Your thyroid needs several nutrients to work normally. Iodine is the building block of thyroid hormones, selenium helps activate them and protect the gland, and iron and zinc are part of the enzymes that make them [1][2][7]. Most adults need 150 mcg of iodine and 55 mcg of selenium a day, and both have upper limits, so more is not better [1][2].
If you have a thyroid condition or take thyroid medication, talk to your doctor before taking iodine, kelp or selenium. Extra iodine can change thyroid function in some people, and minerals such as iron and calcium can affect how well thyroid medicine is absorbed [1][3][4].
Key Takeaways
- Iodine is an essential part of the thyroid hormones T4 and T3. Adults need 150 mcg a day, and the upper limit is 1,100 mcg from all sources.
- Selenium helps convert T4 into active T3. Adults need 55 mcg a day; the upper limit is 400 mcg, and a single ounce of Brazil nuts can exceed it.
- Iron and zinc also take part in thyroid hormone metabolism. Low iron can blunt the benefit of getting enough iodine.
- For most people who get enough iodine, normal amounts of soy, broccoli or cabbage are not a concern for the thyroid.
- Research on gluten-free diets, selenium and vitamin D for thyroid health is small and mixed, so check with your clinician before making big changes.
Evidence grade: Strong. The roles of iodine and selenium in thyroid hormone production and their intake levels are established NIH reference values; research on supplements and special diets for thyroid health is far more limited.
How Nutrients Work Together in the Thyroid
The thyroid makes two main hormones, thyroxine (T4) and triiodothyronine (T3), which help regulate metabolism, protein synthesis and enzyme activity throughout the body [1]. Its activity is steered by thyroid-stimulating hormone (TSH) from the pituitary gland [1].
A 2002 review in Thyroid explains that several minerals are essential for normal thyroid hormone metabolism, including iodine, iron, selenium and zinc, and that falling short on more than one at a time can impair thyroid function [7]:
- Iodine is built directly into T4 and T3 [1].
- Iron is needed by thyroid peroxidase, a heme-containing enzyme that helps make thyroid hormone. Iron deficiency lowers its activity [7].
- Selenium is part of the deiodinase enzymes that turn T4 into active T3, and of antioxidant enzymes that help protect the gland [2][7].
- Zinc is listed among the trace elements essential for normal thyroid hormone metabolism [7].
Thyroid Nutrients at a Glance
| Nutrient | Thyroid role | Adult daily need | Adult upper limit | Good food sources |
|---|---|---|---|---|
| Iodine | Part of T4 and T3 | 150 mcg | 1,100 mcg | Cod, dairy, eggs, iodized salt, seaweed [1] |
| Selenium | Converts T4 to T3; protects thyroid cells | 55 mcg | 400 mcg | Brazil nuts, tuna, sardines, shrimp, meat [2] |
| Iron | Needed by thyroid peroxidase to make hormone | 8 mg (men and women 51+); 18 mg (women 19 to 50) | 45 mg | Meat, seafood, beans, fortified grains [3] |
| Zinc | Involved in thyroid hormone metabolism | 11 mg (men); 8 mg (women) | 40 mg | Oysters, meat, seafood, dairy, beans, nuts [6] |
| Vitamin D | No direct hormone role; involved in immune function | 600 IU (800 IU over 70) | 4,000 IU | Fatty fish, egg yolks, fortified milk [5] |
Needs are higher during pregnancy and breastfeeding for iodine (220 and 290 mcg) and selenium (60 and 70 mcg) [1][2].
Iodine: The Core Building Block
Iodine is the one nutrient the thyroid cannot work without. When intake is too low, TSH stays high and the gland can enlarge as it tries to trap more iodine [1]. Seafood, eggs, milk and milk products are among the best food sources, along with iodized salt. A quarter teaspoon of iodized salt supplies about 78 mcg, and 3 ounces of baked cod about 146 mcg [1]. Seaweed varies enormously, from 16 to nearly 3,000 mcg per gram [1].
We cover daily needs, food sources, kelp and who may fall short in our full guide to iodine and thyroid function. The short version for this article: aim for the RDA, count every source, and do not assume that more iodine means better thyroid function, because excess iodine can cause some of the same problems as too little [1].
