Thyroid Supplements: What May Help and What May Harm
- , by SANUSq Research team
- 13 min reading time
Thyroid supplements are one of the few corners of the supplement aisle where the wrong choice can make things worse rather than better. That's what sets this topic apart: before talking about what may help, it is worth being clear about what may harm.
What the thyroid does, in brief
The thyroid, a small gland at the front of the neck, makes hormones that set the pace at which the body runs: temperature, heart rate, digestion, energy and mood.
When the gland makes too little hormone, a condition called hypothyroidism, everything slows down. Typical signs include tiredness, feeling cold, weight gain, dry skin, constipation and low mood. In countries where people get enough iodine, the most common cause is Hashimoto's thyroiditis, an autoimmune condition in which the immune system attacks the gland.
Thyroid disease is a medical condition. It needs a diagnosis from a doctor, based on blood tests, and in many cases prescribed treatment. Supplements are not a substitute for that care, and nothing in this article should be read as a way to manage a thyroid condition on your own.
Iodine: the warning that has to come first
Iodine is the raw material for thyroid hormones. Iodine contributes to the normal production of thyroid hormones and to normal thyroid function, which is why it has a reputation as the thyroid nutrient. That reputation leads people down a risky shortcut.
If you are genuinely short of iodine, getting enough matters. But in countries where salt is iodised and people eat fish, seafood and dairy foods, outright deficiency is uncommon. More is not better. Too much iodine can trigger or worsen thyroid problems, particularly in people who already have an autoimmune thyroid condition, which is exactly the group most likely to go looking for a supplement.
In a review of iodine excess, Leung and Braverman (2014) note that the recommended intake for adults who are not pregnant or breastfeeding is 150 µg a day. Higher intakes are generally well tolerated, but in susceptible people, including those with existing thyroid disease and older adults, excess iodine can lead to either an underactive or an overactive thyroid, and the source of the extra iodine is not always obvious.
Kelp and seaweed supplements
Seaweed-based products, such as kelp, bladderwrack and kombu, are the main trap. Their iodine content is often high and varies enormously between products and batches.
Aakre et al. (2021) analysed 96 seaweed-containing foods and supplements sold in Norway. Supplements delivered anywhere from 5 to 5,600 µg of iodine per daily dose, and for 54 of the products a single portion or dose would exceed the tolerable upper intake level. Labelling of iodine content was inadequate or inaccurate for several products, and the authors concluded that these products are unreliable iodine sources.
Unless a deficiency has been confirmed and your doctor advises it, kelp and seaweed supplements have no place in looking after your thyroid.
Selenium: the most studied of the thyroid supplements
Selenium contributes to normal thyroid function. The thyroid holds more selenium, gram for gram, than most other tissues, and selenium-containing enzymes help protect the gland from the hydrogen peroxide it produces while making hormones. Of all the nutrients sold for the thyroid, selenium has been the most carefully studied in Hashimoto's thyroiditis, which is why people often search for a selenium thyroid supplement.
A systematic review and meta-analysis by Huwiler et al. (2024), covering 35 studies in people with Hashimoto's thyroiditis, found that selenium supplementation lowered thyroid peroxidase (TPO) antibody levels, and lowered TSH slightly in people not taking thyroid hormone. There were no significant changes in the thyroid hormones T4 and T3 themselves, and side effects were similar to placebo. The overall certainty of the evidence was rated moderate.
Read this carefully. Lower antibody levels are a laboratory finding. They don't necessarily mean someone feels better, and they don't change whether a person needs prescribed treatment.
Selenium also has a fairly narrow margin between enough and too much. High intakes over time can cause problems of their own, such as brittle nails and hair loss. If you have a thyroid condition and are thinking about selenium, discuss the amount with your doctor rather than choosing a high-strength product on your own.
Vitamins for thyroid health: iron, vitamin D and zinc
Lists of vitamins for thyroid health are long. The research points to testing rather than guessing.
In a review of nutritional factors and Hashimoto's thyroiditis, Hu and Rayman (2017) describe evidence linking high iodine intake and shortfalls of selenium and iron with the condition. Iron is needed by the enzyme that makes thyroid hormones, and people with Hashimoto's are often low in iron. Low vitamin D status was common too, but the authors suggest it is more likely a consequence of the autoimmune process than a cause. They advise doctors to check iron, particularly in women who menstruate, and vitamin D, and to correct any deficiency.
In practice, three nutrients are worth a blood test rather than blind supplementation:
- Iron — deficiency is common in women, and iron is involved in making thyroid hormones.
- Vitamin D — often low, especially in winter and in people who spend little time outdoors.
- Zinc — a shortfall is less common but can contribute to tiredness.
The approach is the same each time: test, correct if needed, then re-check.
Supplements and thyroid medication: the line not to cross
Confirmed hypothyroidism is managed with thyroid hormone replacement, usually levothyroxine, with the dose adjusted according to regular blood tests. No supplement replaces this treatment. Stopping it or lowering the dose on your own can have serious consequences.
Timing matters
One practical point is often overlooked. Calcium, iron and magnesium taken too close to levothyroxine can reduce how much of the medicine is absorbed.
Singh et al. (2000) asked 20 people on stable levothyroxine to take calcium carbonate with their tablet for three months. Their thyroxine levels fell and TSH rose, with one in five moving above the normal TSH range, and levels recovered once the calcium was stopped. Laboratory tests showed that levothyroxine binds to calcium carbonate in acidic conditions.
A review of levothyroxine absorption by Skelin et al. (2017) lists calcium carbonate, calcium citrate, calcium acetate, iron sulfate and aluminium-containing antacids among the substances that clearly reduce absorption, along with coffee and soya. The authors note that most of these interactions can be avoided by taking levothyroxine and the interfering product at different times.
