What is a toxic level of iodine?

November 16, 2025 •

3 min read

For adults, the Tolerable Upper Intake Level (UL) for iodine is 1,100 micrograms (mcg) per day, while children and adolescents have lower limits. A toxic level of iodine is defined by exceeding these established upper limits, which can lead to various adverse health effects, particularly affecting thyroid function.

Quick Summary

The Tolerable Upper Intake Level (UL) for iodine is set by health authorities to prevent adverse effects from excess intake. Exceeding this limit, particularly through supplements or certain foods, can lead to thyroid dysfunction, including hypothyroidism and hyperthyroidism. Severe acute overdose can cause burning sensations, fever, and digestive upset.

Key Points

Adult Upper Limit: For most healthy adults, the Tolerable Upper Intake Level (UL) for iodine is 1,100 micrograms (mcg) per day.
Primary Risk: The main health risk from excessive iodine intake is disruption of normal thyroid function, potentially causing either hypothyroidism or hyperthyroidism.
Vulnerable Groups: Individuals with pre-existing thyroid conditions, pregnant women, and the elderly are more susceptible to adverse effects from excess iodine.
Supplement Caution: Many iodine or kelp supplements contain doses far higher than the UL, making them the most common cause of excess intake.
Acute Overdose: Rare, severe acute poisoning can occur from ingesting many grams of iodine, leading to symptoms like burning of the mouth and stomach, fever, and coma.
Treatment Focus: Management for iodine toxicity involves immediately ceasing the source of excess iodine and providing supportive medical care to manage symptoms and thyroid function.
Medication Awareness: Medications like amiodarone and iodinated contrast dyes can contain significant amounts of iodine and should be monitored under a doctor's care.

Understanding Iodine Intake and Toxicity

Iodine is a crucial mineral for the body, playing a key role in the production of thyroid hormones, which regulate growth and metabolism. However, too much iodine can be harmful, and understanding the thresholds for toxicity is vital for maintaining health. While it is difficult to reach toxic levels through a normal diet alone, excessive intake from supplements, medications, or high-iodine foods is a concern.

What is a Tolerable Upper Intake Level (UL)?

The Tolerable Upper Intake Level (UL) is the maximum daily amount of a nutrient that is likely to pose no risk of adverse health effects to most healthy people. For iodine, the UL for adults (19+ years) is 1,100 mcg per day. This level is a crucial benchmark for preventing toxicity, but it does not apply to individuals under medical supervision who may be prescribed higher doses for treatment.

Vulnerable Populations and Increased Sensitivity

Certain groups are more susceptible to the adverse effects of iodine excess.

Children and Adolescents: The UL varies by age, with lower limits for younger individuals.
Pregnant and Lactating Women: Despite increased needs, they should be cautious with high intake. The American Thyroid Association suggests avoiding supplements exceeding 500 mcg daily.
Individuals with Thyroid Disease: Conditions like Hashimoto's or Graves' disease make people more sensitive to high iodine intake, potentially worsening their condition.

Chronic and Acute Toxicity

Chronic Toxicity

Chronic toxicity results from consistent intake above the UL, primarily affecting the thyroid. High iodine levels can trigger the Wolff-Chaikoff effect, temporarily reducing thyroid hormone production. This can lead to prolonged hypothyroidism in sensitive individuals. Symptoms of chronic exposure can be subtle:

Goiter
Hypothyroidism or hyperthyroidism
Acne-like skin lesions
Thyroiditis

Acute Poisoning

Acute iodine poisoning is rare and typically occurs from ingesting a massive, single dose (grams, not micrograms), often from non-food sources like tinctures. The estimated mean lethal dose of free iodine is around 2–4 grams. Symptoms are severe and require immediate medical attention:

Burning in the mouth, throat, and stomach
Fever and abdominal pain
Nausea, vomiting, and diarrhea
Weak or rapid pulse
Coma in severe cases

Sources of Excess Iodine Exposure

Dietary Supplements: Many supplements, especially kelp-based ones, can provide iodine amounts far exceeding the UL.
Iodized Salt: While beneficial, excessive consumption can contribute to high intake, particularly in areas with mandatory iodization.
Medications: Certain drugs, like amiodarone, contain significant amounts of iodine.
Contrast Dyes: Used in medical imaging, these contain substantial iodine.
Environmental Factors: High natural levels of iodine in water or soil in some regions can lead to elevated intake.

Comparison of Iodine Deficiency vs. Toxicity


Feature	Iodine Deficiency	Iodine Toxicity
Cause	Insufficient intake	Consistent intake above UL, supplements, medications
Mechanism	Inadequate thyroid hormone synthesis	Disruption of normal thyroid function
Common Signs	Goiter, fatigue, weight gain	Goiter, hypothyroidism, hyperthyroidism, acneiform rash
Severe Consequences	Cretinism, intellectual impairment	Life-threatening acute poisoning (rare), potential for long-term thyroid dysfunction
Treatment	Increased dietary iodine, supplements	Reduce iodine intake, supportive care, management of thyroid abnormalities

Treatment for Iodine Toxicity

Treating iodine toxicity focuses on reducing intake and providing supportive care; there is no specific antidote. Acute overdose requires immediate medical intervention.

Reduce Iodine Source: Identify and eliminate the source of excess iodine.
Supportive Care: May include activated charcoal, monitoring vital signs, and managing symptoms.
Thyroid Management: Address any resulting thyroid issues; function often normalizes after reducing intake, but some may need ongoing medication.

Conclusion

While essential, iodine must be consumed in balance. For most healthy adults, a toxic level of iodine is above the UL of 1,100 mcg daily. Chronic excess can cause thyroid problems, while acute, massive doses are life-threatening. Prevent toxicity by monitoring supplement use and being mindful of iodine in medications and high-iodine foods. Suspected toxicity requires immediate medical attention. For more information, consult the National Institutes of Health's Office of Dietary Supplements fact sheet on iodine.

Disclaimer: This article is for informational purposes only and should not replace professional medical advice. Always consult a healthcare provider before starting or stopping supplements or medications.

Frequently Asked Questions

Consistently taking more than 1,100 mcg of iodine per day can cause thyroid dysfunction, such as goiter, hypothyroidism, or hyperthyroidism, and other side effects like a metallic taste in the mouth or digestive issues.

It is extremely difficult to get iodine poisoning from food alone. Most cases of toxicity result from overconsumption of dietary supplements, medications, or high-iodine compounds.

Mild symptoms can include nausea, vomiting, a burning sensation in the mouth and throat, abdominal pain, and diarrhea. In more severe cases, symptoms can escalate to include fever, weak pulse, and seizures.

Individuals with pre-existing thyroid disorders, such as Hashimoto's or Graves' disease, and those with a history of iodine deficiency are more sensitive. Infants, the elderly, and pregnant women also need to be careful with their intake.

If you suspect iodine poisoning, especially from an acute overdose, seek immediate medical help or contact a poison control center. Do not induce vomiting unless advised by a healthcare professional.

Diagnosis is based on a patient's symptoms and history of intake. Blood tests are used to measure thyroid hormone levels and thyroid-stimulating hormone (TSH). Urine tests can also check for high iodine levels.

While the effects of excess iodine often resolve once intake is reduced, chronic overconsumption can lead to long-term thyroid problems. Some individuals may require ongoing medication to manage their thyroid function.