Can Supplements Affect Thyroid Function and Thyroid Blood Tests?
Some supplements can affect your thyroid gland. Others can affect thyroid blood test accuracy. Knowing the difference can help prevent confusing results.
Contents
Quick answer
Yes, some supplements can affect thyroid function. Others can interfere with thyroid blood tests. This means the result may not reflect your true thyroid status.
Biotin is the best-known supplement affecting thyroid test accuracy. It can cause falsely low TSH. It can also cause falsely high FT4 and FT3. [1]
Iodine, kelp, sea moss, thyroid support products, and ashwagandha may affect thyroid function itself. Iron and calcium can reduce levothyroxine absorption. [2] [3]
Two different problems
Supplements can affect thyroid care in two main ways. These are not the same.
| Problem | What it means | Examples |
|---|---|---|
| Test interference | The blood test result may be misleading. | Biotin. |
| True thyroid effect | The supplement may alter thyroid hormone production. | Iodine, kelp, sea moss, thyroid extracts, ashwagandha. |
| Medication absorption problem | The supplement may reduce levothyroxine absorption. | Iron, calcium, some multivitamins, soya supplements. |
This is why supplement history matters. A simple label check can prevent unnecessary worry.
Biotin and thyroid tests
Biotin is also called vitamin B7. It is found in many hair, skin, and nail supplements. It is also found in some multivitamins.
Biotin does not usually cause thyroid disease. The main concern is thyroid test interference. It can make results look overactive. [1]
| Thyroid test | Possible biotin effect |
|---|---|
| TSH | Falsely low. |
| FT4 | Falsely high. |
| FT3 | Falsely high. |
This pattern can wrongly suggest hyperthyroidism. It can also suggest too much levothyroxine. The American Thyroid Association advises stopping biotin before thyroid testing. [1]
Read our full biotin guide
We have written a separate guide on biotin and thyroid tests. It explains TSH, FT4, FT3, and washout timing in more detail.
We have written a separate guide on biotin and thyroid tests.
It explains how biotin can affect TSH, FT4 and FT3 results.
It also explains when to stop biotin before thyroid blood testing.
Read: Can biotin affect thyroid blood tests? TSH, FT4 and FT3 explained
Read our full biotin guide
Iodine, kelp and sea moss
Iodine is needed to make thyroid hormones. Adults need a regular intake from food. Too little iodine can affect thyroid hormone production. [4]
However, more iodine is not always better. Excess iodine can trigger thyroid dysfunction in susceptible people. This may include either hypothyroidism or hyperthyroidism. [5]
Kelp and sea moss can contain high iodine levels. Their iodine content can vary between products. They are often marketed as thyroid boosters.
The British Thyroid Foundation advises avoiding kelp and sea moss products. This is especially important with existing thyroid disease. [2]
Important warning
Avoid using kelp or sea moss to “boost” the thyroid. This can be risky in Graves’ disease. It can also be risky in Hashimoto’s thyroiditis.
“Thyroid support” supplements
Be careful with products labelled “thyroid support”. Some contain iodine. Some contain glandular extracts. Some may contain active thyroid hormones.
One study found clinically relevant amounts of T3 and T4. These were found in several commercial thyroid health supplements. This could alter thyroid blood tests. It could also cause thyrotoxicosis. [6]
The American Thyroid Association also highlighted this concern. It warned that thyroid support supplements may expose people to thyroid hormones. [7]
Be cautious with labels such as:
- Thyroid support.
- Thyroid booster.
- Metabolism booster.
- Glandular thyroid extract.
- Iodine complex.
- Kelp thyroid formula.
Ashwagandha
Ashwagandha is a herbal supplement. It is often used for stress, sleep, and wellbeing. It may affect thyroid function in some people.
Case reports have linked ashwagandha with thyrotoxicosis. Thyrotoxicosis means excess thyroid hormone effect. This can cause palpitations, anxiety, sweating, and weight loss. [8]
Another case report described painless thyroiditis after ashwagandha use. This suggests ashwagandha may rarely trigger thyroid disturbance. [9]
The evidence is mostly case reports and small studies. So the risk is not fully quantified. However, it should be mentioned before thyroid testing.
Selenium
Selenium is needed for normal thyroid hormone metabolism. The thyroid gland uses selenium-containing proteins. These help thyroid hormone processing and antioxidant defence. [10]
Selenium has been studied in Hashimoto’s thyroiditis. Some evidence suggests it may reduce thyroid antibody levels. However, benefits vary between studies. It is not a guaranteed treatment. [11]
Too much selenium can be harmful. Possible toxicity symptoms include hair loss, nail changes, and digestive symptoms. It is best not to take high-dose selenium without advice. [10]
Iron, calcium and levothyroxine
Iron and calcium do not usually distort thyroid tests directly. They can affect treatment if you take levothyroxine.
