Iodine
Iodine
Also known as: Potassium iodide, Sodium iodide, Iodide, Lugol's solution, Povidone-iodine
Iodine is an essential trace mineral required for thyroid hormone synthesis. Deficiency causes goiter, hypothyroidism, and intellectual disability in infants. Iodized salt has nearly eliminated deficiency in many regions.
Introduction
Iodine is an essential trace mineral discovered in 1811 by French chemist Bernard Courtois. It is required for the synthesis of thyroid hormones thyroxine (T4) and triiodothyronine (T3), which regulate metabolism, growth, and development.
Iodine deficiency is the most common preventable cause of intellectual disability worldwide. During pregnancy, severe deficiency causes cretinism - a condition characterized by severe mental retardation, stunted growth, and physical abnormalities. Even mild deficiency can impair cognitive development.
The introduction of iodized salt in the 1920s virtually eliminated goiter in the United States and many other countries, representing one of the most successful public health interventions in history. However, deficiency remains a significant problem in many parts of the world, affecting an estimated 2 billion people.
In developed countries, deficiency has reemerged in some populations due to reduced salt intake, use of non-iodized salt, and declining consumption of iodine-rich dairy products. Vegetarians and vegans are at higher risk.
While deficiency is harmful, excessive iodine can also cause thyroid dysfunction. Both hypo- and hyperthyroidism can result from iodine excess, particularly in susceptible individuals.
Main Benefits
Essential for synthesis of thyroid hormones T3 and T4; required for normal thyroid function and metabolism regulation.
Prevents goiter (thyroid enlargement) and hypothyroidism caused by iodine deficiency.
Critical for fetal brain development; deficiency during pregnancy causes irreversible intellectual disability (cretinism).
Supports normal growth and development in children; deficiency impairs physical and cognitive development.
May have antimicrobial properties when applied topically (povidone-iodine); used for wound disinfection.
Mechanism of Action
Iodine functions primarily through thyroid hormone synthesis:
Thyroid Hormone Synthesis: The thyroid gland actively concentrates iodide from blood via the sodium-iodide symporter (NIS). Iodide is oxidized and incorporated into tyrosine residues on thyroglobulin, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). These combine to form T3 (triiodothyronine) and T4 (thyroxine).
Metabolic Regulation: T3 and T4 regulate metabolism in virtually all cells by binding to nuclear thyroid hormone receptors. They influence:
- Basal metabolic rate
- Protein, fat, and carbohydrate metabolism
- Body temperature regulation
- Heart rate and contractility
- Brain development and function
- Bone growth
Brain Development: Thyroid hormones are essential for neuronal migration, myelination, and synapse formation in the developing brain. Deficiency during pregnancy and early childhood causes irreversible brain damage.
Reproductive Function: Thyroid hormones are required for normal reproductive function in both sexes.
Antimicrobial Effects: Elemental iodine and iodophors (like povidone-iodine) disrupt microbial cell walls and proteins, providing topical antiseptic effects.
Natural Sources
Iodine content in foods varies by soil and farming practices. Iodized salt is the primary source in many countries. Seafood and dairy are naturally rich sources.
Examples:
Iodized salt (primary source in many countries)
Seaweed and kelp (extremely high, variable)
Cod and other fish
Shrimp and seafood
Dairy products (milk, yogurt, cheese)
Eggs
Beef liver
Chicken
Iodized salt widely available; deficiency eliminated in many regions through salt fortification; vegetarians/vegans may need attention; declining use of iodized salt increases risk.
Deficiency Symptoms
Iodine deficiency causes goiter, hypothyroidism, and in severe cases during pregnancy, cretinism in offspring.
Common Symptoms:
Goiter (enlarged thyroid gland)
Hypothyroidism symptoms (fatigue, weight gain, cold intolerance)
Intellectual disability (in offspring of deficient mothers)
Growth retardation
Developmental delays
Increased risk of thyroid nodules
Eliminated in many developed countries through salt iodization; still affects ~2 billion globally; reemerging in some populations with reduced salt intake.
Leading cause of preventable intellectual disability; cretinism irreversible; affects entire populations; highest impact of any micronutrient deficiency.
Recommended Daily Intake
RDA: adults 150 mcg/day. Higher during pregnancy (220 mcg) and lactation (290 mcg). Upper Limit (UL) is 1,100 mcg/day.
