Understanding the Tolerable Upper Intake Level (UL)
For most healthy adults, the Tolerable Upper Intake Level (UL) for iodine is 1,100 mcg per day. The UL is the maximum daily intake unlikely to cause adverse health effects for the general population over time. While the risk of exceeding this amount from food alone is low, it becomes a concern for those using dietary supplements containing iodine, especially kelp supplements, which can have highly variable concentrations. Doses several times higher than the UL, typically in the multi-milligram range, are associated with acute poisoning.
How Excess Iodine Affects the Thyroid
Excessive iodine primarily impacts the thyroid gland, which uses iodine to produce thyroid hormones. When intake becomes too high, the thyroid's normal regulation can be overwhelmed, leading to several potential conditions:
- Goiter: The thyroid gland may become enlarged, a condition known as goiter. Ironically, this can also be a symptom of iodine deficiency, but in cases of excess, it often results from the thyroid attempting to suppress hormone production to cope with the high iodine load.
- Hyperthyroidism: In susceptible individuals, high iodine intake can trigger an overproduction of thyroid hormones, leading to hyperthyroidism. This is particularly a risk for people with a history of iodine deficiency or pre-existing thyroid nodules.
- Hypothyroidism: Paradoxically, too much iodine can cause a temporary, and sometimes chronic, suppression of thyroid hormone production, resulting in hypothyroidism. This is known as the Wolff-Chaikoff effect, where the body temporarily shuts down thyroid hormone synthesis to protect against an overabundance of iodine.
- Autoimmune Thyroid Conditions: Some studies suggest that high iodine intake can increase the risk of developing or worsening autoimmune thyroid diseases, such as Hashimoto's thyroiditis and Graves' disease.
Symptoms and Risks of Excess Iodine
Symptoms of iodine excess can vary significantly depending on the amount and duration of intake. Acute, very high doses (several grams) can be severely toxic, while chronic, moderately high intake causes more gradual issues.
Mild to Moderate Symptoms
- Metallic taste in the mouth
- Nausea and vomiting
- Diarrhea
- Stomach pain
- Burning sensation in the mouth and throat
- Skin rashes
Severe or Chronic Symptoms
- Goiter (enlarged thyroid)
- Thyroid gland inflammation
- Symptoms of hyperthyroidism, like a fast heartbeat and muscle weakness
- Symptoms of hypothyroidism, such as fatigue, dry skin, and weight gain
- Weak pulse
- Coma (in very rare, acute poisoning cases)
Who is at a Higher Risk?
Certain groups are more susceptible to the harmful effects of excess iodine and should exercise greater caution with their intake:
- Infants and Children: Have lower ULs and are more vulnerable to adverse effects. For instance, the UL for children aged 1–3 is 200 mcg per day.
- Pregnant and Lactating Women: Require higher intake but are also sensitive to excess, with a UL of 1,100 mcg per day (for those over 18). High intake can impact the developing fetus.
- Individuals with Autoimmune Thyroid Disease: People with conditions like Hashimoto's or Graves' disease may be particularly sensitive to high iodine levels.
- People with Pre-existing Thyroid Nodules or History of Deficiency: May develop iodine-induced hyperthyroidism more easily.
High Iodine Sources: Dietary vs. Supplements
While it is difficult to get excessive iodine from a typical diet, certain foods and supplements can pose a risk. Supplements are the most common cause of excess intake.
| Source Category | Examples | Potential for Excess Intake |
|---|---|---|
| Dietary Sources | Seaweed (Kelp), Cod, Dairy Products (Milk, Yogurt), Iodized Salt | Variable and typically low, but high consumption of seaweed can cause excess. |
| Dietary Supplements | Kelp supplements, Iodine-specific supplements, Multivitamins | High risk, as dosage can vary widely and exceed daily ULs significantly. |
| Medications | Amiodarone (heart medication), Iodinated Contrast Dyes | Very high risk, as some contain large amounts of iodine. Requires medical supervision. |
A Word on Potassium Iodide
In a radiation emergency involving radioactive iodine release, potassium iodide (KI) can be administered under medical guidance to protect the thyroid. This is a very specific, high-dose medical application and is not a substitute for dietary supplementation. The doses used are far higher than the UL and are only for use under expert care.
Conclusion
While iodine is an essential mineral for proper thyroid function, especially in the production of crucial hormones, it is not without risk when consumed in excessive amounts. For the general adult population, regularly exceeding the Tolerable Upper Intake Level of 1,100 mcg per day can lead to adverse health effects, primarily affecting the thyroid gland. Symptoms can range from mild gastrointestinal upset to serious conditions like hyperthyroidism or goiter, and individuals with pre-existing thyroid disorders or pregnant women are at heightened risk. Excess intake is most often caused by the unregulated use of dietary supplements, particularly kelp products. Given the narrow window between adequate and excessive intake for some individuals, it is crucial to monitor consumption from all sources. For those considering supplementation, especially at high doses, consultation with a healthcare professional is strongly recommended to ensure safety.
Visit the NIH Office of Dietary Supplements for more information on iodine