The usual advice is to leave a gap of several hours between your thyroid tablet and any calcium, iron or magnesium supplement. Ask your doctor or pharmacist how best to time yours.
Biotin and thyroid blood tests
Biotin, common in hair, skin and nail products, causes a different problem. It doesn't affect the thyroid itself, but at high doses it can interfere with the laboratory tests used to check thyroid function, making results look abnormal when they are not, or normal when they are not.
Li et al. (2017) gave six healthy adults 10 mg of biotin a day for a week, a dose common in over-the-counter supplements. Biotin interfered with 9 of the 23 biotin-based laboratory tests studied, which included TSH and thyroid hormone tests, producing falsely high results in some and falsely low results in others.
If you take biotin, tell your doctor before any blood test. They may ask you to stop it for a few days beforehand.
Products with hidden hormones
Be wary of products sold as thyroid support with vague or proprietary ingredient lists, particularly those containing glandular extracts. Some have been found to contain actual thyroid hormone, which means taking a medicine without knowing it or measuring it.
Kang et al. (2013) tested ten thyroid support supplements bought in the United States. Nine contained detectable T3 and five contained T4. At the recommended dose, several delivered amounts comparable to or above those used in prescribed treatment, enough to risk an overactive thyroid.
A sensible everyday approach
What is reasonable, and carries little risk, is a varied diet that naturally provides iodine, selenium, iron and zinc. Good sources include fish and seafood, eggs, dairy foods, pulses and meat. Brazil nuts are very rich in selenium, so one or two are plenty; eating handfuls every day can push intake too high.
Enough sleep and regular activity won't change how the gland works, but they help with tiredness. After a treatment adjustment, symptoms can take several weeks to respond.
Cruciferous vegetables such as cabbage, broccoli and cauliflower are often said to be bad for the thyroid. In normal amounts, and especially when cooked, they are not a concern for someone with adequate iodine intake. Cutting them out brings no benefit.
Tiredness isn't always the thyroid
Many people reach for thyroid supplements because of one symptom: tiredness. But fatigue is one of the least specific symptoms there is. Low iron, sleep apnoea, depression, diabetes, side effects of medicines and simple lack of sleep are all more common than a thyroid problem, and all can be addressed once they are identified.
For someone whose thyroid is working normally, no thyroid supplement will help their energy. That is why a blood test comes first: it answers in one step what could otherwise take months of trial and error.
When to see your doctor
Ongoing tiredness, feeling unusually cold, unexplained weight gain or loss, swelling at the front of the neck, a racing or irregular heartbeat, or a lasting change in mood all justify a thyroid blood test, which is simple to arrange. Start with your doctor rather than the supplement shelf, where, with iodine, you risk worsening the very thing you hoped to fix.
Frequently asked questions
What is the best supplement for the thyroid?
There is no single one to take blindly. Selenium is the most studied in Hashimoto's thyroiditis, where it lowers antibody levels, though what that means for symptoms is uncertain. Iron, vitamin D and zinc are best corrected after a blood test shows a shortfall, not taken on principle.
Is iodine good for the thyroid?
Iodine contributes to the normal production of thyroid hormones, but extra iodine only helps if you are deficient, which is uncommon where salt is iodised. Too much can trigger or worsen thyroid problems, especially in autoimmune thyroid disease. Kelp and seaweed supplements are particularly risky because their iodine content varies so widely.
Can supplements replace thyroid medication?
No. Confirmed hypothyroidism is managed with thyroid hormone replacement, adjusted according to blood tests. Reducing or stopping your medication without medical advice can have serious consequences.
Which supplements interfere with levothyroxine?
Calcium, iron and magnesium can reduce its absorption if taken at the same time, so keep several hours between them. Biotin doesn't affect the medicine but can distort thyroid blood test results. Your pharmacist can help you plan the timing.
Should I avoid broccoli and cabbage?
No. In normal amounts, and especially when cooked, they are not a problem for someone getting enough iodine.
When should I have my thyroid checked?
If you have ongoing tiredness, feel unusually cold, have unexplained changes in weight, notice swelling in your neck, have heart rhythm changes or a lasting change in mood. A blood test is simple and far more informative than trying a supplement.
References
- Leung AM, Braverman LE. Consequences of excess iodine. Nature Reviews Endocrinology. 2014;10(3):136–142. PMID 24342882
- 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;65:7584. PMID 33889064
- Huwiler VV, Maissen-Abgottspon S, Stanga Z, et al. Selenium supplementation in patients with Hashimoto thyroiditis: a systematic review and meta-analysis of randomized clinical trials. Thyroid. 2024;34(3):295–313. PMID 38243784
- Hu S, Rayman MP. Multiple nutritional factors and the risk of Hashimoto's thyroiditis. Thyroid. 2017;27(5):597–610. PMID 28290237
- Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA. 2000;283(21):2822–2825. PMID 10838651
- Skelin M, Lucijanić T, Amidžić Klarić D, et al. Factors affecting gastrointestinal absorption of levothyroxine: a review. Clinical Therapeutics. 2017;39(2):378–403. PMID 28153426
- 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):1150–1160. PMID 28973622
- Kang GY, Parks JR, Fileta B, et al. Thyroxine and triiodothyronine content in commercially available thyroid health supplements. Thyroid. 2013;23(10):1233–1237. PMID 23758055
Read more on the blog
- Hypothyroidism: Symptoms and How to Support Your Thyroid →
- Vitamin D and Thyroid Health: Is There a Link? →
- Top Causes of Unexplained Fatigue →
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The team at SANUSq.
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