Iodine

Vitamins & Minerals

What is Iodine?

Iodine is an essential trace element required almost exclusively for the synthesis of thyroid hormones—thyroxine (T4) contains four iodine atoms and triiodothyronine (T3) contains three. The thyroid gland actively concentrates iodine from the blood to levels 20–50 times higher than plasma, using the sodium-iodide symporter (NIS) on the basolateral membrane of thyroid follicular cells. Once inside the thyroid, iodine is oxidized by thyroid peroxidase (TPO), incorporated into tyrosine residues on thyroglobulin, and coupled to form T3 and T4.

Iodine is obtained primarily from iodized salt, seafood, dairy products, and bread made with iodate-containing dough conditioners. Iodine status is most reliably assessed at the population level using urinary iodine concentration (UIC), as approximately 90% of ingested iodine is excreted in urine. For individual assessment, urinary iodine-to-creatinine ratio and serum thyroglobulin levels are used. Iodine deficiency remains the leading preventable cause of intellectual disability worldwide, affecting approximately 2 billion people globally, particularly in landlocked and mountainous regions far from oceanic iodine sources.

Why It Matters

Iodine is indispensable for thyroid hormone production, and thyroid hormones regulate metabolic rate, growth, and development in virtually every organ system. Iodine deficiency during pregnancy and early childhood causes irreversible brain damage—the most severe form, cretinism, involves profound intellectual disability, deaf-mutism, and motor impairment. Even mild deficiency during pregnancy is associated with reduced IQ in offspring. In adults, iodine deficiency causes hypothyroidism and goiter (thyroid enlargement). Paradoxically, excess iodine can also cause thyroid dysfunction—both hypothyroidism (Wolff-Chaikoff effect) and hyperthyroidism (Jod-Basedow phenomenon)—particularly in individuals with preexisting thyroid disease.

Normal Reference Ranges

Group	Range	Unit
Adults (urinary iodine concentration)	100–299	µg/L
Pregnant Women (UIC)	150–249	µg/L
Adults (serum iodine)	40–92	µg/L

Reference ranges may vary by laboratory. Always compare results to the ranges provided by your testing facility.

What High I Levels Mean

Common Causes

Iodine-containing medications (amiodarone—contains 37% iodine by weight)
Iodinated contrast media from CT scans
Excessive kelp, seaweed, or iodine supplement consumption
Povidone-iodine (Betadine) application to large wounds
Iodine-containing expectorants

Possible Symptoms

Thyrotoxicosis/hyperthyroidism (Jod-Basedow phenomenon, especially in areas of prior deficiency)
Hypothyroidism (Wolff-Chaikoff effect, especially in those with Hashimoto thyroiditis)
Metallic taste in mouth
Excessive salivation
Acneiform skin eruptions (iododerma)
Gastrointestinal irritation

What to do: Identify and remove the source of excess iodine. Monitor thyroid function closely, as both hyperthyroidism and hypothyroidism can result from iodine excess. Patients with underlying autoimmune thyroid disease (Hashimoto or Graves) are particularly susceptible to iodine-induced thyroid dysfunction. For amiodarone-induced thyrotoxicosis, endocrinology consultation is essential as treatment depends on the type (type 1 vs type 2). The tolerable upper limit for iodine is 1,100 µg/day for adults.

What Low I Levels Mean

Common Causes

Inadequate dietary intake (no iodized salt, low seafood/dairy)
Living in iodine-depleted geographic areas (mountainous regions, flood plains)
Pregnancy and lactation (increased requirements)
Selenium deficiency (impairs iodine metabolism)
Goitrogenic foods consumed in excess without adequate iodine (cassava, millet)
Vegan diets without iodine supplementation

Possible Symptoms

Goiter (enlarged thyroid gland)
Hypothyroidism (fatigue, weight gain, cold intolerance, constipation)
Cognitive impairment and reduced IQ
Cretinism in severe prenatal deficiency (intellectual disability, growth failure)
Increased susceptibility to radioactive iodine uptake after nuclear events
Pregnancy complications: miscarriage, stillbirth, preterm delivery

What to do: Mild to moderate deficiency is addressed by using iodized salt and increasing dietary intake of seafood, dairy, and eggs. The WHO recommends 150 µg/day for adults and 250 µg/day during pregnancy and lactation. Potassium iodide supplements are available and widely used in pregnancy. In areas of endemic deficiency, iodized salt programs and iodized oil supplementation are public health interventions. Do not rapidly correct severe, long-standing deficiency with large iodine doses, as this can precipitate thyrotoxicosis in individuals with autonomous thyroid nodules.

When Is I Testing Recommended?

When goiter is present without known cause
In pregnant women in regions with possible iodine deficiency
When evaluating thyroid dysfunction, especially in areas with variable iodine intake
Before and after administration of iodine-containing contrast or medications
In patients on amiodarone therapy
In population-level epidemiological surveys of iodine sufficiency

Frequently Asked Questions

The recommended daily intake is 150 µg for adults, 220 µg during pregnancy, and 290 µg during lactation. A quarter teaspoon of iodized salt contains approximately 95 µg of iodine. Most people in countries with salt iodization programs get sufficient iodine. However, sea salt, kosher salt, and Himalayan pink salt are typically NOT iodized unless specifically labeled as such. Vegans who avoid dairy and do not use iodized salt are at particular risk for deficiency and should consider supplementation.

Yes, some seaweeds—particularly kelp (kombu)—contain extremely high amounts of iodine, sometimes exceeding 2,000–3,000 µg per gram of dried seaweed, far above the tolerable upper limit of 1,100 µg/day. Regular consumption of large amounts of kelp has caused thyroid dysfunction, including both hypothyroidism and hyperthyroidism. Japanese populations who consume seaweed regularly have adapted somewhat, but thyroid abnormalities from iodine excess are still documented. Nori and wakame contain lower iodine levels and are generally safer. Moderation and monitoring thyroid function are advisable.

Thyroid hormones are essential for fetal brain development, particularly during the first trimester when the fetus depends entirely on maternal thyroid hormones. Iodine requirements increase by approximately 50% during pregnancy due to increased maternal thyroid hormone production, increased renal iodine clearance, and transfer of iodine to the fetus. Even mild iodine deficiency during pregnancy has been associated with lower IQ scores (average 7–10 point reduction) in offspring. The WHO recommends 250 µg/day during pregnancy. The American Thyroid Association recommends that pregnant and breastfeeding women take prenatal vitamins containing 150 µg of iodine daily.

Related Biomarkers

TSHThyroid-Stimulating Hormone Free T4Free Thyroxine Free T3Free Triiodothyronine TgThyroglobulin SeSelenium

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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult your healthcare provider for interpretation of your specific test results.