Is Your Multivitamin Making Your Hashimoto’s or Hypothyroidism Worse?

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When you’re managing a thyroid condition like Hashimoto’s thyroiditis or hypothyroidism, you’re probably already taking levothyroxine or another thyroid hormone replacement drug. You may also be taking a multivitamin. After all, your body needs nutrients to function correctly, and a daily multivitamin promises a convenient safety net.

But did you know that your daily multivitamin could interfere with your thyroid function or medication or worsen your underactive thyroid? Certain ingredients—like iodine, iron, calcium, and soy—can interfere with thyroid function or thyroid medication. That means your efforts to support your health could worsen your symptoms or interfere with your treatment.

Let’s explore how common supplement ingredients can impact your thyroid health and what you should consider before popping your daily multivitamin.

Your thyroid is a small gland at the base of your neck that plays a significant role in your health. It produces hormones regulating metabolism, energy levels, mood, heart rate, and even menstrual cycles.

In hypothyroidism, your thyroid doesn’t produce enough hormones. In Hashimoto’s disease, the most common cause of hypothyroidism in the U.S., your immune system mistakenly attacks your thyroid, slowly impairing its ability to function.

Managing these conditions often requires thyroid hormone replacement therapy, lifestyle changes, and careful attention to nutrient intake. That’s where things can get complicated—because not all nutrients are helpful, and some can even interfere with your thyroid medication or worsen your symptoms.

Iodine is essential for thyroid hormone production. In fact, your thyroid needs iodine to make T3 (triiodothyronine) and T4 (thyroxine). But here’s the twist. In areas with iodized salt, most people get adequate iodine from their diet, and supplements may push levels into a range that worsens your thyroid condition, particularly autoimmune Hashimoto’s thyroiditis.

Iodine supplements are also associated with what’s known as an “iodine crash,” where iodine supplementation initially boosts energy but later causes fatigue, body aches, and autoimmune flare-ups by increasing inflammation and lymphocyte infiltration in the thyroid.

Many multivitamins contain 150 to 290 mcg of iodine, close to or exceeding the Recommended Daily Allowance (RDA) of 150 mcg for adults. While that’s generally safe for the average person, it may be too much for someone with autoimmune thyroid disease. For Hashimoto’s patients, even modest amounts may worsen symptoms, as the immune system is pushed into overdrive and thyroid inflammation increases.

Even small doses of iodine (250 mcg daily) have been shown to cause subclinical hypothyroidism or hyperthyroidism in predisposed individuals. This phenomenon, known as the Wolff–Chaikoff effect, occurs when high iodine levels temporarily suppress thyroid hormone production.

What can you do?

• Avoid multivitamins with iodine unless your doctor specifically recommends it.

• If an iodine deficiency is suspected, work with a practitioner to test your iodine levels and determine a safe dosage for supplementation.

• Watch for hidden iodine in kelp or seaweed-based supplements.

THYROID GUIDE

Iodine and the thyroid

Iodine is a crucial component of the molecular structure of thyroid hormones. Learn more about the connection between Hashimoto's and iodine.

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Iron is crucial in oxygen transport, energy production, and even thyroid hormone synthesis. If you’re iron-deficient, your thyroid may struggle. But too much iron—or taking it at the wrong time—can be problematic.

Iron deficiency is common in people with hypothyroidism and can impair thyroid function by reducing thyroid peroxidase (TPO) enzyme activity. TPO which helps produce the key thyroid hormones thyroxine (T4) and triiodothyronine (T3). Animal studies show that iron-deficient diets can reduce TPO activity by 33% to 56%.

Many women with an underactive thyroid gland also have low ferritin (a marker of iron storage), which can lead to fatigue, hair loss, and cold intolerance.

But here’s the catch: iron supplements can interfere with the absorption of thyroid hormone replacement drugs.

That means your thyroid hormone levels could remain low, even if you’re taking your medication faithfully.

What can you do?

·        Space iron supplements at least 4 hours apart from your thyroid medication. 

• Before supplementing, ask your doctor to test your ferritin, serum iron, and transferrin saturation.

• Avoid multivitamins with iron unless you’re deficient.

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Calcium is another mineral that sounds harmless—after all, it’s essential for strong bones, nerve function, and muscle contractions. But when it comes to thyroid health, calcium can interfere with your medication like iron does.

Calcium binds to thyroid medication in your gut and prevents it from being absorbed. Calcium supplements and calcium-fortified foods can reduce the absorption of levothyroxine – the most commonly prescribed thyroid medication – by 30 to 40% if taken simultaneously.

That means even if you take your medicine, your body may not benefit from it fully if calcium is around.

This is especially important for:

• People taking calcium-fortified antacids, juices, or dairy-based supplements

• Post-menopausal women who take calcium for bone health

• Anyone taking multivitamins with calcium in the morning

What can you do?

• Check your multivitamin for calcium. If it’s included, ensure it’s under 500 mg and take the multivitamin at a different time than your thyroid medication.
• Take your thyroid medication on an empty stomach at least 30–60 minutes before food or supplements.
• Avoid calcium-rich foods or supplements within 4 hours of your medication.
• Consider switching to a multivitamin without calcium or taking it at night.

