How to Choose a Thyroid Supplement: 7 Key Criteria

Written by Dr. Arnaud BERNARD and reviewed by the Phytocea Scientific Committee. Information checked on 21 September 2026.

Key takeaways

• Always read the amounts for the daily serving, not just for one capsule.
• Check the units: 1 mg equals 1,000 µg. Confusing the two completely changes the comparison.
• For a multi-ingredient formula, check iodine, selenium and L-tyrosine first, followed by the cofactors.
• A clearly named form and traceable origin matter more than a long list with no amounts.
• Compare the daily cost, number of doses and compatibility with your medicines.

When choosing a thyroid supplement, treat the label as a technical specification. Check the daily serving, doses in µg or mg, the forms of iodine and selenium, number of capsules, allergens, daily cost and any duplicated ingredients. This method lets you compare two formulas without being swayed by the length of the ingredient list or the size of the promises.

The 60-second check

Put the pack in front of you and work through this checklist from top to bottom. If an essential detail is missing, make a note before moving to the next product.

You do not need to memorise dozens of thresholds. The aim of this first pass is to establish whether the label gives you enough information to make a decision.

Step 1: read the daily serving, not the dose per capsule

The serving changes the entire comparison. A brand may highlight “150 µg of iodine” when that amount requires three capsules. Another may provide the same dose in a single capsule. The ingredient looks identical, but the routine, pack duration and cost are not.

Example A. A fictional pack contains 60 capsules and recommends 2 capsules per day. Its label lists 150 µg of iodine and 60 µg of selenium per serving. The pack therefore lasts 30 days; one capsule provides only half the listed amounts.

Example B. Another pack contains 90 capsules and lists 50 µg of iodine per capsule, with a recommendation of 3 per day. The daily dose is also 150 µg, but the pack still lasts only 30 days. Comparing “90 capsules” with “60 capsules” without reading the serving would create a false impression of value.

For units, remember one reference point: 1 mg = 1,000 µg. Iodine and selenium are normally expressed in micrograms; tyrosine, zinc and iron often appear in milligrams. Never compare two figures directly until the units match.

Step 2: check the core active ingredients

Iodine: dose, source and context

Iodine is required for the normal production of thyroid hormones. On the label, check the daily amount and then the source: algae standardised for iodine, potassium iodide or another precisely named form. Iodine from algae should be standardised; the total weight of the algae alone does not tell you how much iodine you are taking.

More is not automatically better. French and European dietary reference values help put a dose in context, but your individual circumstances matter, especially during pregnancy or if you have an autoimmune condition or take thyroid medication. Our guide to iodine and thyroid health explains the sources and precautions.

Selenium: amount and stated form

Selenium contributes to normal thyroid function and to the protection of cells from oxidative stress. The label should state the daily amount in µg. Then check the form: selenomethionine, selenium-enriched yeast, selenite or selenate. These names describe different sources; on their own, they do not establish that one product is better suited to your circumstances.

Selenium also appears in multivitamins, antioxidant complexes and some hair supplements. Add up all your sources before choosing. For more detail, see our page on the forms and role of selenium.

L-tyrosine: amount and exact name

Tyrosine is an amino acid that, together with iodine, forms part of the structure of thyroid hormones. Look for “L-tyrosine” or “N-acetyl-L-tyrosine” on the pack, then check the amount per serving. Two products that simply display “tyrosine” on the front can provide very different doses.

Do not assume that a high dose necessarily suits your priority better. The formula still needs to make sense alongside your overall intake and personal circumstances. Our L-tyrosine ingredient guide covers its role and the main precautions.

Zinc, iron and B vitamins: the cofactors to check

Zinc and iron often appear in multi-ingredient formulas. Check the amount and stated form, then determine whether you already take a multivitamin. Iron calls for particular attention: you should not add multiple sources without knowing your total intake, and according to their instructions some thyroid medicines must be taken separately from it.

B vitamins can complement a formula, but they do not make up for missing information about the main active ingredients. A long series of vitamins at very small doses should not distract you from the iodine, selenium, tyrosine and daily cost.

Step 3: compare ingredient forms and traceability

Algal iodine or iodide? Algae provide an ingredient of natural origin, but its iodine content should be standardised and stable from one batch to the next. Iodide allows for a highly standardised dose. The useful criterion is therefore not “natural versus synthetic”; it is dose precision and the manufacturer's ability to document it.

Organic or mineral selenium? Selenomethionine and enriched yeasts are organic forms; selenite and selenate are mineral forms. Their absorption and metabolism differ. A serious brand names the form without implying that one word on the label settles the entire question of quality.

