Originally written by ZRT Laboratory author. Reproduced with permission. Last reviewed: May 2026.
Iodine is an essential trace element that most people associate with iodised salt or seafood — yet the full picture of where dietary iodine comes from is far more nuanced. Changing food habits, agricultural practices, and food manufacturing trends are quietly shifting iodine intake across the population, with significant implications for thyroid health.
This article explains the main dietary sources of iodine, why the body needs it, how it is absorbed and processed, and how to confirm whether your intake is adequate.
What Foods Contain the Most Iodine?
It may come as a surprise that dairy products — not seafood or salt — are the largest single source of dietary iodine in many Western countries, including the UK.
Dairy Products
Iodine is used in cattle farming both as an additive in feed and as a sanitising agent during milking. As a result, iodine passes into milk and dairy products at meaningful concentrations. On average, one cup (240 ml) of cow's milk provides approximately 56 µg of iodine — more than a third of the adult recommended daily intake. Cheese and yogurt also contribute, though at lower concentrations than liquid milk.
Iodine levels in dairy vary seasonally. Because cows graze on iodine-rich pasture during summer and consume indoor feed during winter, dairy iodine content tends to be higher in winter months — an often-overlooked source of variability in dietary intake.
Seafood and Marine Products
The ocean contains approximately 60 µg of iodine per litre, and marine organisms concentrate this iodine efficiently. Marine fish iodine content ranges from as little as 2 µg per 100 g to over 1,000 µg per 100 g of wet tissue, with cod among the richest sources. Shellfish such as prawns, mussels, and oysters are also good sources.
Seaweed is the most concentrated dietary source of iodine available. Content varies dramatically by species: a sheet of nori provides around 16 µg/g (dry weight), while kelp granules can contain over 8,000 µg/g — enough to cause iodine excess if consumed regularly. Other variables affecting seaweed iodine content include the time of year, water quality, and whether the seaweed is eaten raw or cooked.
Eggs
Eggs are a reliable and often underappreciated source of iodine, providing approximately 25 µg per egg. Importantly, iodine is concentrated in the yolk — at roughly five times the concentration found in the egg white. Dietary trends that favour egg whites over whole eggs therefore reduce iodine intake more than most people realise.
Bread and Cereals
Bread was historically a significant source of dietary iodine, when iodine-based compounds were used as dough conditioners. A single slice of bread once provided around 150 µg of iodine. However, many commercial bakeries have now replaced iodine with bromine-based conditioners, effectively eliminating this source from the modern diet. Bromine competes with iodine for uptake in the thyroid gland, compounding the problem by actively interfering with iodine utilisation.
Iodised Salt
Iodised salt remains a key public health tool for preventing iodine deficiency in many countries. In the UK, iodised salt is available but not universally used. In the US, iodised salt contains approximately 77 µg of iodine per gram. However, public health guidance to reduce sodium intake — particularly for those at risk of high blood pressure — has led to a meaningful reduction in iodine intake from this source for a significant portion of the population.
Why Is Dietary Iodine Intake Declining?
Several converging trends are reducing iodine intake across Western populations:
- Reduced dairy consumption. Per capita milk consumption has fallen significantly over recent decades. In the US, consumption dropped from 30 gallons per person per year in 1968 to 19 gallons by 2008. As dairy is the primary iodine source for many people, this decline has a direct impact on overall intake.
- Egg white preference. The trend towards consuming egg whites rather than whole eggs — driven by cholesterol concerns — removes the iodine-rich yolk from the diet.
- Bromine replacing iodine in bread. The shift away from iodine-based dough conditioners in commercial baking has eliminated a historically significant dietary source.
- Lower salt intake. Sodium reduction guidelines, while beneficial for cardiovascular health, reduce iodine intake from iodised salt.
- Plant-based and dairy-free diets. Vegan and dairy-free diets eliminate the two largest iodine sources (dairy and eggs) without always replacing them adequately. Iodine is one of the most commonly deficient nutrients in vegan diets.
If you follow a dairy-free, vegan, or low-sodium diet, assessing your iodine status is particularly important. Our Iodine & Creatinine Dried Urine Test Kit provides a simple, accurate at-home method for measuring urinary iodine — the most reliable indicator of daily iodine intake.
Why Does the Body Need Iodine?
Iodine is required primarily for the synthesis of thyroid hormones. It is the main constituent of both thyroxine (T4) and triiodothyronine (T3) — the hormones that regulate metabolism, energy production, body temperature, heart rate, and neurological development.
If the body does not receive sufficient iodine, thyroid hormone synthesis is impaired. The thyroid gland compensates by enlarging in an attempt to capture more circulating iodine — a condition known as goitre. Prolonged deficiency leads to hypothyroidism, with symptoms including fatigue, weight gain, cold intolerance, hair loss, dry skin, constipation, brain fog, and depression.
Iodine is also essential during pregnancy and early childhood. Deficiency during foetal development and infancy can cause irreversible cognitive impairment, developmental delays, and in severe cases, cretinism. Even mild-to-moderate deficiency during pregnancy is associated with lower IQ scores and behavioural difficulties in children.
