Seafood with high iodine content provides essential support for thyroid function and metabolic health. Iodine is a trace element required for the synthesis of thyroid hormones, which regulate energy production, temperature control, and cellular metabolism throughout the body. In the UK, where salt iodisation is not mandatory, dietary sources—particularly seafood and dairy products—represent the primary means of obtaining adequate iodine. Understanding which seafood options deliver substantial iodine helps individuals make informed dietary choices that support optimal thyroid function whilst maintaining nutritional balance. This article examines high-iodine seafood varieties, daily requirements, and practical guidance for safe consumption.

Why Iodine Matters for Your Health

Iodine is an essential trace element that plays a critical role in human physiology, particularly in the synthesis of thyroid hormones—thyroxine (T4) and triiodothyronine (T3). These hormones regulate numerous metabolic processes throughout the body, including energy production, temperature regulation, and cellular metabolism. Without adequate iodine, the thyroid gland cannot produce sufficient quantities of these hormones, potentially leading to hypothyroidism and associated complications.

The importance of iodine extends beyond thyroid function. During pregnancy and early childhood, adequate iodine intake is crucial for proper neurological development and cognitive function. Maternal iodine deficiency has been associated with impaired foetal brain development, and severe deficiency can result in congenital iodine deficiency disorders characterised by intellectual disability and growth failure.

In the United Kingdom, mild iodine deficiency may be a concern, particularly among women of childbearing age, though overt deficiency is uncommon. Unlike many countries, the UK does not have a mandatory salt iodisation programme, making dietary sources—especially dairy products and seafood—the primary means of obtaining this essential nutrient. The British Thyroid Foundation notes that whilst overt iodine deficiency disorders are rare in the UK, subclinical deficiency may affect thyroid function and overall health. It's important to note that increasing salt intake to obtain iodine is not recommended, as the UK promotes salt reduction for cardiovascular health. Understanding which foods provide substantial iodine, particularly seafood options, enables individuals to make informed dietary choices that support optimal thyroid function and metabolic health throughout life.

Seafood with High Iodine Content

Seafood represents one of the richest natural sources of dietary iodine, with marine species accumulating this element from seawater. The iodine content varies considerably depending on the species, habitat, season, and preparation method, but several types consistently provide substantial amounts.

Fish with particularly high iodine content include:

• Cod – A single 100g portion of cod can provide approximately 100–170 micrograms of iodine, making it one of the more reliable fish sources. Both fresh and frozen cod retain their iodine content well.

• Haddock – Similar to cod, haddock offers approximately 110–250 micrograms per 100g serving, representing a significant contribution towards daily requirements.

• Pollock – This white fish provides roughly 80–130 micrograms per 100g portion and serves as an economical alternative to cod.

Shellfish and other seafood options:

• Prawns and shrimp – These crustaceans contain approximately 30–50 micrograms per 100g, with variations depending on whether they are wild-caught or farmed.

• Mussels – Providing around 100–150 micrograms per 100g, mussels represent an excellent iodine source alongside other minerals like selenium and zinc.

• Scallops – These molluscs offer moderate iodine levels, typically 50–100 micrograms per 100g.

Seaweed and sea vegetables deserve special mention, as they can contain extraordinarily high iodine concentrations—sometimes exceeding 2,000 micrograms per serving. Brown seaweeds like kelp and kombu are particularly high in iodine, while nori (used in sushi) contains more moderate levels. The NHS and Food Standards Agency advise caution with seaweed consumption and recommend avoiding kelp-based supplements, as excessive intake can disrupt thyroid function.

Pregnant women should follow NHS guidance on fish consumption, which includes limiting tuna intake and avoiding shark, marlin, and swordfish due to mercury concerns, as well as avoiding raw shellfish. When incorporating seafood into your diet for iodine, variety and moderation ensure both adequate intake and nutritional balance.

How Much Iodine Do You Need Daily?

The recommended daily iodine intake varies according to age, physiological status, and individual health circumstances. Understanding these requirements helps ensure adequate intake whilst avoiding potential complications from excessive consumption.

UK Reference Nutrient Intakes (RNIs) for iodine:

• Adults (19+ years) – 140 micrograms per day for both men and women, including during pregnancy and breastfeeding

• Children (11–14 years) – 130 micrograms per day

• Children (7–10 years) – 110 micrograms per day

• Children (4–6 years) – 100 micrograms per day

• Children (1–3 years) – 70 micrograms per day

These recommendations are set by the Scientific Advisory Committee on Nutrition (SACN) and are designed to maintain optimal thyroid function whilst providing a safety margin. It is worth noting that the tolerable upper intake level—the maximum amount unlikely to cause adverse effects—is set at 600 micrograms per day for adults by the European Food Safety Authority (EFSA). The NHS advises not exceeding 500 micrograms per day from supplements and avoiding kelp supplements, which can contain excessive amounts.

For most individuals following a varied diet that includes dairy products, eggs, and occasional seafood, meeting these requirements presents little difficulty. However, those following vegan diets, individuals with restricted food choices, or people living in areas with historically low soil iodine may require particular attention to iodine sources. Some plant-based milk and yoghurt alternatives are now fortified with iodine—check product labels for this information.

