Iodine

What Is Iodine?

Iodine is a trace mineral that is uniquely essential for the synthesis of thyroid hormones. The adult body contains approximately 15-20 mg of iodine, with 70-80 percent concentrated in the thyroid gland. Iodine is found naturally in the ocean and in soil near coasts; inland and mountainous regions often have iodine-poor soils, making iodine deficiency a significant global public health concern — it affects approximately 2 billion people worldwide.

Key Functions

• Thyroid hormone synthesis: Thyroxine (T4) contains four iodine atoms and triiodothyronine (T3) contains three. The thyroid gland actively concentrates iodide from the blood to produce these hormones.
• Metabolic regulation: Thyroid hormones regulate basal metabolic rate, thermogenesis, and the metabolism of carbohydrates, proteins, and lipids in virtually every cell.
• Growth and development: Adequate iodine is critical during fetal development and infancy for normal brain development and growth. Thyroid hormones are essential for myelination of the nervous system.
• Cardiovascular function: Thyroid hormones influence heart rate, cardiac contractility, and peripheral vascular resistance.

Dietary Reference Values and Food Sources

The RDA for iodine is 150 mcg/day for adults, 220 mcg/day during pregnancy, and 290 mcg/day while breastfeeding. The UL is 1,100 mcg/day. Iodized salt (100-130 mcg per quarter teaspoon in the US) has been the most successful public health intervention against iodine deficiency since its introduction in the 1920s. Other food sources include:

• Seaweed (nori, kelp): variable, from 16 mcg to over 2,000 mcg per serving — kelp can far exceed safe limits
• Cod (3 oz): ~99 mcg
• Plain low-fat yogurt (1 cup): ~75 mcg
• Milk (1 cup): ~56 mcg
• Shrimp (3 oz): ~35 mcg
• Egg (1 large): ~24 mcg

Deficiency and Toxicity

Iodine deficiency causes the thyroid gland to enlarge — a condition called goiter — as it attempts to trap more iodide from the blood. Severe deficiency during pregnancy causes cretinism in offspring, characterized by intellectual disability, deafness, and stunted growth. Mild-to-moderate deficiency impairs cognitive development and thyroid function. Paradoxically, excess iodine can also impair thyroid function. Very high iodine intakes (from supplements or excessive seaweed consumption) can trigger or worsen autoimmune thyroid disease (Hashimoto's or Graves' disease) in susceptible individuals.