Thyroid Supplements: Iodine, Selenium and What the Trials Show

Two nutrients carry a thyroid wording in Great Britain: iodine contributes to the normal production of thyroid hormones and normal thyroid function, and selenium contributes to the normal thyroid function.¹ No supplement treats a thyroid condition or replaces levothyroxine, and the NHS lists calcium, iron, antacids, St John's wort and biotin among the things that may not mix with it.²

Evidence at a glance

• Trials: four systematic reviews of selenium in autoimmune thyroid disease, the largest of 35 studies.³
• Largest single trial of thyroid function: 491 Danish adults aged 60 to 74, five years.⁴
• Strongest measured outcome: thyroid peroxidase antibodies lower with selenium across 29 trial groups and 2,358 people (standardised mean difference −0.96; overall certainty moderate).³
• Not tested: none of the four trials in the 2013 Cochrane review measured a change in levothyroxine dose.⁵

Sources checked on 13 October 2026.

What can a thyroid supplement lawfully claim?

Only those two sentences. No other wording on the Great Britain health claims register mentions the thyroid, and a label may use them only when the product is a "source" of the nutrient.¹ Food law bars any food from claiming the property of "preventing, treating or curing a human disease".⁶ Nothing on a shelf stands in for levothyroxine, which replaces the hormone an underactive gland "cannot produce" and is usually taken for life;⁷ any change to it is your doctor's decision.

How much iodine do you need, and when is more harmful?

Adults need 140 µg a day, and most people should be able to get it from "a varied and balanced diet": milk and dairy foods, eggs, sea fish and shellfish.⁸ The NHS says strict vegans "may want to consider foods fortified with iodine or consider taking an iodine supplement", with a healthcare professional's advice, and that "0.5mg or less a day of iodine supplements is unlikely to cause any harm."⁸ In the 2019 to 2023 national survey, 18% of women aged 19 to 64 ate less iodine from food than the lower reference intake.⁹

Most people tolerate high intakes, but people with thyroid disease, older adults and babies before and after birth are more susceptible, and excess can push the thyroid underactive or overactive.^{10, 11} Seaweed can be a hidden source: see what sea moss is and why its iodine varies and sea moss side effects. Iodine in pregnancy is covered in pregnancy vitamins: what the NHS recommends.

How much selenium do you need, and how much is too much?

The NHS puts adult needs at 75 µg a day for men and 60 µg for women, and says "350μg or less a day of selenium supplements is unlikely to cause any harm."¹² The European Food Safety Authority's upper level, set in January 2023, is 255 µg a day from all sources, with hair loss an early sign of excess; the adults most likely to exceed it are regular high-dose supplement users and regular Brazil nut eaters.¹³ The NHS line is the UK advice. In the national survey, 57% of women aged 19 to 64 ate less selenium from food than the lower reference intake.⁹

Does selenium help Hashimoto's? What the trials measured

Selenium contributes to the normal thyroid function; in a 2024 review of 35 studies it lowered thyroid antibodies, and whether people felt better is unproven.^{3, 15} The reviews, in order:

• 2013: four trials, 463 people, unclear to high risk of bias; antibodies fell in three; pooling not possible; clinical relevance "unclear".⁵
• 2016: 16 trials; antibodies lower at 3, 6 and 12 months on levothyroxine; low-quality evidence; more side effects reported.¹⁴
• 2017: without levothyroxine, no change in TSH or quality of life; well-being gains in three trials with it could not be pooled.¹⁵
• 2024: antibodies lower with or without thyroid hormone; TSH slightly lower in people not taking it; side effects similar.³

Antibodies are a blood marker, not a symptom. In people with normal thyroids, five years of selenium lowered TSH by 0.066 mIU/L per 100 µg a day.⁴ In 1,202 US adults followed for an average of 7.7 years, 200 µg a day went with more type 2 diabetes than placebo (12.6 against 8.4 cases per 1,000 person-years).¹⁶ In 159 people with mild Graves' eye disease, six months of selenium improved quality of life against placebo;¹⁷ its use there is a specialist decision. This page gives no dose for Hashimoto's; that belongs with your GP or endocrinologist.

Which supplements interact with levothyroxine?

The NHS levothyroxine page names "antacids and other indigestion treatments", "calcium and iron supplements", "the herbal supplement St John's wort" and "biotin (vitamin B7) as it can affect your thyroid tests", and asks you to tell your doctor about any vitamins or supplements.² In small studies, calcium carbonate taken with levothyroxine raised average TSH from 1.6 to 2.7 mIU/L in 20 people,¹⁸ and ferrous sulfate from 1.6 to 5.4 mU/L in 14 people over 12 weeks.¹⁹ Your pharmacist will tell you how far apart to take them.

Can a supplement affect a thyroid blood test?

Biotin can, because many laboratory tests use biotin in their chemistry. When 6 healthy adults took 10 mg a day for a week, 9 of 23 biotin-based tests, on a panel that included TSH and thyroid hormones, gave falsely high or low results.²⁰ The European Thyroid Association's 2026 guideline lists biotin among causes of misleading thyroid results.²¹ Tell whoever orders the test what you take; pausing is their call.

Can vitamin D, magnesium, B12 or iron affect your thyroid?

