Iodine

Medically reviewed by Prof. Giorgi Pkhakadze, MD, MPH, PhD — GMJ Editorial Board

What is Iodine?

Iodine is an essential trace mineral that plays a critical role in human health, particularly in thyroid function and metabolism. As a halogen element found naturally in seawater, soil, and certain foods, iodine cannot be produced by the human body and must be obtained through diet or supplementation. The body contains approximately 15-20 mg of iodine, with about 70-80% concentrated in the thyroid gland.

Iodine deficiency was historically widespread, leading to conditions like goiter and cretinism. The introduction of iodized salt in the 1920s significantly reduced these deficiency-related disorders in many countries, though iodine deficiency remains a global health concern affecting nearly 2 billion people worldwide.

Health benefits & uses

Thyroid function (Strong evidence): Iodine is essential for the synthesis of thyroid hormones thyroxine (T4) and triiodothyronine (T3), which regulate metabolism, growth, and development. Adequate iodine intake prevents goiter and maintains normal thyroid function.

Submit Your Paper

Cognitive development in children (Strong evidence): Sufficient iodine during pregnancy and early childhood is crucial for proper brain development. Iodine deficiency during these critical periods can lead to intellectual disabilities and developmental delays.

Metabolic regulation (Moderate evidence): Through its role in thyroid hormone production, iodine helps regulate metabolic rate, body temperature, and energy production at the cellular level.

Immune system support (Limited evidence): Some research suggests iodine may have antimicrobial properties and support immune function, though more studies are needed to establish definitive benefits.

How it works

Iodine is absorbed in the small intestine and transported to the thyroid gland via the bloodstream. The thyroid actively concentrates iodine through the sodium-iodide symporter, a specialized transport protein. Within the thyroid, iodine undergoes oxidation by thyroid peroxidase enzyme and is incorporated into the amino acid tyrosine to form thyroglobulin.

This process creates monoiodotyrosine and diiodotyrosine, which then couple to form the active thyroid hormones T3 and T4. These hormones are released into circulation and bind to thyroid hormone receptors throughout the body, influencing gene expression and regulating numerous metabolic processes including protein synthesis, carbohydrate metabolism, and oxygen consumption.

Dosage

The Recommended Dietary Allowance (RDA) for iodine varies by age and life stage:

• Adults: 150 mcg/day
• Pregnant women: 220 mcg/day
• Breastfeeding women: 290 mcg/day
• Children 1-8 years: 90 mcg/day
• Children 9-13 years: 120 mcg/day

The Tolerable Upper Intake Level (UL) is 1,100 mcg/day for adults. Supplement doses typically range from 150-300 mcg daily, though some therapeutic applications may use higher amounts under medical supervision. Always consult with a healthcare provider before starting iodine supplementation, especially if you have thyroid conditions.

Food sources

Iodine content in foods varies significantly based on soil iodine levels and processing methods. Rich dietary sources include:

• Seaweed and sea vegetables (exceptionally high, varies widely)
• Fish and shellfish (cod, shrimp, tuna)
• Dairy products (milk, yogurt, cheese)
• Iodized salt
• Eggs
• Bread made with iodized salt

Sea vegetables like kelp can contain extremely variable amounts of iodine, sometimes exceeding safe levels, making them unreliable sources for consistent intake.

Safety & side effects

Iodine is generally safe when consumed within recommended amounts. However, both deficiency and excess can cause thyroid problems. Acute high-dose exposure may cause metallic taste, mouth and throat irritation, nausea, vomiting, and diarrhea.

Chronic excessive intake can lead to:

• Hyperthyroidism or hypothyroidism
• Thyroid inflammation
• Goiter
• Autoimmune thyroid disease

Some individuals may experience allergic reactions, including skin rashes or respiratory symptoms, though true iodine allergy is rare.

Interactions

Medications: Antithyroid drugs (methimazole, propylthiouracil) may interact with iodine supplementation. Lithium can enhance iodine’s thyroid-suppressing effects. Some heart medications and diuretics may affect iodine levels.

Supplements: High doses of calcium, iron, or fiber supplements may interfere with iodine absorption. Selenium works synergistically with iodine in thyroid function. Soy isoflavones may affect thyroid hormone synthesis when iodine intake is marginal.

Who should be cautious

Individuals with autoimmune thyroid conditions (Hashimoto’s thyroiditis, Graves’ disease) should exercise caution with iodine supplementation. Those with kidney disease, dermatitis herpetiformis, or known iodine sensitivity should avoid supplementation.

People taking thyroid medications should consult their healthcare provider before using iodine supplements. Residents of areas with naturally high iodine levels in water or soil should be monitored for excessive intake.

Frequently asked questions

Is iodized salt enough to meet my iodine needs?

For most people, regular use of iodized salt provides adequate iodine. However, those following low-sodium diets, using non-iodized specialty salts, or living in areas with iodine-poor soil may need additional sources.

Can I get too much iodine from seaweed?

Yes, seaweed can contain extremely high amounts of iodine, sometimes thousands of times the daily requirement in a single serving. Regular consumption of large amounts may lead to thyroid dysfunction.

Do I need iodine supplements if I have hypothyroidism?

Not necessarily. Most cases of hypothyroidism in iodine-sufficient areas are due to autoimmune conditions rather than iodine deficiency. Taking iodine supplements when not deficient may worsen some thyroid conditions.

How do I know if I’m iodine deficient?

Iodine deficiency can be assessed through urine iodine concentration tests, thyroid function tests, or clinical evaluation by a healthcare provider. Symptoms may include fatigue, weight gain, cold sensitivity, and enlarged thyroid.

Is it safe to take iodine during pregnancy?

Adequate iodine is crucial during pregnancy for fetal brain development. The RDA increases to 220 mcg daily during pregnancy. However, supplementation should be discussed with your healthcare provider to ensure appropriate dosing.