Reference Values:
| Adult men | 150 mcg/day |
| Adult women | 150 mcg/day |
| Pregnancy | 220 mcg/day |
| Lactation | 290 mcg/day |
| Upper Limit (UL) | 1,100 mcg/day |
Sources for RDI/AI:
1/4 teaspoon iodized salt provides ~100 mcg iodine. Kelp/seaweed supplements highly variable and can exceed UL. Prenatal vitamins should contain 150 mcg iodine.
Effectiveness for Specific Focuses
Absolutely essential for thyroid hormone synthesis; deficiency causes thyroid dysfunction; foundational for metabolic regulation.
Critical for pregnancy; deficiency causes cretinism; increased requirements during pregnancy and lactation; prenatal essential.
Essential for fetal brain development; deficiency causes irreversible intellectual disability; supports lifelong cognitive function.
Thyroid hormones regulate metabolism; deficiency causes hypothyroidism and metabolic slowing; supports healthy weight.
Safety Information
Potential Side Effects
Thyroid dysfunction (both hypo- and hyperthyroidism at excess)
Acne-like skin reactions (high doses)
Metallic taste
Sore teeth and gums
Burning mouth and throat
Contraindications
Hyperthyroidism (unless supervised)
Thyroid nodules (caution)
Dermatitis herpetiformis (worsens with iodine)
Overdose Information
UL 1,100 mcg/day; excess can cause thyroid dysfunction; narrow therapeutic window relative to some minerals; kelp supplements often exceed UL.
Thyroid dysfunction, metallic taste, sore mouth, acne-like rash, gastrointestinal distress; severe excess can cause thyroid storm.
Documented Overdose Symptoms:
Thyroid dysfunction
Metallic taste
Skin reactions
Gastrointestinal upset
Toxicity Thresholds: UL: 1,100 mcg/day. Toxicity occurs above UL, particularly in susceptible individuals. Some tolerate higher amounts.
Both deficiency and excess cause thyroid problems. Kelp supplements vary wildly in iodine content. Monitor thyroid function if supplementing.
Interactions
Drug Interactions:
Antithyroid medications (methimazole, PTU) - oppose iodine effects
Lithium - may affect thyroid function
ACE inhibitors and potassium-sparing diuretics - increase potassium iodide effects
Significant interactions with thyroid medications; caution with ACE inhibitors when using potassium iodide.
Other Supplement Interactions:
Selenium - synergistic for thyroid function
Soy and cruciferous vegetables - may interfere with thyroid function at low iodine status
Selenium synergistic; goitrogens in soy/crucifers may affect thyroid at low iodine status.
Kelp and seaweed supplements highly variable in iodine content - may provide excessive amounts. People with thyroid conditions should consult healthcare provider before supplementing. Ensure adequate but not excessive intake.
Forms and Bioavailability
Iodine supplements typically provide iodide or iodine/iodide combinations. Potassium iodide is the standard form.
Potassium Iodide
Standard supplemental form; nearly 100% absorbed; stable and well-characterized.
Excellent bioavailability; iodide form readily taken up by thyroid and other tissues.
Standard form in supplements and iodized salt. Preferred for general supplementation.
Iodine/Potassium Iodide (Lugol's)
Combination of elemental iodine and potassium iodide; used for specific therapeutic purposes.
Excellent bioavailability; provides both forms; used historically for various conditions.
Higher concentration - use with caution. Not typically needed for general supplementation. Historical use for various conditions.
Kelp/Seaweed Extracts
Natural source but highly variable iodine content; food matrix may affect absorption.
Variable content; difficult to dose accurately; natural food matrix with other minerals.
NOT recommended due to extreme variability. Some kelp supplements provide 1000+ mcg per serving. Avoid unless specifically analyzed for content.
Warnings & Suitability
Did You Know...?
Iodized salt was introduced in the US in 1924, reducing goiter rates by ~90% in the "goiter belt" states around the Great Lakes.
The thyroid gland is uniquely efficient at concentrating iodine - it can concentrate iodine up to 50 times higher than blood levels.
The word "goiter" comes from the Latin "guttur" meaning throat, referring to the characteristic neck swelling.
Iodine deficiency remains the leading cause of preventable intellectual disability worldwide, affecting an estimated 2 billion people.
General Scientific Sources
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Content Verification
Last Medical Review: 2/13/2026
Reviewed by: Editorial Team