Selenium: Activating Thyroid Hormone
The thyroid keeps high selenium concentrations even when selenium intake is low [7]. Selenium-containing proteins help convert T4 into the more active T3 and help protect the gland from damage, including from excess iodide [2][7].
Most people get enough selenium from food. Brazil nuts are the richest source, averaging 544 mcg per ounce (6 to 8 nuts), although amounts vary widely; that is more than the 400 mcg upper limit in one handful. A 3-ounce serving of yellowfin tuna has about 92 mcg [2]. Selenium-only supplements often contain 100 to 400 mcg [2].
What selenium research shows
Selenium has been tested in people with autoimmune thyroid conditions. A 2013 Cochrane review found four small trials with 463 participants. In some, 200 mcg a day of selenomethionine lowered thyroid antibody levels, but the reviewers said the clinical meaning of that change was unclear and the evidence was too limited for confident decisions [8]. The NIH Office of Dietary Supplements describes the trial results as mixed [2]. These studies do not show that selenium supplements improve thyroid function in healthy people.
Iron and Zinc
Iron is easy to overlook in thyroid conversations. Iron deficiency reduces the activity of thyroid peroxidase, and in people who are low in both iodine and iron, correcting the iron improves how well iodine works [7]. The NIH also notes that iron deficiency may contribute to goiter [1]. Women of childbearing age need the most iron, 18 mg a day [3].
Zinc is another trace mineral that supports normal thyroid hormone metabolism [7]. Adults need 8 to 11 mg a day, and oysters, meat, seafood and dairy are the best sources [6]. Neither mineral is a reason to take high-dose supplements without a measured need; both have upper limits [3][6].
Vitamin D and the Thyroid
Vitamin D helps regulate immune function, among other roles [5]. A 2015 meta-analysis of 20 case-control studies found that people with autoimmune thyroid conditions had lower vitamin D levels than people without them, and were about three times as likely to be deficient [9]. Studies like these show an association, not cause and effect: they cannot tell whether low vitamin D contributes to thyroid problems or is a result of them.
The practical step is simply to meet the vitamin D RDA of 600 IU a day (800 IU over age 70) from fatty fish, egg yolks, fortified foods or a supplement, without going over 4,000 IU [5]. Our list of foods high in vitamin D can help.
Gluten, Soy and Other Food Questions
Gluten-free diets
Many people with autoimmune thyroid conditions try a gluten-free diet on their own. A 2023 meta-analysis of four small studies (87 people without celiac disease) found that about six months gluten-free was followed by a small drop in TSH; the decreases in thyroid antibodies did not reach statistical significance. The authors concluded the evidence is not yet enough to recommend the diet to everyone with the condition [10]. If you are considering it, talk to your clinician, and do not cut gluten before any celiac testing they may want to do.
Soy
Soy contains isoflavones, and soy is one of the foods that can act as a goitrogen, meaning it can interfere with iodine uptake [1]. A review of 14 trials found little evidence that soy foods or isoflavones affect thyroid function in healthy people who get enough iodine [11]. The NIH agrees that for most people with adequate iodine, reasonable amounts of goitrogen-containing foods such as soy, cabbage and broccoli are not a concern [1].
An overall eating pattern
No single "thyroid diet" has strong evidence behind it. A varied diet that includes seafood, dairy or another iodine source, eggs, legumes, nuts and plenty of vegetables covers most of the nutrients above. Our anti-inflammatory diet guide shows one balanced way to put that together. A registered dietitian can tailor it to you.
Where an Iodine Supplement Fits
Many multivitamin and mineral supplements already contain 150 mcg of iodine [1]. If you use a separate iodine product, check every label and add the amounts together.
OPA Advanced Iodine+ is a capsule formula with iodine from kelp, made to support normal energy metabolism. A two-capsule serving lists 300 mcg of iodine, which is 200% of the adult RDA. That is below the 1,100 mcg upper limit on its own [1], but it is more than many people need once food is counted, so total all your iodine sources, including any multivitamin, iodized salt and seaweed. The formula also contains a kelp and greens blend (alfalfa, chlorella and milk thistle) and an herbal and mineral blend (turmeric, reishi, guggul, red algae, hyssop and milk thistle extract). It contains fish and crustacean shellfish ingredients. It does not list selenium. If you have a thyroid condition or take any medication, ask your doctor before using it.
