Supplements With an Underactive Thyroid: A Plain Guide to What to Avoid

When medication has brought your results back into range but not your energy, reaching for a supplement makes complete sense. The shelf is full of bottles promising "thyroid support", and most of them look reassuringly similar. Some are fine. Some are pointless. A few can quietly work against you, or against the tablet you already take.

This is a plain guide to telling the difference, written for anyone with an underactive thyroid, Hashimoto's, or no thyroid at all after surgery. It is not a list of things to fear. It is a list of things to check.

First, timing

Before what is in a supplement, there is when you take it. Iron, calcium, magnesium and other minerals can bind to thyroid medication in the gut and reduce how much of your dose you absorb. The usual advice is to keep them around four hours away from your tablet. We have covered this in detail in supplements and levothyroxine: the timing that matters. The rest of this guide is about what goes in the bottle.

Added iodine and kelp

Iodine is one of the ingredients most often added to thyroid supplements, frequently as kelp or other seaweed. It sounds logical, because the thyroid uses iodine to make hormone. But since 2009, bread in Australia has been made with iodised salt, and the national health survey that followed found Australian adults, as a population, now get enough iodine. In Hashimoto's, extra iodine can work against you, because excess iodine can aggravate an already struggling thyroid rather than help it.

Seaweed-based products add another problem: you cannot be sure how much iodine you are getting. One analysis of kelp and seaweed supplements found daily doses ranging from 5 to 5,600 micrograms of iodine, with several labels that did not match the contents. The top of that range is more than thirty times the recommended daily intake for an adult.

If you have no thyroid at all, your medication supplies the finished hormone, so extra iodine has little to do there either. Our guide to iodine and why more isn't better goes deeper.

Thyroid extract and "glandular" ingredients

Some products sold as supplements contain dried animal thyroid tissue, listed as "thyroid", "bovine thyroid", "thyroid glandular" or "thyroid tissue". The problem is that thyroid tissue can contain actual thyroid hormone. When US researchers tested ten over-the-counter thyroid supplements, nine contained T3 and five contained T4, and several delivered daily amounts in the range of a prescribed dose. For someone already on thyroid medication, that is an unmeasured extra dose, and it can push levels too high without anyone understanding why.

Biotin before a test

Biotin (also called vitamin B7 or vitamin H) is common in hair, skin and nail products and in many B-complex supplements, often at high doses. It does not harm your thyroid. What it can do is interfere with the way many laboratory tests measure hormones. In a study where healthy adults took 10mg of biotin a day for a week, 9 of 23 biotin-based tests gave wrong results. For thyroid tests, the pattern can look exactly like an overactive thyroid, and there are published cases of people being mistakenly diagnosed with Graves' disease because of it. That can lead to confusion, or even a dose change you did not need.

If you take biotin, tell whoever orders your thyroid test, and ask how long to pause it beforehand. There is no single agreed answer: Australian advice ranges from at least 72 hours to at least a week, depending on the dose.

Herbs that can nudge hormone levels

Ashwagandha (also sold as withania) is popular for stress and sleep, and it is often marketed alongside thyroid products. It may also raise thyroid hormone levels. In a small eight-week trial of 50 people with a mildly underactive thyroid who were not on medication, ashwagandha raised thyroid hormone levels compared with a placebo. That might sound like a good thing. But if you are already on medication, adding something that pushes hormone levels up could tip a well-balanced dose too high, and there are published case reports of people taking ashwagandha developing an overactive thyroid.

Separately, Australia's Therapeutic Goods Administration has issued safety advice about a very rare risk of liver injury from ashwagandha, and says people with liver problems should avoid it. If you are considering it, talk to your doctor or pharmacist first.

Stimulants and "metabolism boosters"

Products sold for energy, fat loss or "metabolism" often rely on caffeine or other stimulants. They can make your heart race, mask how tired you really are, and make it harder to tell whether symptoms come from your dose or the product. If you feel wired, shaky or unusually anxious after starting something new, stop and speak to your doctor.

Any promise to replace your medication

This is the clearest warning sign of all. If a product suggests it can reduce, replace or "free you from" thyroid medication, walk away. After thyroid surgery or radioactive iodine, your body cannot make enough thyroid hormone on its own, and no supplement changes that. In Hashimoto's that has become an underactive thyroid, stopping prescribed medication usually brings the symptoms back. A good supplement will say plainly that it sits alongside your medication.