Beyond the thyroid, iodine plays a role in breast tissue health, immune function, and the regulation of oestrogen metabolism. Iodine deficiency has been associated with fibrocystic breast changes and may influence hormonal balance more broadly.
How Does the Body Absorb and Use Dietary Iodine?
When iodine-containing food is consumed, absorption occurs primarily in the stomach and small intestine within one to two hours. The absorbed iodine enters circulation and is taken up by the thyroid gland via a specialised transport protein called the sodium-iodide symporter. The thyroid uses iodine — alongside the enzyme thyroid peroxidase and the protein thyroglobulin — to synthesise T4 and T3.
Approximately 97% of dietary iodine not retained by the thyroid or other tissues is excreted in urine, with small amounts lost through sweat, tears, saliva, and faeces. This makes urinary iodine concentration a reliable proxy for daily dietary intake. In iodine-deficient individuals, the thyroid increases its uptake efficiency — absorbing up to twice as much circulating iodine as in iodine-sufficient individuals — as a compensatory mechanism.
Selenium is a critical co-factor in this process. The deiodinase enzymes responsible for converting inactive T4 into active T3 are selenoproteins — meaning adequate selenium is required alongside iodine for optimal thyroid hormone function. Our Thyroid & Iodine Test Kit measures both iodine and key thyroid markers together, providing a comprehensive view of how your iodine intake is affecting thyroid function.
How Much Iodine Do You Need Each Day?
The recommended daily intake (RDI) for iodine in non-pregnant adults is 150 µg per day. Requirements increase during pregnancy (250 µg/day) and breastfeeding (250–290 µg/day), reflecting the critical role of iodine in foetal and infant neurological development.
Reaching 150 µg per day is achievable through a varied diet that includes dairy, eggs, and occasional seafood. However, for those following restrictive diets, or who have reason to suspect deficiency, testing is the only reliable way to confirm adequacy. Supplementation without testing carries its own risks — excessive iodine intake can trigger or worsen autoimmune thyroid conditions such as Hashimoto's thyroiditis in susceptible individuals.
For a broader assessment of thyroid function alongside iodine, selenium, and other essential elements, our Thyroid, Heavy Metals & Essential Elements Test provides a comprehensive at-home panel using dried urine and blood spot samples analysed by ICP-MS — the gold standard in trace element analysis.
How to Test Your Iodine Levels at Home
Because approximately 97% of absorbed iodine is excreted in urine, urinary iodine concentration is the most practical and validated method for assessing daily intake. A single spot urine sample collected at home and analysed by ICP-MS provides a reliable estimate of iodine status without the need for a clinic visit or 24-hour urine collection.
Testing is particularly recommended for:
- Individuals following vegan, dairy-free, or low-sodium diets
- Women who are pregnant, planning pregnancy, or breastfeeding
- Those with symptoms of hypothyroidism or thyroid dysfunction
- People taking iodine supplements who want to confirm they are within a safe range
- Anyone with a family history of thyroid disease
Frequently Asked Questions
What is the best dietary source of iodine?
Dairy products — particularly milk — are the largest single source of dietary iodine in the UK and many Western countries, providing around 56 µg per cup. Seafood, especially cod and shellfish, is also rich in iodine. Seaweed is the most concentrated source but varies enormously in content and should be consumed with caution to avoid excess.
Can you get enough iodine on a vegan diet?
It is possible but requires careful planning. Dairy and eggs — the two most reliable everyday iodine sources — are excluded from vegan diets. Seaweed can provide iodine but its content is highly variable and potentially excessive. Iodised salt and iodine supplements are the most practical options for vegans, but testing is recommended to confirm adequacy.
What are the symptoms of iodine deficiency?
Symptoms of iodine deficiency include fatigue, unexplained weight gain, feeling cold, hair thinning or loss, dry skin, constipation, brain fog, depression, and a swelling at the base of the throat (goitre). Many of these symptoms overlap with other conditions, which is why testing is important for accurate diagnosis.
Is too much iodine harmful?
Yes. While iodine deficiency is the more common concern, excessive intake — particularly from high-dose supplements or large amounts of kelp — can suppress thyroid function, trigger autoimmune thyroid disease, or worsen existing Hashimoto's thyroiditis. The tolerable upper intake level for adults is 1,100 µg per day. Testing before supplementing is strongly recommended.
Does cooking affect the iodine content of food?
Yes. Iodine is water-soluble and heat-sensitive, meaning boiling or prolonged cooking can reduce iodine content in foods. Boiling milk, for example, can reduce its iodine content by up to 30%. Eating iodine-rich foods in less processed forms — such as raw dairy, lightly cooked fish, or soft-boiled eggs — helps preserve iodine content.
Why has iodine been replaced by bromine in bread?
Bromine-based compounds (potassium bromate) became popular as dough conditioners in commercial baking because they are cheaper and more stable than iodine-based alternatives. However, bromine competes directly with iodine for uptake by the thyroid gland, meaning high bromine exposure can actively worsen iodine status even when dietary intake appears adequate.
Originally written by ZRT Laboratory author. Reproduced with permission. Last reviewed: May 2026.