Most adults need 150 micrograms (mcg) of iodine per day to keep their thyroid functioning properly. That number rises to 250 mcg during pregnancy and lactation. Children need less, ranging from 90 mcg for toddlers up to 120 mcg for adolescents. These are the amounts recommended by the WHO and the U.S. Institute of Medicine, and they reflect the minimum your thyroid needs to produce its hormones reliably.
Why Your Thyroid Needs Iodine
Your thyroid gland absorbs iodine from your bloodstream and uses it as a raw building block for its two main hormones: T4 and T3. These names actually describe their iodine content. T4 contains four iodine atoms per molecule, and T3 contains three. Without enough iodine coming in, the gland simply cannot manufacture adequate hormone, and everything downstream suffers. Thyroid hormones regulate your metabolism, body temperature, heart rate, and brain function, so the effects of running low touch nearly every system.
Any iodine your thyroid doesn’t capture gets filtered out through your kidneys. That’s why doctors use urine iodine concentration to gauge whether a population is getting enough. A median reading between 100 and 199 mcg/L indicates adequate intake, while anything below 100 signals some degree of deficiency.
Daily Requirements by Age and Life Stage
- Children ages 1 to 8: 90 mcg/day
- Children ages 9 to 13: 120 mcg/day
- Adults (14+): 150 mcg/day
- Pregnant women: 250 mcg/day
- Breastfeeding women: 250 mcg/day
During pregnancy, your body produces significantly more thyroid hormone to support fetal brain and nervous system development. Your kidneys also flush out iodine faster, and the fetus depends entirely on your iodine supply. Falling short during this window is linked to congenital abnormalities, lower IQ in children, and in severe cases, a condition called cretinism involving intellectual disability and motor impairments. The WHO recommends supplementation for pregnant women in regions where iodized salt isn’t widely available.
Best Food Sources of Iodine
A quarter teaspoon of iodized table salt contains about 78 mcg of iodine, roughly half the daily adult requirement. That single fact explains why iodine deficiency became rare in countries that adopted salt iodization programs. But if you’ve cut back on salt, or if you mostly use sea salt or kosher salt (which are typically not iodized), you may be getting less than you think.
Some of the richest food sources, per serving:
- Cod (baked, 3 oz): 146 mcg
- Dried nori seaweed (2 tbsp, flaked): 116 mcg
- Greek yogurt (plain, nonfat, ¾ cup): 87 mcg
- Nonfat milk (1 cup): 84 mcg
- Iodized salt (¼ tsp): 78 mcg
- Fish sticks (cooked, 3 oz): 57 mcg
Dairy is a surprisingly reliable source because iodine-based sanitizers are used on dairy equipment, and cattle feed is often supplemented with iodine. Fish and shellfish concentrate iodine from seawater naturally. Seaweed is the most variable: nori (a red seaweed) contains moderate amounts, while brown kelp species like sugar kelp can contain over 6,000 mcg per gram, which is enough to push you well past safe limits from a single serving.
Supplements: Potassium Iodide vs. Kelp
If you’re considering a supplement, the two main options are potassium iodide tablets and kelp-based capsules. They deliver iodine differently. In a controlled trial comparing a potassium iodide tablet (225 mcg) with a seaweed-based meal containing a similar amount of iodine (231 mcg), the supplement showed 97% bioavailability compared to 75% from the seaweed. Potassium iodide also produces a faster spike in blood levels, peaking within the first two hours, while seaweed-sourced iodine absorbs more gradually over two to five hours.
The lower absorption from seaweed happens because iodine gets trapped within the plant’s cell walls and bound to organic compounds like alginate, which can form insoluble gels that pass through your gut unabsorbed. This doesn’t make seaweed a bad source, but it does mean the amount listed on a kelp supplement label may overstate what your body actually takes in. The bigger concern with kelp supplements is inconsistency. Iodine content varies enormously between species and even between batches, making it harder to control your dose precisely.
The Upper Limit: How Much Is Too Much
The tolerable upper intake level for adults is 1,100 mcg per day. Going above this threshold regularly increases the risk of thyroid dysfunction. Your thyroid has a built-in safety mechanism: when iodine levels surge, it temporarily shuts down hormone production to prevent overload. This response normally resolves on its own as the gland adjusts. But in some people, particularly those with pre-existing thyroid nodules or autoimmune thyroid conditions, this regulation fails. The result can be either iodine-induced hypothyroidism (too little hormone) or hyperthyroidism (too much).
Excessive iodine also makes the thyroid’s key protein, thyroglobulin, more visible to the immune system. This can trigger or worsen autoimmune thyroid disease, including Hashimoto’s thyroiditis. The populations most vulnerable to excess iodine include the elderly, pregnant women, newborns, and anyone with a history of goiter or prior iodine deficiency. If you’ve been deficient for a long time, suddenly flooding your system with iodine can paradoxically cause a temporary hyperthyroid state.
Signs of Deficiency and Excess
Iodine deficiency typically shows up as goiter first: a visible swelling at the front of the neck where the thyroid enlarges in an attempt to trap more iodine from the blood. As deficiency worsens, hypothyroidism develops, bringing fatigue, weight gain, cold sensitivity, dry skin, and sluggish thinking. In severe cases during pregnancy or early childhood, the consequences extend to permanent intellectual disability, hearing loss, and speech deficits.
Excess iodine can look confusingly similar. It may cause either hypothyroidism or hyperthyroidism depending on your individual thyroid health. Hyperthyroid symptoms include rapid heartbeat, anxiety, weight loss, and tremors. Some people develop thyroid inflammation (thyroiditis) from chronic overexposure. The overlap in symptoms between deficiency and excess is one reason that randomly taking high-dose iodine supplements “for thyroid health” without knowing your actual iodine status can backfire.
Practical Takeaways for Daily Intake
For most adults eating a varied diet that includes dairy, seafood, or iodized salt, hitting the 150 mcg daily target happens without much effort. A cup of milk plus a quarter teaspoon of iodized salt already puts you at about 160 mcg. If you follow a vegan diet, avoid dairy, or use non-iodized salt exclusively, you’re at higher risk of falling short and may benefit from a supplement providing 150 mcg of potassium iodide.
Pregnant and breastfeeding women should aim for 250 mcg daily. Many prenatal vitamins contain 150 mcg of iodine, which combined with dietary sources typically covers the increased need. If your prenatal doesn’t include iodine, check the label, as not all do.
If you have a thyroid condition, particularly Hashimoto’s or Graves’ disease, your iodine needs may differ from the general population. High-dose iodine supplements (especially kelp products delivering hundreds or thousands of micrograms) carry real risk for people with autoimmune thyroid disease and are generally not recommended without medical guidance.