A single 100g portion of cod or haddock can provide a substantial portion of an adult's daily requirement, whilst two portions of seafood weekly—as recommended by NHS dietary guidelines—contribute significantly towards adequate iodine status. NICE does not recommend routine iodine supplementation during pregnancy; instead, a balanced diet is emphasised. Pregnant women should discuss their individual nutritional needs with their midwife or GP, as prenatal vitamins in the UK do not universally contain iodine.

When to Limit High-Iodine Seafood

Whilst iodine is essential for health, certain circumstances warrant careful monitoring or restriction of high-iodine seafood consumption. Both excessive iodine intake and specific medical conditions can necessitate dietary modifications.

Thyroid disorders requiring iodine restriction:

Individuals with hyperthyroidism (overactive thyroid) or Graves' disease may need to limit iodine intake, as excess iodine can exacerbate thyroid hormone overproduction. Those preparing for radioactive iodine treatment must follow a low-iodine diet for 1–2 weeks beforehand to maximise treatment efficacy—this involves avoiding seafood, dairy, and iodised products. Follow your nuclear medicine department's specific guidance, as protocols vary between centres. Patients with autoimmune thyroid conditions, including Hashimoto's thyroiditis, may experience variable responses to high iodine intake; some individuals find that excessive iodine triggers or worsens symptoms, though there is no universal recommendation to avoid seafood entirely.

Other considerations for limiting high-iodine seafood:

• Reactions to seafood – True iodine allergy does not exist. Shellfish allergies relate to specific proteins rather than iodine content, and contrast media reactions are unrelated to dietary iodine. However, some individuals may have sensitivities to concentrated iodine sources.

• Medication interactions – Certain medications require careful monitoring with dietary iodine. Amiodarone (a heart rhythm medication containing significant iodine) requires regular thyroid function monitoring. Lithium can also affect thyroid function. Patients taking these medications should avoid iodine supplements and seaweed products unless specifically advised by their healthcare provider, and discuss seafood consumption with their GP or specialist.

• Pregnancy complications – Women with pre-existing thyroid conditions require individualised advice during pregnancy, as both insufficient and excessive iodine can affect maternal and foetal thyroid function.

Seek urgent medical attention if you experience a rapidly enlarging neck mass, difficulty breathing or swallowing, or severe palpitations, as these may indicate serious thyroid dysfunction.

The key principle is that moderation serves most people well. Consuming 2–3 portions of varied seafood weekly, as per NHS guidance, provides health benefits without risking iodine excess. If you have been diagnosed with a thyroid condition, always consult your endocrinologist or GP before making significant dietary changes. They can arrange thyroid function tests and provide personalised guidance based on your specific circumstances and treatment plan.

Balancing Iodine Intake Safely

Achieving optimal iodine status requires a balanced approach that considers both dietary sources and individual health needs. For most people in the UK, this involves incorporating iodine-rich foods as part of a varied, nutritious diet rather than relying on single sources or supplements.

Practical strategies for balanced iodine intake:

• Diversify seafood choices – Rather than consuming the same high-iodine fish repeatedly, rotate between different species. Combining white fish (cod, haddock) with oily fish (salmon, mackerel—which contain moderate iodine) provides iodine alongside omega-3 fatty acids and other nutrients.

• Include complementary iodine sources – Dairy products (milk, yoghurt, cheese) contribute significantly to UK iodine intake. Eggs provide moderate amounts. For those following plant-based diets, check labels for iodine-fortified plant milks and yoghurt alternatives, as iodine content in non-fortified plant foods is generally low.

• Exercise caution with seaweed – Whilst nori used in sushi contains moderate iodine, kelp and some seaweed supplements can provide excessive amounts in a single serving. The NHS advises avoiding kelp supplements. If you enjoy seaweed, consume it occasionally rather than daily, and be particularly cautious with brown seaweeds like kelp and kombu.

• Consider your individual circumstances – Pregnant women, those planning pregnancy, and breastfeeding mothers should focus on dietary sources of iodine. A balanced diet is the preferred approach, as NICE does not recommend routine iodine supplementation during pregnancy. If considering supplements, discuss with your midwife or GP first.

• Don't increase salt intake – Although some countries use iodised salt, the UK does not have a universal salt iodisation programme. Follow NHS guidance on reducing salt intake for cardiovascular health rather than increasing salt consumption to obtain iodine.

When to seek professional advice:

Contact your GP if you experience symptoms potentially related to thyroid dysfunction, such as unexplained weight changes, persistent fatigue, temperature sensitivity, or neck swelling. These symptoms warrant thyroid function testing regardless of dietary iodine intake. If you are considering iodine supplements, discuss this with a healthcare professional first—the NHS advises not exceeding 500 micrograms per day from supplements, and excessive supplementation can paradoxically impair thyroid function.

If you experience any suspected side effects from medicines or supplements, report them through the MHRA Yellow Card scheme.

For most individuals, following the NHS Eatwell Guide—which recommends two portions of fish weekly, including one oily variety—naturally provides adequate iodine whilst delivering broader nutritional benefits. This balanced approach supports thyroid health without the risks associated with either deficiency or excess.

Frequently Asked Questions