How much is too much iodine per day? A guide to safe intake

December 20, 2025 •

3 min read

The Tolerable Upper Intake Level (UL) for adults aged 19 and over is 1,100 micrograms (mcg) per day, a threshold established to prevent adverse effects. Understanding how much is too much iodine per day is critical for maintaining proper thyroid function and overall health.

Quick Summary

For adults, exceeding 1,100 mcg of iodine daily can lead to thyroid problems like goiter or dysfunction. While excess from food is rare, high-dose supplements are a common cause of toxicity.

Key Points

Adult UL: The Tolerable Upper Intake Level for adults is 1,100 mcg per day.
Risk Factors: Excess iodine can cause both hypothyroidism and hyperthyroidism, especially in individuals with pre-existing thyroid conditions.
Supplement Danger: Dietary supplements, particularly kelp, are the most common cause of iodine excess, as they can contain significantly more than the UL.
Wolff-Chaikoff Effect: High iodine levels can temporarily inhibit thyroid hormone synthesis, a mechanism known as the Wolff-Chaikoff effect.
Goiter Risk: One of the signs of excessive iodine is an enlarged thyroid gland, or goiter.
Medical Sources: High iodine intake can also result from medical sources like contrast dyes and certain heart medications.

The Crucial Role of Iodine

Iodine is an essential mineral that the body cannot produce on its own. Its sole known physiological role is to act as a building block for the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). These hormones are vital for regulating metabolism, growth, and neurological development, especially during fetal and early childhood stages. Inadequate intake leads to insufficient hormone production, causing deficiency issues like goiter and hypothyroidism. However, the opposite is also true: excessive intake can disrupt thyroid function and lead to adverse health outcomes.

Recommended Intake vs. Upper Limit

To manage iodine intake, health organizations define two key levels: the Recommended Dietary Allowance (RDA) and the Tolerable Upper Intake Level (UL).

Recommended Dietary Allowance (RDA)

The RDA for non-pregnant, non-lactating adults is 150 mcg per day.
This level is sufficient to meet the needs of nearly all healthy individuals.

Tolerable Upper Intake Level (UL)

The UL is the maximum daily intake that is unlikely to cause adverse health effects in the general population.
For adults aged 19 and over, the UL is 1,100 mcg (1.1 mg) per day.
Consistent intake above this level increases the risk of thyroid disorders.

Who Needs to Be Careful with Iodine Intake?

Individuals with Pre-existing Thyroid Conditions: People with autoimmune thyroid diseases (like Hashimoto's or Graves' disease) or a history of iodine deficiency may be more sensitive to excess iodine.
Infants and Children: Younger individuals have lower ULs. Preterm infants are particularly vulnerable to iodine overload.
Pregnant and Lactating Women: While their iodine needs are higher, they should still be mindful of the UL to avoid potential harm to themselves and their infant.
High-Dose Supplement Users: Many supplements, especially kelp-based ones, can contain significantly more iodine than the UL.

Health Risks of Excessive Iodine

Excessive iodine intake can interfere with thyroid function in several ways, primarily by triggering the Wolff-Chaikoff effect, a temporary inhibition of thyroid hormone synthesis. If the thyroid fails to 'escape' this effect, it can lead to chronic dysfunction.

Here are some of the health risks associated with too much iodine:

Thyroid Dysfunction: Excess iodine can cause both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid), sometimes causing symptoms similar to deficiency.
Goiter: The thyroid gland may enlarge as it attempts to compensate for the hormonal imbalance.
Autoimmune Thyroiditis: High iodine intake has been linked to the development and progression of autoimmune thyroid diseases.
Papillary Thyroid Cancer: Chronic exposure to high iodine levels has been associated with an increased risk of this type of thyroid cancer.
Gastrointestinal Issues: Mild symptoms can include nausea, vomiting, and diarrhea.
Other Symptoms: Chronic high intake can lead to a metallic taste in the mouth, soreness of the teeth and gums, and skin issues like acneiform lesions.
Acute Poisoning: Ingestion of many grams of iodine (far exceeding typical exposure) can be toxic, causing severe symptoms like fever, coma, and stomach burns.

Sources of Excessive Iodine Intake

While the American diet generally provides adequate iodine, high intake typically comes from a few specific sources:

Dietary Supplements: This is the most common cause of iodine excess. Many supplements, particularly kelp, can contain hundreds or thousands of micrograms per pill.
Seaweed Consumption: People who regularly consume large quantities of seaweed, common in some traditional diets like in Japan, can have very high iodine intake.
Medications and Contrast Dyes: Certain medications (like amiodarone for heart rhythm) and iodinated contrast media used for CT scans contain very high levels of iodine, requiring medical supervision.
Over-iodized Salt: While regulated in many areas, over-iodization can contribute to high population-level intake.

Comparison of Iodine Intake Levels


Life Stage	Recommended Dietary Allowance (RDA) per day	Tolerable Upper Intake Level (UL) per day
Adults (19+ years)	150 mcg	1,100 mcg
Pregnant women	220 mcg	1,100 mcg (19+ years)
Lactating women	290 mcg	1,100 mcg (19+ years)
Adolescents (14-18 years)	150 mcg	900 mcg
Children (9-13 years)	120 mcg	600 mcg

Conclusion

Iodine is a double-edged sword: vital for health at recommended levels but harmful in excess. For most people, a balanced diet including iodized salt provides sufficient iodine without risk of toxicity. The primary concern for excess intake comes from dietary supplements, which often contain amounts far exceeding the Tolerable Upper Intake Level, or from certain medical treatments. It is crucial to read labels, understand your specific needs based on life stage, and consult a healthcare provider before taking any iodine supplements, especially if you have an underlying thyroid condition. For more in-depth information on dietary intake levels, consult the NIH Office of Dietary Supplements' Iodine fact sheet(https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/).

Frequently Asked Questions

The Tolerable Upper Intake Level (UL) for adults aged 19 and over is 1,100 micrograms (mcg) per day.

It is very difficult to get iodine poisoning from food alone. However, consuming large quantities of very iodine-rich foods like certain seaweeds can contribute to excessive intake over time.

Symptoms of chronic excess iodine can include a metallic taste in the mouth, increased salivation, GI upset, and skin lesions. Very high doses can lead to fever, abdominal pain, and weak pulse.

While many multivitamins contain iodine, you should check the label to ensure the dose does not exceed your UL. High-dose iodine supplements, especially kelp, are a greater risk factor for excessive intake.

Individuals with pre-existing thyroid conditions, autoimmune thyroid disease, or a history of severe iodine deficiency are more susceptible to adverse effects from high iodine intake.

The Wolff-Chaikoff effect is a temporary inhibition of thyroid hormone synthesis that occurs in response to high concentrations of inorganic iodide within thyroid cells.

Exceeding the UL for iodine during pregnancy is potentially unsafe and should be avoided. The UL for pregnant women aged 19+ is 1,100 mcg per day. Pregnant women should consult their doctor before taking iodine supplements.

Yes, both too little and too much iodine can cause the thyroid gland to enlarge, a condition known as a goiter.