Levothyroxine is absorbed through the gut. Calcium-rich foods and soya can reduce its absorption. NHS advice recommends leaving a gap where needed. [3]
Oral iron can also reduce levothyroxine absorption. The BNF advises separating oral iron and levothyroxine by at least four hours. [12]
If levothyroxine absorption falls, TSH may rise. This can look like under-treated hypothyroidism. It may happen even when the dose has not changed.
| Supplement | Main concern | Practical advice |
|---|---|---|
| Biotin | Can distort thyroid blood tests. | Stop before testing, unless prescribed. |
| Kelp or sea moss | May contain high iodine. | Avoid with thyroid disease. |
| Iodine | Too much can trigger thyroid dysfunction. | Use only when clinically appropriate. |
| Thyroid support products | May contain thyroid hormones or high iodine. | Avoid unless medically supervised. |
| Ashwagandha | Rare reports of thyrotoxicosis. | Mention before thyroid testing. |
| Selenium | Possible benefit, but excess can harm. | Avoid high-dose use without advice. |
| Iron | Can reduce levothyroxine absorption. | Separate from levothyroxine. |
| Calcium | Can reduce levothyroxine absorption. | Separate from levothyroxine. |
What should you do before a thyroid blood test?
Before your thyroid blood test, check all supplement labels. This includes tablets, gummies, powders, drops, and herbal blends.
- Tell the testing provider about all supplements.
- Bring a photo of each supplement label.
- Look specifically for biotin, iodine, kelp, and sea moss.
- Do not stop prescribed supplements without medical advice.
- Tell the provider if you take levothyroxine.
- Repeat testing may be needed if results look unusual.
When should supplement interference be suspected?
It should be suspected when symptoms and results do not match. It is also important when results change suddenly. A supplement history can explain unexpected thyroid results.
AIS Health Check thyroid tests
AIS Health Check offers convenient private thyroid blood tests. These can help assess thyroid hormone levels and thyroid autoimmunity.
Please tell us about supplements before your blood test. This helps support accurate interpretation.
Frequently asked questions
Can supplements cause thyroid disease?
Some can contribute to thyroid dysfunction. This is more likely with excess iodine. It can also occur with thyroid support products.
Which supplement most affects thyroid blood tests?
Biotin is the main supplement affecting test accuracy. It can make results look overactive.
Are kelp and sea moss good for the thyroid?
Not usually. They can contain high iodine levels. They may worsen thyroid problems in susceptible people.
Can selenium treat Hashimoto’s thyroiditis?
Selenium may reduce thyroid antibodies in some studies. It is not a guaranteed treatment. High doses can cause harm.
Can iron or calcium affect thyroid tablets?
Yes. Iron and calcium can reduce levothyroxine absorption. They should usually be taken at a different time.
Should I stop all supplements before thyroid testing?
Do not stop prescribed supplements without advice. However, you should always tell the provider. Non-essential biotin is commonly stopped before testing.
Book a thyroid blood test with AIS Health Check
Check your thyroid function with clear private blood testing. Tell us about supplements before your appointment. This helps support accurate and reliable interpretation.
References
- American Thyroid Association. Biotin supplement use is common and can lead to the false measurement of thyroid hormone in commonly used assays . Clinical Thyroidology for the Public. 2018;11(12):3-4.
- British Thyroid Foundation. Diets and supplements for thyroid disorders . British Thyroid Foundation. Updated 2024.
- NHS. Levothyroxine: a medicine for an underactive thyroid . NHS medicines information.
- NHS. Iodine: vitamins and minerals . NHS health information.
- Sohn SY, Lee SY, Kim JH, et al. Risks of iodine excess . Endocrine Reviews. 2024;45(6):858-887.
- Kang GY, Parks JR, Fileta B, et al. Thyroxine and triiodothyronine content in commercially available thyroid health supplements . Thyroid. 2013;23(10):1233-1237.
- American Thyroid Association. Thyroid health supplements contain significant amounts of thyroid hormones . Clinical Thyroidology. 2013;25(8):166-167.
- Kamal HI, Patel K, Braden G, et al. Ashwagandha as a unique cause of thyrotoxicosis . Cureus. 2022;14(4):e24150.
- Hayashi M, Ito K, Kato K, et al. Painless thyroiditis by Withania somnifera, Ashwagandha . Cureus. 2024;16(2):e54390.
- National Institutes of Health Office of Dietary Supplements. Selenium: fact sheet for consumers . NIH Office of Dietary Supplements.
- Huwiler VV, Weller A, Blum MR, et al. Selenium supplementation in patients with Hashimoto thyroiditis . Thyroid. 2024;34(3):295-313.
- British National Formulary. Levothyroxine interactions . BNF. National Institute for Health and Care Excellence.
This article is for general information only. It is not a substitute for personalised medical advice. Speak to a healthcare professional about abnormal thyroid results.