For most adults, iodine becomes potentially harmful above 1,100 micrograms (mcg) per day, which is the tolerable upper intake level set by the Food and Nutrition Board. That’s roughly seven times the 150 mcg daily recommendation for adults. But the threshold drops significantly for children, and people with certain thyroid conditions can run into trouble at much lower amounts.
Upper Limits by Age
The tolerable upper intake level (UL) represents the highest daily intake unlikely to cause harm in the general population. These limits apply to iodine from both food and supplements combined:
- Ages 1–3: 200 mcg per day
- Ages 4–8: 300 mcg per day
- Ages 9–13: 600 mcg per day
- Ages 14–18: 900 mcg per day
- Adults 19 and older: 1,100 mcg per day
- Pregnant or breastfeeding (any age): 900–1,100 mcg per day, depending on age
No upper limit has been established for infants under 12 months because there isn’t enough data. For babies, formula and breast milk should be the only iodine sources.
These limits have a built-in safety margin, so briefly exceeding them on a single day is different from doing so consistently. It’s the pattern that matters. Long-term intakes above the UL increase the risk of thyroid dysfunction and other adverse effects.
What Happens When You Get Too Much
Your thyroid gland has a built-in safety mechanism. When it detects a sudden flood of iodine, it temporarily shuts down the process of incorporating iodine into thyroid hormones. This protective response prevents the gland from producing dangerously high levels of thyroid hormones all at once. In a healthy thyroid, this shutdown lasts only a few days before the gland adapts, reduces how much iodine it absorbs, and resumes normal hormone production.
The problem arises when this adaptation fails. In some people, the thyroid can’t escape the shutdown, and hormone production stays suppressed. The result is hypothyroidism: fatigue, weight gain, cold sensitivity, and sluggish metabolism. In others, particularly those with underlying thyroid nodules or autoimmune thyroid conditions, excess iodine can have the opposite effect, pushing the gland into overdrive and causing hyperthyroidism with symptoms like rapid heartbeat, weight loss, tremors, and anxiety.
Chronic Excess and Thyroid Disease
The relationship between iodine and thyroid health follows a U-shaped curve. Too little iodine causes problems, but so does too much. Both ends of the spectrum increase the prevalence of thyroid disorders.
Consistently high iodine intake is linked to higher rates of autoimmune thyroiditis, a condition where the immune system attacks the thyroid. Evidence from population studies supports this. When Poland introduced universal salt iodization in 1997, the rate of chronic thyroiditis diagnosed through biopsies rose from 1.5% in 1992 to 5.7% by 1999. Animal studies show a similar dose-dependent pattern: higher iodine intake leads to progressively more immune-related thyroid damage in genetically susceptible subjects.
Population-level data also ties excessive iodine to mortality risk. A study using U.S. national health data found that people with urinary iodine concentrations at or above 400 mcg per liter, a marker of excessive intake, had an increased risk of death from all causes compared to those in the normal range of 100–299 mcg/L.
Who Is Most Vulnerable
People with existing thyroid conditions face risks at iodine levels that would be perfectly safe for the general population. In one study, patients with Hashimoto’s thyroiditis who took just 250 mcg of supplemental iodine per day, well below the adult UL, developed thyroid dysfunction. Of 40 patients, seven developed subclinical hypothyroidism and one became fully hypothyroid within months. For someone without thyroid antibodies, 250 mcg would be unremarkable. For someone with autoimmune thyroid disease, it was enough to shift their hormone levels.
Other groups at elevated risk include the elderly, newborns, fetuses, and people with chronic kidney disease. Because the kidneys are the main route for clearing excess iodine, impaired kidney function means iodine builds up more easily. People with a history of thyroid surgery, latent Graves’ disease, or nontoxic goiter are also more susceptible to iodine-induced hyperthyroidism.
Seaweed: The Biggest Dietary Wildcard
Most people won’t approach the upper limit through a typical Western diet. Iodized salt, dairy, eggs, and seafood contribute modest, predictable amounts. Seaweed is the exception, and the variation between types is enormous.
Nori, the thin sheets used in sushi rolls, contains about 18 mcg of iodine per gram. A single portion delivers roughly 144 mcg, which is close to the daily recommendation and well within safe limits. Wakame, common in miso soup, jumps to about 172 mcg per gram, and a single portion can deliver around 1,376 mcg, already above the adult UL.
Kombu is in a different league entirely. This thick kelp used to make dashi broth averages 2,267 mcg of iodine per gram. One portion can deliver over 18,000 mcg, exceeding the adult upper limit by roughly 16 times. Testing of commercial kelp products found that a single serving of kombu, sugar kelp, or oarweed could exceed the UL by 30 to 104 times. If you regularly consume kelp supplements or cook with kombu, you may be taking in far more iodine than you realize.
Medical Sources of Excess Iodine
Some medications and procedures deliver iodine loads that dwarf anything in the diet. The heart rhythm drug amiodarone contains 75 mg of iodine per 200 mg tablet. That’s 75,000 mcg, nearly 70 times the daily upper limit, though only about 10% of the iodine in an oral dose actually enters the bloodstream. Contrast dyes used in CT scans also deliver large iodine loads in a single session. These medical exposures are the most common triggers of iodine-induced hyperthyroidism, particularly in patients with preexisting thyroid nodules or autoimmune thyroid disease.
Signs You May Be Getting Too Much
Chronic iodine excess often looks like a thyroid problem because it is one. Symptoms of iodine-induced hypothyroidism include fatigue, weight gain, constipation, dry skin, and feeling unusually cold. Iodine-induced hyperthyroidism presents differently: rapid or irregular heartbeat, unintentional weight loss, nervousness, heat intolerance, and difficulty sleeping.
Acute iodine poisoning from swallowing concentrated iodine solutions causes more immediate symptoms: burning in the mouth, nausea, vomiting, diarrhea, and fever. In severe cases, it can progress to delirium and shock. This type of poisoning is rare and typically involves accidental ingestion of antiseptic solutions rather than food or supplements.
If you suspect your iodine intake is excessive, a urinary iodine test can help clarify the picture. Levels in the normal range fall between 100 and 299 mcg/L. Readings of 300–399 mcg/L are considered high, and anything at or above 400 mcg/L indicates excessive intake.