Underactive Thyroid Diet: What to Eat and What to Avoid

There is no special diet that research shows fixes an underactive thyroid or Hashimoto's. The diet studies so far are small, short and point in different directions. What the evidence does support is simple: eat a normal balanced diet, get your iodine from ordinary food rather than supplements, enjoy broccoli and soy in normal amounts, and time your food around your thyroid tablet. Very few foods need avoiding.

If you have just been told your thyroid is underactive, or you have Hashimoto's and keep reading lists of "foods to avoid", here is what the research and Australian guidance actually say.

Is there a "hypothyroid diet"?

Not one that research backs. The only systematic review of diet trials in Hashimoto's, published in 2023, found nine small studies that each tested something different, and concluded it is still unclear whether any particular diet helps. A 2022 review in a journal of the US Endocrine Society found that many of the most popular approaches have no proven benefit or have not been well studied. The Australian Thyroid Foundation also notes that elimination diets lack good evidence.

The best base is the Australian Dietary Guidelines: plenty of vegetables and legumes, wholegrains, fruit, protein foods like eggs, fish, meat and tofu, and dairy or calcium-fortified alternatives.

What to eat

• Ordinary iodine foods. In Australia, bread has been made with iodised salt by law since 2009, which adds about as much iodine as a large glass of milk each day. Dairy, eggs and seafood add more.
• Vegetables, including broccoli, kale and cabbage. More on these below.
• Enough fibre. Constipation is common with an underactive thyroid, and fibre helps. Keep your intake steady.
• Protein at each meal, from whatever sources suit you.
• Dairy, or lactose-free dairy if lactose doesn't agree with you. It keeps the iodine and calcium. Most plant milks don't contain iodine.

What to avoid, or watch

Extra iodine: tablets, kelp and seaweed supplements

With Hashimoto's, the caution is not about iodine in food. It is about extra iodine on top. In a European study, a small daily iodine supplement changed thyroid function in 9 of 40 people with Hashimoto's, compared with 1 of 43 people without it. The American Thyroid Association advises against iodine supplements above 500 micrograms a day. Kelp is the biggest wildcard: its iodine content varies enormously, from about 16 micrograms per gram in nori to over 8,000 in kelp granules. Don't take iodine, kelp or seaweed supplements unless your doctor says so. Our guide to iodine, kelp and your thyroid explains more.

That also means you don't need to cut bread, dairy or iodised salt to protect your thyroid. A low-iodine diet is only for a short, prescribed period before radioactive iodine.

Broccoli, kale and cabbage: fine in normal amounts

These vegetables contain natural compounds called goitrogens, which can affect how a thyroid gland uses iodine. That sounds alarming, but a 2024 review of 123 studies found that eating them as part of a daily diet causes no harm to thyroid function, especially when iodine intake is adequate. The well-known case reports involved eating around one to one and a half kilograms of raw bok choy a day. Cooking reduces the effect. So the honest caution is about very large amounts, mostly raw, not a serve of broccoli with dinner.

Soy: fine, just not next to your tablet

A review of 18 trials found soy had no effect on thyroid hormone levels and only a very small effect on TSH. You may have read that soy "triples your risk". That came from one 2011 study of 60 people with a mildly underactive thyroid who were not on medication, and the same team's later trial at a higher dose found no increase. The real issue is absorption: soy can reduce how much levothyroxine you absorb if it is too close to your tablet. The Australian levothyroxine leaflet says your dose may need adjusting when you start or stop soy supplements, so tell your doctor if you do.

Gluten: only if you have coeliac disease

Coeliac disease is more common with Hashimoto's, affecting about 2.7% of adults with autoimmune thyroid disease in a large review. If you have it, going gluten-free matters. If you don't, trials have not shown that cutting gluten changes Hashimoto's. Get tested before you cut gluten, because cutting it first can hide the result. Our guide to gluten-free for Hashimoto's covers this in detail.

Lactose: worth asking about if your dose keeps rising

Coeliac disease and lactose intolerance can both affect how well levothyroxine is absorbed. In one small study of people with Hashimoto's and lactose intolerance, cutting lactose lowered TSH. If your dose keeps going up or your results won't settle, ask your doctor whether either could be playing a part. Lactose-free dairy is usually the answer, not cutting dairy out.

