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Nutrients and interactions

Supplements if you take levothyroxine

The main problem with levothyroxine is not depletion but absorption: calcium, iron, magnesium, soy and fiber all bind it in the gut and cut how much reaches your blood. The fix is timing, usually at least 4 hours apart, and a routine you keep the same so your dose stays stable. Picks are supplements you may still need, ranked by evidence and with the spacing that keeps them from undermining treatment. This page is about nutrients and interactions: never change your levothyroxine dose without your prescriber, and tell them when you start or stop any of these.

This page covers nutrients and supplement interactions. Never stop or change a prescribed medication without your prescriber, and check any new supplement with your pharmacist.

Evidence last reviewed: October 2026

Situational

OptionEvidenceBest forThe honest note
Selenomethionine Moderate evidence Hashimoto's antibodies, low selenium intake A meta-analysis of 35 trials found selenium lowered thyroid peroxidase antibodies, but free T4 and T3 did not change, and antibodies are a lab number rather than a symptom. Trials used 100–200 mcg a day; stay under the 400 mcg upper limit from all sources. View
Calcium Citrate Moderate evidence needed calcium, 4 hours apart In 20 people on stable levothyroxine, taking 1,200 mg of calcium as carbonate with the tablet raised TSH, and 20% went above the normal range. Separate any calcium by at least 4 hours, and supplement only if food falls short. View
Iron Bisglycinate Moderate evidence confirmed iron deficiency only Iron binds levothyroxine and markedly reduces absorption, so take it at least 4 hours apart. Supplement only with a confirmed low ferritin; 14–30 mg of elemental iron a day is the studied range for this gentler form. View
Magnesium Moderate evidence evening dose, far from levothyroxine Magnesium also reduces levothyroxine absorption. If you take it, take levothyroxine on waking and magnesium in the evening, at least 4 hours apart, and keep the routine constant. View

Skip, or avoid with this medication

  • Thyroid Support Formulas Limited evidence

    In testing, nine of ten products contained real T3, which adds to your prescribed dose and can cause thyrotoxicosis, while their minerals block levothyroxine.

  • Biotin (Vitamin B7) Limited evidence

    At 5–10 mg a day it distorts thyroid immunoassays and can mimic Graves' disease on paper, which can mislead dose decisions; stop it 48–72 hours before blood tests.

  • Ashwagandha Moderate evidence

    It can raise thyroid hormone levels, and case reports describe thyrotoxicosis, so it can push someone on replacement into overtreatment.

  • Kelp (Iodine Source) Limited evidence

    Iodine content varies more than 100-fold, and excess iodine can cause both hypothyroidism and hyperthyroidism, especially with Hashimoto's.

  • L-Carnitine Moderate evidence

    Carnitine acts as a peripheral antagonist of thyroid hormone and can blunt your replacement dose.

  • Soy Isoflavones Moderate evidence

    Soy reduces levothyroxine absorption and can destabilize a well-controlled dose; if you use it, separate by at least 4 hours and keep intake consistent.

Our verdict

Take levothyroxine on an empty stomach and keep calcium, iron, magnesium, multivitamins, soy and fiber supplements at least 4 hours away. Stop biotin 48–72 hours before thyroid blood tests, and avoid thyroid support formulas, which often contain real thyroid hormone.

Key studies