Iodine: thyroid hormones, tests and safety
A practical guide to iodine in T4 and T3 synthesis, food sources, deficiency and excess, thyroid testing and safe supplement decisions.
Biological role
Iodine is part of thyroxine (T4) and triiodothyronine (T3). These hormones regulate metabolism, body temperature, heart function, growth and nervous-system development. The thyroid takes iodine from blood and uses it to build hormones. Pregnancy and early childhood are especially sensitive periods because thyroid hormones support brain development. More iodine is not a substitute for treating iron, selenium or other nutritional problems.
Food sources
Reliable sources include marine fish, seafood, dairy, eggs and iodised salt. Ordinary sea salt and Himalayan salt may contain unpredictable amounts. Kelp and other seaweed products can provide either very little or excessive iodine, and labels may not reflect batch variation. Store iodised salt dry and covered, and add it near the end of cooking. Sodium reduction does not require abandoning iodised salt; total salt still matters.
Deficiency risk
Risk rises in regions with low iodine intake, diets excluding fish and dairy, avoidance of iodised salt, and malabsorption disorders. Prolonged deficiency may enlarge the thyroid and contribute to hypothyroidism. Tiredness, feeling cold, dry skin, constipation and weight gain are non-specific and also occur with anaemia, depression and other conditions. Symptoms or urine colour cannot diagnose deficiency.
Excess and medicines
High doses and kelp supplements may trigger hyperthyroidism or hypothyroidism, particularly with nodules, Graves’ disease or Hashimoto thyroiditis. Medicines such as amiodarone can contain substantial iodine and alter thyroid function. Palpitations, tremor, anxiety, sweating, weakness, sleepiness and swelling are possible. Iodine should not be used to treat a ‘slow metabolism’ without a diagnosis.
Testing
Thyroid function is usually assessed first with TSH and free T4; antibodies and ultrasound are added when indicated. Spot urinary iodine reflects recent intake and is more useful for population monitoring than an individual treatment decision. Salt, supplements, pregnancy, collection timing and kidney function affect results. A single urine value is not a reason to start tablets.
Pregnancy and children
Discuss diet and prevention before conception. A clinician may recommend a standard daily supplement when food intake is inadequate; thyroid disease changes the plan. Children should receive iodine only when prescribed. High doses are not more reliable prevention and can harm both parent and child.
Practical sequence
- Review region, salt, seafood, dairy, medicines and thyroid history. 2. Discuss TSH and free T4 when symptoms suggest dysfunction. 3. Do not use one urine test as the sole reason for supplements. 4. Choose iodised salt within overall sodium guidance. 5. Avoid kelp and megadoses. 6. Seek urgent help for severe palpitations, weakness, swelling or confusion. This page is educational, not an endocrine prescription.