Iodine-Rich Foods, Kelp and Your Thyroid: Why More Isn't Better

Foods high in iodine in Australia include seafood (especially oysters, mussels and sushi made with seaweed), milk, yoghurt, cheese, eggs and bread, which by law is made with iodised salt. Most adults need 150 micrograms a day, and an ordinary Australian diet usually covers it. With Hashimoto's or an underactive thyroid, the caution is about extra iodine from tablets, kelp and seaweed supplements, not these everyday foods. Don't cut them out unless your doctor tells you to.

Walk down the supplement aisle, pick up almost anything with the word thyroid on it, and you will often find iodine on the label, frequently as kelp. It seems to make sense: iodine is what the thyroid uses to make its hormones. For many of the people buying those bottles, it is the wrong answer to the right worry. Here is why.

Why iodine is a Goldilocks nutrient

Your thyroid needs iodine, and deficiency is a real problem in parts of the world. But the relationship is not a straight line where more keeps helping. Too little iodine causes thyroid trouble, and so does too much, so the body works best in a middle range.

Australia as a whole is classed as iodine sufficient, largely thanks to iodised salt in bread since 2009, although levels have slipped since 2011-12. A major 2014 review found extra iodine is well tolerated by most people, but people with existing thyroid disease, including autoimmune thyroid disease, are more likely to develop thyroid problems from too much.

How much iodine do you need a day?

Australia's Nutrient Reference Values set the recommended intake for adults at 150 micrograms a day. The upper level, the most an adult should have each day from all sources, is 1,100 micrograms. The same reference values note that people with thyroid disorders may react to amounts below that upper level, which is why the advice for a thyroid condition is enough iodine, not extra.

Needs are higher during pregnancy and breastfeeding, and the NHMRC recommends an iodine supplement at those times; with a thyroid condition, talk to your doctor or midwife before starting one.

Iodine in everyday Australian foods

The figures below are approximate. Food Standards Australia New Zealand notes that the iodine in food varies with where it is grown and how it is made, so use them as a guide.

Food	Typical serve	Approximate iodine (micrograms)
Mussels, steamed	100 g	about 270
Oysters	6 oysters (90 g)	about 145 to 180
Sushi made with seaweed	1 roll	about 90
Canned pink salmon	100 g	about 60
Milk (cow's, regular)	1 cup (250 mL)	about 55 to 60
Eggs	2 large eggs	about 25 to 70
Snapper, steamed	100 g	about 40
Prawns, cooked	100 g	about 5 to 40
Flavoured yoghurt	1 tub	about 30
Bread made with iodised salt	1 slice	about 15 to 25
Cheddar cheese	2 slices	about 10
Canned tuna	100 g	about 10
Fresh Atlantic salmon or farmed barramundi	100 g	close to none
Organic bread	1 slice	about 1
Soy, almond or oat milk	1 cup (250 mL)	about 3
Iodised table salt	1 g	about 45
Dried seaweed	1 g	about 9 (nori) to 4,300 (kombu)

A few things stand out. A glass of milk gives you more than a third of a day's iodine, and a serve of oysters or mussels can cover the whole day. Bread is the quiet achiever: since 2009, Australian law has required iodised salt in all bread except organic bread and bread mixes for making at home, so check the label if you buy organic. Plant milks are not fortified with iodine, and sea salt doesn't contain much, so people who skip dairy and bread can fall short. Not all fish is high in iodine either: in the national food database, fresh Atlantic salmon and farmed barramundi carry very little. Eggs vary the most between sources: the newest database analysis puts two large eggs well above older figures, so treat that row as a range.

Salt is a balancing act. The Australian Thyroid Foundation suggests keeping salt intake as low as you can and making sure the salt you do use is iodised. For the rest of the plate, see what to eat with an underactive thyroid. And if you have read that broccoli, kale or soy block iodine, here is what the evidence on goitrogens actually shows.

Seaweed is the outlier. An Australian survey of dried seaweed found iodine ranging from about 9 micrograms per gram in nori to 4,300 micrograms per gram in kombu. On those figures, a single gram of dried kombu can carry nearly four times the adult upper level for a whole day.

Should you avoid iodine if you have Hashimoto's?

Avoid extra iodine, not normal dietary iodine. The evidence that raises concern is about intakes above what the body needs:

• In a five-year Chinese study, people who already had thyroid antibodies were more likely to develop a raised TSH where iodine intake was high, not where it was normal-to-low.
• In a German study, about one in five people with Hashimoto's who took a 250 microgram iodine supplement daily developed a change in thyroid function, compared with one in 43 who did not. Several of those changes reversed when the iodine was stopped.

Food regulators in Australia say the iodine added to bread is unlikely to harm people with a thyroid condition, and that people with thyroid disorders should follow their doctor's advice on iodine. The Australian Thyroid Foundation is more cautious about iodised salt for people with diagnosed autoimmune thyroid disease. Because Australian sources differ in tone, the safe step is simple: talk to your doctor before changing your iodine intake in either direction.

You may see studies where cutting iodine helped people with Hashimoto's. Those were done in Korea and Japan, where iodine intake from seaweed can be many times higher than a typical Australian diet. They do not show that cutting normal Australian food iodine helps. For the wider food question, here is what the evidence says about Hashimoto's diets.

Is kelp OK for an underactive thyroid?