None carries a thyroid wording.¹ "Vitamin D contributes to the maintenance of normal bones." "Magnesium contributes to a reduction of tiredness and fatigue." "Vitamin B12 contributes to the reduction of tiredness and fatigue." "Iron contributes to normal formation of red blood cells and haemoglobin." Each is covered in the Wellgard vitamins and minerals guide. B12 for women covers deficiency signs, one cause of lingering tiredness.

When should you see a GP about your thyroid?

The NHS lists tiredness, feeling cold, weight gain, constipation, low mood and dry skin for an underactive thyroid,⁷ and nervousness, difficulty sleeping, heat sensitivity, weight loss, palpitations and a swollen neck for an overactive one, where about 4 in 5 people have Graves' disease.²² A blood test settles it; a supplement is never the first step.

What we don't know

• No trial in the 2013 review measured whether selenium changed health-related quality of life or the levothyroxine dose.
• The NHS gives one iodine figure for all adults and none for people with autoimmune thyroid disease.
• The national survey measured selenium intake in UK women, not their thyroid function.
• No pooled analysis here compared selenium forms; trials used selenomethionine, sodium selenite or selenium yeast.

Frequently asked questions

What supplements should I avoid with a thyroid condition?

On levothyroxine, the NHS list: antacids, calcium and iron supplements, St John's wort, and biotin because of thyroid tests.² With any thyroid condition, high-dose iodine and seaweeds such as kelp can tip the thyroid either way.¹¹ Ask your pharmacist first.

Can I take levothyroxine and selenium together?

Selenium is not on the NHS levothyroxine list, and several Hashimoto's trials gave it alongside levothyroxine.¹⁴ Tell your doctor, and stay within the NHS 350 µg supplement line.¹²

Can I take vitamin D3 with my thyroid medication?

Vitamin D, as D3 or D2, is not on the NHS levothyroxine list.² What not to take with vitamin D covers the medicines that do matter.

Is iodine good for an underactive thyroid?

Iodine contributes to the normal production of thyroid hormones and normal thyroid function, but iodine shortage is a rare cause of an underactive thyroid in the UK, where Hashimoto's is the main one.⁷ People with thyroid disease are more susceptible to problems from extra iodine,¹⁰ so ask your doctor first.

What does a thyroid peroxidase antibody test show?

Thyroid peroxidase (TPO) antibodies are a blood marker of autoimmune thyroid disease such as Hashimoto's, and a main measure in the selenium trials.^{3, 14} The NHS diagnoses an underactive thyroid with TSH and T4 blood tests.⁷

Sources

1. Regulation (EU) No 432/2012, GB text, annex: authorised health claims. Read 13 October 2026. legislation.gov.uk.
2. NHS. Levothyroxine. Page last reviewed 6 July 2026. nhs.uk.
3. Huwiler VV, et al. Pooled analysis of 35 randomised trials, 2,358 people for antibodies. Thyroid, 2024. PubMed 38243784.
4. Winther KH, et al. Randomised trial, 491 adults, 5 years. European Journal of Endocrinology, 2015. PubMed 25740851.
5. van Zuuren EJ, et al. Systematic review, 4 randomised trials, 463 people. Cochrane Database of Systematic Reviews, 2013. PubMed 23744563.
6. Regulation (EU) No 1169/2011, GB text, article 7. Read 13 October 2026. legislation.gov.uk.
7. NHS. Underactive thyroid (hypothyroidism). Page last reviewed 28 April 2025. nhs.uk.
8. NHS. Vitamins and minerals: iodine. Page last reviewed 3 August 2020 (review overdue). nhs.uk.
9. National Diet and Nutrition Survey 2019 to 2023, tables 5.43 and 5.44. National survey, 863 women aged 19 to 64, published 11 June 2025. gov.uk.
10. Leung AM, Braverman LE. Narrative review. Nature Reviews Endocrinology, 2014. PubMed 24342882.
11. Farebrother J, et al. Narrative review. Annals of the New York Academy of Sciences, 2019. PubMed 30891786.
12. NHS. Vitamins and minerals: others (selenium). Page last reviewed 3 August 2020 (review overdue). nhs.uk.
13. EFSA NDA Panel. Upper intake level for selenium. Scientific opinion, January 2023. doi 10.2903/j.efsa.2023.7704.
14. Wichman J, et al. Pooled analysis of 16 controlled trials. Thyroid, 2016. PubMed 27702392.
15. Winther KH, et al. Pooled analysis of 9 controlled trials. Endocrine, 2017. PubMed 27683225.
16. Stranges S, et al. Randomised trial, secondary analysis, 1,202 adults, average 7.7 years. Annals of Internal Medicine, 2007. PubMed 17620655.
17. Marcocci C, et al. Randomised trial, 159 people, 6 months. New England Journal of Medicine, 2011. PubMed 21591944.
18. Singh N, et al. Prospective study, 20 people, 3 months. JAMA, 2000. PubMed 10838651.
19. Campbell NR, et al. Uncontrolled trial, 14 people, 12 weeks. Annals of Internal Medicine, 1992. PubMed 1443969.
20. Li D, et al. Non-randomised crossover study, 6 adults, 7 days. JAMA, 2017. PubMed 28973622.
21. Campi I, et al. Practice guideline. European Thyroid Journal, 2026. PubMed 42466561.
22. NHS. Overactive thyroid (hyperthyroidism). Page last reviewed 11 April 2023 (review overdue). nhs.uk.

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