Safety and Interactions
Iodine
Too much iodine can cause some of the same effects as too little, including an enlarged thyroid, raised TSH and low thyroid hormone output [1]. People with autoimmune thyroid disease may react badly to iodine intakes that are safe for most people [1]. Stay under 1,100 mcg a day from all sources unless your doctor directs otherwise.
- Antithyroid medicines such as methimazole: high doses of iodine taken with them can have an additive effect and could lower thyroid hormone too far [1].
- ACE inhibitors (such as lisinopril) and potassium-sparing diuretics (such as spironolactone): potassium iodide taken with these can raise blood potassium [1].
Selenium
Long-term high intake causes selenosis. The most common signs are hair loss and brittle nails; garlic breath, a metallic taste, rash, nausea and fatigue can also occur [2]. Keep total intake under 400 mcg a day. The American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity [2].
Thyroid hormone medicine
- Iron supplements can reduce the absorption of levothyroxine. Product labels advise taking levothyroxine at least 4 hours apart from iron [3].
- Calcium carbonate supplements can also interfere with levothyroxine; its label advises separating them by 4 hours [4].
- Soy foods may reduce absorption and raise the dose of thyroid hormone some people need, though people taking it do not need to avoid soy [11]. Keep your soy intake steady and tell your doctor about big diet changes.
If you have a thyroid condition or take thyroid medication, talk to your doctor or pharmacist before taking iodine, kelp, selenium, iron or calcium supplements. Never change or stop thyroid medicine on your own.
Frequently Asked Questions
What nutrients does the thyroid need to work normally?
Iodine is the main one, since it is built into thyroid hormones. Selenium, iron and zinc are also needed for making and activating those hormones [1][2][7]. Most people get enough from a varied diet that includes an iodine source.
How much iodine and selenium do adults need?
Adults need 150 mcg of iodine and 55 mcg of selenium a day. The upper limits are 1,100 mcg for iodine and 400 mcg for selenium from all sources combined [1][2].
Does a gluten-free diet support thyroid health?
The evidence is limited. A 2023 meta-analysis of four small studies found a small change in TSH and no statistically significant change in thyroid antibodies, and the authors said it is not enough to recommend the diet to everyone [10]. Talk to your clinician before trying it.
Is soy bad for the thyroid?
For healthy people who get enough iodine, a review of 14 trials found little evidence that soy affects thyroid function [11]. People taking thyroid hormone should keep soy intake consistent and tell their doctor, because soy may affect absorption [11].
Should I take selenium for thyroid support?
Most people get enough selenium from food, and too much is harmful [2]. Trials of selenium supplements have had mixed results, so ask your doctor first, especially if you have a thyroid condition or are pregnant [2][8].
Can I take an iodine supplement if I take thyroid medication?
Only with your doctor's approval. Extra iodine can affect thyroid function in some people and can interact with antithyroid medicines [1]. Your doctor can check whether you need any iodine beyond food and a multivitamin.
Sources
- NIH Office of Dietary Supplements. Iodine: Fact Sheet for Health Professionals.
- NIH Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals.
- NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals.
- NIH Office of Dietary Supplements. Calcium: Fact Sheet for Health Professionals.
- NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals.
- NIH Office of Dietary Supplements. Zinc: Fact Sheet for Health Professionals.
- Zimmermann MB, Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism: biochemistry and relevance to public health. Thyroid. 2002.
- van Zuuren EJ, Albusta AY, Fedorowicz Z, Carter B, Pijl H. Selenium supplementation for Hashimoto's thyroiditis. Cochrane Database Syst Rev. 2013.
- Wang J, Lv S, Chen G, et al. Meta-analysis of the association between vitamin D and autoimmune thyroid disease. Nutrients. 2015.
- Piticchio T, Frasca F, Malandrino P, et al. Effect of gluten-free diet on autoimmune thyroiditis progression in patients with no symptoms or histology of celiac disease: a meta-analysis. Front Endocrinol (Lausanne). 2023.
- Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