No good evidence supports it, and several health bodies advise against it:

• It can push your tests the wrong way. In a small trial, healthy people taking kelp capsules for four weeks saw their TSH rise, the same direction as an underactive thyroid.
• The dose is unpredictable. In a 2021 study of kelp supplements, iodine per daily dose ranged from 5 to 5,600 micrograms, and labels were often wrong.
• It has caused harm in Australia. In 2009-2010 an Australian soy milk made with kombu, a type of kelp, caused a cluster of thyroid problems. A national survey then found dried kombu at up to 4,300 milligrams of iodine per kilo.
• Experts advise a limit. The American Thyroid Association advises against iodine or kelp supplements above 500 micrograms a day, and lists Hashimoto's among the conditions most sensitive to excess iodine.

Iodine also turns up where you might not expect it. The Australian Thyroid Foundation lists vitamins, supplements and herbal medicines containing iodine or kelp among the things people on levothyroxine should be aware of, so read the label of any multivitamin or thyroid blend for iodine, potassium iodide, kelp or seaweed. Here is what to look for in a thyroid supplement in Australia.

If you take levothyroxine, your thyroid hormone comes from the tablet. Kelp does not add to it, and it can make your thyroid tests harder to read.

After a thyroidectomy or radioactive iodine

After a total thyroidectomy, your hormone comes from your tablet, though small amounts of thyroid tissue often remain. That is why iodine matters around radioactive iodine (RAI) treatment. After a partial thyroidectomy, the remaining lobe still uses iodine and can be more sensitive to too much.

Before RAI, your treating team will usually ask you to follow a low-iodine diet for one to two weeks so the remaining thyroid cells take up more of the treatment. It is short, it is specific, and it is theirs to prescribe. If you are looking for foods low in iodine because RAI is coming up, here is how the low-iodine diet before RAI works. Outside that window, there is no reason to restrict iodine on your own.

Questions to ask your doctor

• With my thyroid condition, should I change anything about the iodine I get from food?
• I don't eat dairy or regular bread. Am I getting enough iodine?
• Do any of my supplements contain iodine or kelp, and should I keep taking them?
• If I am having radioactive iodine, when should the low-iodine diet start and end?

Where ThyroBase fits

ThyroBase is a daily morning and evening plant-based drink powder, a food rather than a medicine, made with no added iodine and no kelp. The spirulina in the AM serve carries a trace of naturally occurring iodine, which is why the claim is "no added iodine", and why it should be paused during any low-iodine diet before radioactive iodine. It is designed to sit alongside your thyroid medication, never instead of it, and because both serves contain minerals, keep them several hours away from your tablet. Here is who ThyroBase is, and is not, designed for.

With a thyroid condition, aim for enough iodine from everyday food, not extra from kelp or tablets.

Frequently asked questions

What foods are high in iodine in Australia?

Seafood is the richest everyday source, especially oysters, mussels and sushi made with seaweed. Milk, yoghurt, cheese and eggs add more, and bread is a steady source because Australian law requires it to be made with iodised salt, except organic bread. Plant milks, sea salt and organic bread contain very little.

How much iodine do you need a day?

Australia's Nutrient Reference Values recommend 150 micrograms a day for adults, with an upper level of 1,100 micrograms. Needs are higher in pregnancy and breastfeeding. People with thyroid conditions can react below the upper level, so ask your doctor before adding any iodine supplement.

Should I avoid iodine if I have Hashimoto's?

Avoid extra iodine, such as iodine tablets, kelp and seaweed supplements, rather than the iodine in a normal diet. Studies link high intakes, not normal ones, to thyroid changes in people with thyroid antibodies. Don't cut out iodised salt or iodine foods unless your doctor advises it.

Is it OK to take kelp for an underactive thyroid?

No good evidence supports it. Kelp can raise TSH, its iodine content varies hugely from product to product, and the American Thyroid Association advises against iodine or kelp supplements above 500 micrograms a day. If you take levothyroxine, your hormone comes from the tablet, not from kelp.

Do I need iodine after a thyroidectomy?

After a total thyroidectomy, your thyroid hormone comes from your levothyroxine tablet. Small amounts of thyroid tissue often remain, which is why your team may ask for a short low-iodine diet before radioactive iodine. After a partial thyroidectomy, the remaining lobe still uses iodine. Ask your doctor about your situation.

Is iodised salt safe with a thyroid condition?

Australian food regulators say the iodine added to bread is unlikely to harm people with a thyroid condition, and the Australian Thyroid Foundation says most people tolerate iodised salt, with more caution for diagnosed autoimmune thyroid disease. The Foundation also suggests keeping salt low overall and making the salt you use iodised. Follow your doctor's advice on iodine.

The honest takeaway

Iodine deficiency is real, but more is not safer. Everyday Australian foods like dairy, eggs, seafood and bread give most people what they need, and those foods are not the problem with Hashimoto's or an underactive thyroid. The evidence points to avoiding extra iodine from tablets and kelp, and leaving seaweed-heavy habits for occasional meals. After a thyroidectomy, your hormone comes from your tablet, and any iodine changes around radioactive iodine belong to your treating team. When in doubt, ask your doctor rather than letting a supplement label decide.

ThyroBase is a nutritional supplement and is not intended to diagnose, treat, cure, or prevent any disease. It is designed to sit alongside your thyroid medication, not replace it. This article is general information, not medical advice. Never start, stop or change a supplement or prescription medication without speaking to your doctor or pharmacist first.

References

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