Timing matters more than any food

The Australian levothyroxine leaflet says to take your tablet with water on an empty stomach, at least 30 minutes and preferably 60 minutes before food. That covers breakfast, coffee and milk. Calcium, iron and magnesium supplements need a longer gap of several hours. Keep fibre supplements away from your tablet too, and keep your habits steady so your dose stays right. Our guide to coffee and levothyroxine has the detail.

Weight and an underactive thyroid

An underactive thyroid can add some weight, mostly salt and water, and the American Thyroid Association puts the thyroid-related part at about 2 to 4.5 kg. Starting treatment usually changes weight very little: in one study the median change was about 0.1 kg. A trial that adjusted levothyroxine doses found no effect on weight, and the Australian leaflet says levothyroxine should not be used for weight loss. Once your levels are right, losing weight works the same way as for anyone else. Our guide to weight and an underactive thyroid goes further.

What about Mediterranean, anti-inflammatory or AIP diets?

A Mediterranean-style diet is a sensible way to eat for anyone, and small studies suggest some people with Hashimoto's feel better on it. But these diets haven't been shown to reliably change thyroid tests or antibodies, and the trials are tiny and short. Our guide to the Hashimoto's diet looks at each of the named diets in turn.

Food myths you can let go of

• "Never eat raw kale or broccoli." Normal servings are fine.
• "No soy ever." Soy is fine away from your tablet.
• "Everyone with Hashimoto's should go gluten-free." Only with coeliac disease, after testing.
• "Iodine supplements help an underactive thyroid." In Australia, extra iodine is more likely to cause problems than solve them.
• "Eat right and you can come off your medication." No diet has been shown to do this, apart from treating coeliac disease or lactose intolerance that was blocking absorption. Never change your dose yourself.
• "You must wait 4 hours to eat." Food is 30 to 60 minutes. The longer gap is for mineral supplements.

Where ThyroBase fits

No food or powder changes how your thyroid works, and none should be used to try to lower your dose. ThyroBase is a daily morning and evening nutrition powder, a food rather than a medicine, made with plant protein, greens and minerals, with no added iodine. It sits alongside your thyroid medication, never instead of it. Both serves contain minerals, so keep them several hours away from your tablet. Here is who ThyroBase is, and is not, designed for.

Eat normally. Get iodine from food, not tablets. Time your food around your tablet.

Frequently asked questions

What is the best diet for an underactive thyroid?

No diet has been shown to fix an underactive thyroid. Research supports a normal balanced diet, such as the Australian Dietary Guidelines, with iodine from ordinary food, and your thyroid tablet taken 30 to 60 minutes before food. The diet trials in Hashimoto's so far are small and inconsistent.

What foods should I avoid with an underactive thyroid?

Very few. Avoid iodine, kelp and seaweed supplements unless your doctor recommends them, especially with Hashimoto's. Keep coffee, soy, milk and fibre supplements away from your tablet rather than cutting them out. Only cut gluten if you have coeliac disease.

Can I eat broccoli with an underactive thyroid?

Yes. A 2024 review of 123 studies found broccoli, kale and cabbage cause no harm to thyroid function in normal daily amounts, especially with adequate iodine. The caution is about very large amounts, mostly raw. Cooking reduces the effect further.

Is soy bad for hypothyroidism?

No. A review of 18 trials found soy had no effect on thyroid hormone levels and only a very small effect on TSH. Keep soy away from your levothyroxine tablet, and tell your doctor if you start or stop a soy supplement, as your dose may need checking.

Can diet help me lose weight with an underactive thyroid?

Once your thyroid levels are right, weight loss works the same way as for anyone else: what you eat, how active you are, and sleep all matter. Levothyroxine should not be used for weight loss, and changing your dose to lose weight isn't safe. Talk to your doctor or a dietitian.

The honest takeaway

With an underactive thyroid or Hashimoto's, you don't need a special diet or a long list of banned foods. Eat a balanced diet, get iodine from bread, dairy and eggs rather than supplements, enjoy your vegetables and soy in normal amounts, and take your tablet 30 to 60 minutes before food. If your dose keeps rising or you still don't feel right, ask about coeliac disease, lactose intolerance and other causes.

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.

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