How much iodine per day and the thyroid upper limit

Most healthy adults need 150 mcg of iodine per day, according to the NIH Office of Dietary Supplements. Staying well below the established upper limit of 1,100 mcg per day is important for keeping the thyroid working properly.

What iodine does in the body

Iodine is an essential mineral, meaning the body cannot make it and must obtain it from food or supplements. Its primary role is serving as a building block of two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Without enough iodine, the thyroid gland cannot produce adequate amounts of these hormones.

Those hormones, in turn, regulate a wide range of processes throughout the body, including metabolic activity, protein synthesis, and enzymatic activity. Iodine is also required for proper skeletal and central nervous system development in fetuses and infants, which is why adequate intake during pregnancy is a particular focus of public health guidance.

Daily targets by life stage

The following values come from the NIH Office of Dietary Supplements and reflect Dietary Reference Intake recommendations. These are population-level reference values. Please consult a clinician or registered dietitian for guidance on your personal needs.

• Adults (men and women, 19 and older): 150 mcg per day
• Pregnant people: 220 mcg per day
• Breastfeeding people: 290 mcg per day
• Children ages 1 to 8: 90 mcg per day
• Children ages 9 to 13: 120 mcg per day
• Adolescents ages 14 to 18: 150 mcg per day

The upper limit: 1,100 mcg per day for adults

The NIH Office of Dietary Supplements sets the Tolerable Upper Intake Level (UL) for iodine at 1,100 mcg per day for adults. This is the highest daily intake considered unlikely to cause harm for most healthy people. Separate, lower upper limits apply to children and adolescents.

For most people, iodine intakes from foods and supplements are unlikely to reach the adult UL, and intakes below it are generally considered safe. However, long-term intakes above the UL increase the risk of adverse thyroid effects. These can include goiter, elevated thyroid-stimulating hormone (TSH), hypothyroidism, and iodine-induced hyperthyroidism.

People with autoimmune thyroid conditions, such as Hashimoto's thyroiditis or Graves' disease, may be more sensitive to high iodine intakes and can experience adverse effects even at levels that would not affect most people. If you have a thyroid condition, speak with your doctor before changing your iodine intake.

What the research says: an honest look at the evidence

Not all health claims about iodine carry the same weight of evidence. The NIH Office of Dietary Supplements grades the research, and CareDoctor reflects those grades honestly below.

Strong evidence

• Iodine is an essential component of the thyroid hormones T4 and T3 and is required for normal thyroid function and metabolism. This is well established.
• Adequate iodine is required for normal fetal and infant brain and neurological development. Severe deficiency causes irreversible harm, including a condition called cretinism. This evidence base is strong.
• Potassium iodide reduces the risk of radiation-induced thyroid cancer during nuclear emergencies. This is a specific, evidence-backed use authorized by public health agencies.

Limited evidence

• Correcting mild iodine deficiency in children may improve cognitive function. The research is promising but not yet conclusive.
• Iodine supplementation may help with fibrocystic breast disease symptoms. Current evidence is limited and does not support a firm conclusion.

Insufficient evidence

• Iodine supplementation during pregnancy in areas of only mild-to-moderate deficiency improving infant and child neurodevelopment is plausible, but the evidence is currently insufficient to draw firm conclusions.

Supplements are not intended to diagnose, treat, cure or prevent any disease, and these statements have not been evaluated by the FDA.

Drug interactions to know

Iodine supplements, particularly potassium iodide, can interact with certain medications. Always talk to a doctor or pharmacist before combining any supplement with prescription or over-the-counter drugs. Key interactions identified by the NIH Office of Dietary Supplements include the following.

• Antithyroid medications (for example methimazole, brand name Tapazole): These are used to treat hyperthyroidism. Taking high doses of iodine alongside them can have an additive effect and may cause hypothyroidism.
• Potassium-sparing diuretics (for example spironolactone, brand name Aldactone; amiloride, brand name Midamor): Taking potassium iodide together with these drugs can increase the risk of hyperkalemia, which is abnormally high blood potassium.
• ACE inhibitors (for example benazepril, brand name Lotensin; lisinopril, brand names Prinivil and Zestril; fosinopril, brand name Monopril): Taking potassium iodide with ACE inhibitors can also increase the risk of hyperkalemia.

Common food sources of iodine

Iodine occurs naturally in some foods and is added to others. Seaweed and seafood tend to be among the richest natural sources, though the iodine content of seaweed varies widely and can be very high in certain varieties. Dairy products, eggs, and iodized salt are reliable everyday sources for many people. The iodine content of plant foods depends heavily on the iodine levels in the soil where they were grown.

People who avoid iodized salt, dairy, and seafood, such as those following a vegan diet, may have lower iodine intakes and could benefit from discussing their status with a clinician.

Key takeaways

• The adult daily target for iodine is 150 mcg, according to the NIH Office of Dietary Supplements.
• The adult upper limit is 1,100 mcg per day. Long-term intakes above this level raise the risk of thyroid problems.
• People with thyroid conditions or who take thyroid-related medications face additional considerations and should speak with a doctor.
• Evidence strongly supports iodine's role in thyroid hormone production and fetal brain development. Evidence for other uses ranges from limited to insufficient.
• For personal dosing guidance, consult a clinician or registered dietitian rather than relying on population reference values alone.

Sources: NIH Office of Dietary Supplements, Iodine Fact Sheet for Health Professionals (https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/); Iodine Fact Sheet for Consumers (https://ods.od.nih.gov/factsheets/Iodine-Consumer/); Nutrient Recommendations (https://ods.od.nih.gov/HealthInformation/nutrientrecommendations.aspx).