Zinc and iron. Gluconate, citrate, bisglycinate and fumarate are useful details for comparison. They should still be accompanied by the actual amount of elemental mineral provided. A figure referring to the total weight of a compound is not necessarily the amount of mineral used in dietary reference values.

Traceability extends beyond the nutrition table. Look for the responsible operator's address, batch number, best-before date, country of manufacture, allergens and capsule-shell ingredients. To verify vegan status, check the capsules, vitamin D and any excipients that might be animal-derived.

A batch certificate of analysis, when available, can confirm identity and content. Its absence from a public website does not automatically invalidate a product, but the brand should be able to answer a precise question about origin and testing. Be wary of vague labels invented by the seller and logos with no identifiable certifying body.

Step 4: calculate the true cost

The formula is simple:

Daily cost = pack price ÷ number of days of use.
Monthly cost = daily cost × 30.

Take a fictional €24 pack containing 60 capsules. At one capsule per day, it lasts 60 days: €24 ÷ 60 = €0.40 per day, or €12 for 30 days. At two capsules per day, it lasts 30 days: €0.80 per day, or €24 per month. The shelf price has not changed; the true cost has doubled.

Then factor in ease of use. A four-capsule formula may look appealing on paper, but be less practical if you regularly miss a dose. The best value comes from a formula whose dose you understand, whose cost you accept and which you can fit into your routine over time.

Comprehensive formula or single ingredient?

Choose the approach first, then the product.

• If you want to isolate a clearly identified intake, a single ingredient simplifies label reading and limits duplication.
• If you want iodine, selenium, tyrosine and cofactors in one routine, a comprehensive formula is more convenient.
• If you already take several supplements, add up their shared ingredients before introducing a complex.
• If your priority is still unclear, ask a healthcare professional to assess your diet, test results and medicines with you.

A comprehensive formula simplifies purchasing but multiplies the number of checks. A single ingredient is easier to monitor but serves only one purpose. To see how these differences appear in real products, read our thyroid supplement comparison.

Compatibility, timing and professional advice

A transparent label is not enough if its timing conflicts with a prescription. Iron and calcium can reduce levothyroxine absorption when taken too close together; the exact instructions depend on the medicine leaflet and the associated product. Coffee, soy and fibre can also affect absorption conditions.

Our page Levothyrox and supplements: how long should you leave between doses? brings together official reference points and two sample schedules. Do not change the dose or timing of a medicine without your doctor or pharmacist's agreement.

Ask for advice before taking iodine if you are pregnant, breastfeeding, have a diagnosed thyroid condition or your doctor is adjusting your prescription. Remember to mention biotin too: it can interfere with some thyroid laboratory results.

Your final 7-point checklist

1. I know the exact daily serving.
2. I distinguish µg from mg.
3. I can identify the doses of iodine, selenium and L-tyrosine.
4. The forms, origin and allergens are identifiable.
5. I have calculated the daily and monthly cost.
6. I have checked for duplicated ingredients across my other supplements.
7. I have checked compatibility and timing with my medicines.

See our thyroid supplement comparison to apply this checklist to ten formulas sold in France.

As an example of a detailed label, Thy-Ro publishes the amount of each active ingredient for two capsules: iodine 150 µg, selenium 56 µg, L-tyrosine 250 mg, zinc 7.6 mg, iron 7.5 mg, vitamins and botanical extracts.

This article is for informational purposes only and is not a substitute for professional medical advice. Consult a qualified healthcare professional if you have symptoms or before taking a dietary supplement, particularly if you are pregnant, breastfeeding or taking any medication.

Research and references

1. ANSES. Iodine: functions, dietary sources and dietary reference values.
2. ANSES. Selenium: functions and dietary reference values.
3. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for iodine. EFSA Journal. 2014;12(5):3660.
4. EFSA Panel on Dietetic Products, Nutrition and Allergies. Dietary Reference Values for selenium. EFSA Journal. 2014;12(10):3846.
5. Zimmermann MB, Boelaert K. Iodine deficiency and thyroid disorders. Lancet Diabetes Endocrinol. 2015;3(4):286-295.
6. Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256-1268.
7. Ventura M, Melo M, Carrilho F. Selenium and thyroid disease: from pathophysiology to use. Int J Endocrinol. 2017;2017:1297658.
8. National Institutes of Health, Office of Dietary Supplements. Iodine: Fact Sheet for Health Professionals.
9. National Institutes of Health, Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals.
10. Haute Autorité de Santé. Management of hypothyroidism in adults. 2023.
11. Liu H, et al. Medications and food interfering with the bioavailability of levothyroxine: a systematic review. Ther Clin Risk Manag. 2023;19:503-523.
12. Winther KH, Rayman MP, Bonnema SJ, Hegedüs L. Selenium in thyroid disorders. Front Endocrinol. 2020;11:486.