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Soy often appears in unexpected places, including multivitamins, protein powders, and supplement fillers. While soy has health benefits, it can be problematic for people with thyroid issues.

Soy contains isoflavones, which are plant compounds that can block thyroid peroxidase (TPO) activity—the enzyme we mentioned earlier that’s essential for making thyroid hormones.

While the effect is usually small in healthy individuals, it can be more significant in people with low thyroid function or autoimmune disease.

Soy also interferes with the absorption of thyroid hormone replacement medications like calcium and iron.

What can you do?

• Check dietary supplement labels for soy-derived ingredients, including soy protein, soybean oil, or soy lecithin.

• Avoid taking soy-based products around the time you take your thyroid medication.

• Consider switching to soy-free supplements.

There are other supplement ingredients to be aware of due to their ability to affect your thyroid.

Selenium

While selenium supports antioxidant defenses and may lower TPO antibodies, excessive amounts (over 400 mcg/day from supplements and/or foods combined) can be toxic. At the same time, appropriate amounts of selenium supplementation, especially when combined with myo-inositol, show promise in reducing thyroid antibodies and improving TSH levels.

Vitamin D

A vitamin D deficiency is linked to higher thyroid antibody levels. However, mega-doses (over 10,000 IU/day) aren’t proven to help Hashimoto’s and may interact with calcium metabolism.

B vitamins

Many multivitamins use synthetic forms (e.g., cyanocobalamin for B12), which require conversion to active forms. For those with genetic variations like MTHFR, which is more common in people with Hashimoto’s, methylated B vitamins like methylcobalamin and methylfolate are better absorbed.

There are some benefits of taking a multivitamin. A multivitamin:

• Can correct mild deficiencies in nutrients like selenium, zinc, or B vitamins

• May help with energy, mood, and immune support

• Is a convenient source of multiple nutrients.

At the same time, there are some potential downsides to multivitamins, given that they:

• May contain potentially problematic ingredients like iodine, soy, iron, or calcium

• Can interfere with the effectiveness of thyroid medication

• Have dosages that are often too low or too high to be truly therapeutic

If you still want the convenience of a multivitamin, here are some tips for choosing one that supports your thyroid rather than harming it.

• Free of iodine (unless your provider advises otherwise). Even small amounts (≥100% daily value) can flare Hashimoto’s.
  No iron or calcium, or at clearly labeled and including very low amounts.
  Soy-free
• Contains active B vitamins, like B12 and B6, and uses bioavailable forms like methylcobalamin and pyridoxal-5-phosphate (B6).
• Contains selenium to support T4 to T3 conversion and reduce thyroid antibodies
• Includes zinc to help with hormone production and immune function
• Contains vitamin D to support immune health and reduce inflammation

You can often find these in thyroid-support formulas, like Paloma Health’s Daily Thyroid Care supplement, or as individual supplements—giving you more control over what you’re taking.

Finally, it’s essential to consider the timing. Ideally, you should take your multivitamin at night or several hours after your thyroid medication to avoid interference.

Daily Thyroid Care

$34/month

The reinvented multivitamin that works smarter to help fill the gaps in your thyroid health journey

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Vegan and free of gluten, GMOs, dairy and iodine

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One of the key advantages of being a Paloma Health patient is access to personalized, thyroid-savvy care—including expert guidance on supplements. If you have Hashimoto’s or an underactive thyroid, choosing the wrong multivitamin or dietary supplement can actually do more harm than good. Ingredients like iodine, iron, calcium, and soy isoflavones—common in many formulas—can disrupt thyroid function or interfere with your medication.

As a Paloma patient, you don’t have to guess. Your care team helps you:

• Avoid unnecessary or harmful ingredients like iodine unless specifically needed.

• Time iron and calcium properly to prevent absorption issues with your thyroid meds.

• Understand how and when to consume soy to minimize interference.

With access to convenient thyroid testing using Paloma’s home thyroid test kit to monitor thyroid hormone levels and expert clinical support from Paloma’s providers, you can make confident, informed choices. Instead of a one-size-fits-all approach, you’ll get a supplement strategy customized to your thyroid status, symptoms, and health goals—because your thyroid deserves nothing less!
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• Common multivitamin ingredients like iodine, iron, calcium, and soy can interfere with thyroid function or reduce the effectiveness of thyroid medication.
• Iodine, while essential for thyroid hormone production, can worsen Hashimoto’s symptoms and trigger autoimmune flares when taken in excess.
• Iron deficiency is common in hypothyroidism, but iron supplements can block the absorption of thyroid medication if not spaced apart properly.
• Calcium can reduce thyroid medication absorption by up to 40% when taken at the same time, making proper timing critical.
• Soy contains compounds that may suppress thyroid hormone production and also interfere with medication absorption.
• Other nutrients like selenium, vitamin D, and B vitamins can support thyroid health if used in the correct forms and dosages.
• Multivitamins can be helpful but should be free of iodine, soy, calcium, and iron unless specifically recommended.
• A personalized approach—timing dietary supplements properly and selecting thyroid-friendly formulations—can help support thyroid health without compromising treatment.