A quick label check

If the label lists	Why it is worth a second look
Iodine, kelp, seaweed, bladderwrack	Extra iodine can work against you in Hashimoto's, and seaweed iodine content varies widely.
Thyroid, bovine thyroid, glandular, thyroid tissue	May contain actual thyroid hormone, an unmeasured extra dose on top of your medication.
Biotin, vitamin B7, vitamin H	Can make thyroid tests read wrongly. Tell your doctor before any test.
Ashwagandha, withania	May raise thyroid hormone levels. Check with your doctor or pharmacist if you take medication.
Guarana, high caffeine, "fat burner"	Stimulants can mimic or mask symptoms of a dose that is off.
"Replaces", "instead of", "reduce your medication"	No supplement replaces thyroid hormone. Walk away.
Iron, calcium, magnesium	Useful nutrients, but keep them about four hours from your thyroid tablet.

What a sensible supplement looks like

A trustworthy product tells you exactly what is in it and how much. It gives clear instructions about timing around thyroid medication. It says, plainly, that it sits alongside your medication rather than replacing it. And it does not promise to fix your thyroid, because nothing sold in a bottle can.

That is the standard we built ThyroBase to. It has no added iodine, no kelp, no added biotin, no thyroid extract, no ashwagandha and no added caffeine. The AM serve is designed to be taken at least four hours after your thyroid tablet, so its minerals stay clear of your dose. And it is designed to sit alongside your thyroid medication, never instead of it. Here is who it is, and is not, designed for.

Before you start anything new, take the bottle to your pharmacist.

It takes five minutes, costs nothing, and they can check it against everything else you take.

Frequently asked questions

What supplements should you avoid with an underactive thyroid?

Be careful with added iodine or kelp (especially with Hashimoto's), thyroid extract or glandular products that may contain thyroid hormone, high-dose biotin before thyroid tests, herbs such as ashwagandha that may raise hormone levels, and stimulant-based "metabolism" products. Keep iron, calcium and magnesium about four hours from your thyroid tablet.

Can biotin affect thyroid test results?

Yes. High-dose biotin can interfere with many laboratory tests for thyroid hormones and make results look like an overactive thyroid when nothing has changed. Tell your doctor if you take it, and ask how long to stop before your test. Australian advice ranges from at least 72 hours to at least a week, depending on the dose.

Is ashwagandha safe with levothyroxine?

Ashwagandha may raise thyroid hormone levels, so combining it with thyroid medication could push levels too high. Speak to your doctor or pharmacist before taking it, and let them know if you already do.

Should people with Hashimoto's avoid iodine supplements?

It is worth being cautious. Excess iodine, from supplements or kelp, can aggravate an already struggling thyroid, and Australian adults as a population get enough iodine since bread began to be made with iodised salt in 2009. Pregnancy and breastfeeding are a different situation, so speak to your doctor about your own needs.

The honest takeaway

You do not need to fear the supplement aisle. You need to read it. Check for iodine and kelp, thyroid extract, biotin, hormone-nudging herbs and stimulants. Keep minerals four hours from your tablet. Be wary of anything that promises to replace your medication. And when in doubt, bring the bottle to your pharmacist. Good support should make your medication's job easier, never harder.

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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Australian Bureau of Statistics. Australian Health Survey: Biomedical Results for Nutrients, 2011-12. Iodine. abs.gov.au.
Aakre I, Solli DD, Markhus MW, et al. Commercially available kelp and seaweed products: valuable iodine source or risk of excess intake? Food & Nutrition Research. 2021;65.
Kang GY, Parks JR, Fileta B, et al. Thyroxine and triiodothyronine content in commercially available thyroid health supplements. Thyroid. 2013;23(10):1233-7.
Li D, Radulescu A, Shrestha RT, et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA. 2017;318(12):1150-60.
Elston MS, Sehgal S, Du Toit S, et al. Factitious Graves' disease due to biotin immunoassay interference: a case and review of the literature. Journal of Clinical Endocrinology & Metabolism. 2016;101(9):3251-5.
Trambas CM, Sikaris KA, Lu ZX. A caution regarding high-dose biotin therapy: misdiagnosis of hyperthyroidism in euthyroid patients. Medical Journal of Australia. 2016;205(4):192.
Lee L, Rankin W. Preparation for blood tests: what can go wrong before the sample reaches the lab. Australian Prescriber. 2025;48(4):122-7.
Sharma AK, Basu I, Singh S. Efficacy and safety of ashwagandha root extract in subclinical hypothyroid patients: a double-blind, randomized placebo-controlled trial. Journal of Alternative and Complementary Medicine. 2018;24(3):243-8.
Kamal HI, Patel K, Brdak A, et al. Ashwagandha as a unique cause of thyrotoxicosis presenting with supraventricular tachycardia. Cureus. 2022;14(3):e23494.
Therapeutic Goods Administration. Withania somnifera (ashwagandha) and very rare risk of liver injury. tga.gov.au. 2026.