy for preventing iodine deficiency since the 1990s. In India, the National Iodine Deficiency Disorders Control Programme promotes iodised salt, and legislation requires that salt sold for direct human consumption be iodised. Global progress has been substantial, with the Iodine Global Network reporting that most countries now have adequate iodine nutrition, though some, particularly in parts of Europe, still report mild deficiency in specific groups such as pregnant women. Heavy processing, high rainfall and flooding that leach iodine from soil, and the trend toward non-iodised specialty salts and lower salt intake complicate matters.
It is worth noting the dual message: iodised salt is helpful, but total salt intake should still be kept below 5 g per day (WHO recommendation) for blood pressure and cardiovascular health. Iodisation levels are designed so that people eating within this limit still receive enough iodine.
Iodine Deficiency Disorders
The consequences of iodine deficiency depend on timing and severity:
- Fetus: miscarriage, stillbirth, congenital anomalies and cretinism, characterised by severe intellectual disability, deaf-mutism and short stature.
- Infants and children: impaired cognitive and motor development; even mild deficiency in pregnancy has been associated with lower IQ and school performance in some cohort studies.
- Adults: goitre, hypothyroidism (fatigue, weight gain, cold intolerance, constipation, dry skin), and reduced mental function.
The WHO describes iodine deficiency as the world’s most common cause of preventable brain damage in childhood.
Goitrogens: Do Certain Foods Block Iodine?
Some foods contain goitrogens, compounds that can interfere with thyroid function, particularly when iodine intake is low. Examples include cruciferous vegetables (cabbage, cauliflower, broccoli, kale), cassava, millet, soy and sweet potato. Cooking reduces goitrogenic activity of many of these foods. For people with adequate iodine intake, normal portions of these foods are not a concern, and they offer many health benefits. Soy products may reduce the absorption of thyroid medication, so spacing them from levothyroxine is recommended.
Too Much Iodine
Excess iodine can paradoxically cause thyroid dysfunction: either hypothyroidism or hyperthyroidism, particularly in people with underlying thyroid conditions. Kelp supplements and some seaweed products can provide thousands of micrograms per serving, and several cases of thyroid disease have been reported from regular kelp consumption. Iodine-containing medications (such as amiodarone) and radiographic contrast agents also load the thyroid with iodine. People with autoimmune thyroid disease should be cautious about iodine supplements and kelp.
Who Is Most at Risk?
- Pregnant and breastfeeding women
- Infants who are not breastfed and not given iodine-containing formula
- People living in iodine-poor regions, such as mountainous areas and flood plains, who rely heavily on local food
- Vegans who avoid dairy, fish, eggs and iodised salt (studies show lower iodine status among vegans)
- People who avoid salt completely or use only non-iodised salts
Common Misconceptions
- “Sea salt contains enough iodine.” Most sea salt is not iodised and contains trivial iodine.
- “Goitre is a thing of the past.” It persists in regions with inadequate iodisation, and subtle deficiency still occurs elsewhere.
- “More iodine boosts metabolism and weight loss.” Iodine does not accelerate metabolism in people with normal thyroid function, and excess can harm.
- “Thyroid problems are always from iodine.” Autoimmune disease (Hashimoto’s, Graves’) is the commonest cause of thyroid disorders in iodine-sufficient areas.
Practical Takeaways
- Use iodised salt for cooking in moderate amounts, storing it in a cool, dry, closed container (iodine is lost with moisture, heat and light).
- Include dairy, eggs and fish regularly if your diet allows.
- Pregnant and breastfeeding women should discuss iodine supplements with their doctor; many prenatal vitamins include it.
- Avoid regular kelp supplements without medical advice.
- Consult a clinician about thyroid symptoms rather than self-supplementing.
Summary
Iodine is the raw material for thyroid hormones, which govern metabolism and the development of the brain. Fish, dairy, eggs and iodised salt are the main sources, and deficiency is prevented largely by salt iodisation. Pregnancy and breastfeeding need extra attention, and both deficiency and excess can disturb the thyroid. A balanced approach based on iodised salt, varied foods and professional advice is best.
References and Further Reading
- NIH Office of Dietary Supplements. Iodine Fact Sheet for Health Professionals.
- WHO. Assessment of iodine deficiency disorders and monitoring their elimination.
- Iodine Global Network. Global Scorecard of Iodine Nutrition.
- National Academies. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc.
- Ministry of Health and Family Welfare, India. National Iodine Deficiency Disorders Control Programme.
This article is educational and does not replace medical advice.