Drug class·Thyroid Hormones·Reviewed June 9, 2026
Levothyroxine and supplements: timing is almost everything.
Levothyroxine is the classic timing drug: calcium, iron, magnesium, and fiber all bind it in the gut and can quietly blunt the dose. Biotin does not touch the drug at all but distorts the lab tests used to manage it. Every documented pairing and the separation windows are below.
Thyroid Hormones at a glance.
A quick, data-grounded summary. The per-drug tables are below.
Across the 2 thyroid hormones in the NutriStack database (Levothyroxine and Liothyronine), 20 substances have a documented interaction. Interactions are documented per drug, and members of a class do not all behave the same, so check your exact medication below. None of this replaces your prescriber's advice.
The thyroid hormones we track.
Open any medication for its own full interaction and depletion guide.
Substances that interact with thyroid hormones.
Merged across the class, highest severity first. The 'Documented for' column shows which members carry the interaction record; an absent drug means no documented record, not proven safety.
| Substance | Interaction | Documented for | What happens |
|---|---|---|---|
| Ashwagandha | Caution | All 2 thyroid hormones | Levothyroxine: Ashwagandha has been associated with increased thyroid hormone levels and rare thyrotoxicosis reports. When combined with levothyroxine,... |
| Vitamin B7 | Caution | All 2 thyroid hormones | Levothyroxine: High-dose biotin (>5mg/day) interferes with streptavidin-biotin immunoassays used to measure TSH and free T4. This creates FALSELY NORMAL... |
| Biotin | Caution | Levothyroxine | Biotin (vitamin B7) does not pharmacologically interact with levothyroxine, but it causes clinically dangerous interference with thyroid... |
| DIM | Caution | Levothyroxine | DIM (diindolylmethane) may affect thyroid hormone metabolism by inducing phase II conjugation enzymes. It can alter the estrogen-thyroid... |
| Calcium | Timing Sensitive | Levothyroxine | Calcium supplements significantly reduce levothyroxine absorption by 20-25% through chelation in the gastrointestinal tract. This can lead... |
| Chromium | Timing Sensitive | Levothyroxine | Chromium supplements, particularly chromium picolinate, have been shown in a small human absorption study to reduce levothyroxine... |
| Coenzyme Q10 Ubiquinol | Timing Sensitive | Levothyroxine | CoQ10 may have a minor effect on levothyroxine absorption if taken concurrently. While the interaction is not well-studied, the general... |
| Coffee | Timing Sensitive | Levothyroxine | Coffee significantly reduces levothyroxine absorption when consumed within 60 minutes of taking the medication. Chlorogenic acids and... |
| Iron | Timing Sensitive | Levothyroxine | Iron supplements form an insoluble complex with levothyroxine in the GI tract, reducing thyroid hormone absorption by up to 75% in some... |
| Iron Bisglycinate | Timing Sensitive | Levothyroxine | Iron bisglycinate contains elemental iron and should be handled like other oral iron supplements around levothyroxine. Iron can bind... |
| Magnesium Citrate | Timing Sensitive | Levothyroxine | Magnesium-containing products can interfere with levothyroxine absorption when taken together. The best evidence is for magnesium antacids... |
| Magnesium Glycinate | Timing Sensitive | Levothyroxine | Magnesium supplements, particularly magnesium oxide and magnesium hydroxide (antacid forms), can reduce levothyroxine absorption by... |
| Magnesium L-Threonate | Timing Sensitive | Levothyroxine | Magnesium L-threonate has not been studied directly with levothyroxine, but it supplies magnesium ions that may interfere with thyroid... |
| Magnesium Malate | Timing Sensitive | Levothyroxine | Magnesium malate may reduce levothyroxine absorption if taken at the same time. Direct studies with this exact magnesium salt are lacking,... |
| Magnesium Taurate | Timing Sensitive | Levothyroxine | Magnesium taurate may interfere with levothyroxine absorption when taken together. This is an extrapolation from magnesium-containing... |
| Psyllium Husk | Timing Sensitive | Levothyroxine | Fiber supplements, particularly psyllium, can bind levothyroxine in the gastrointestinal tract and reduce its absorption. Studies have... |
| Soy Isoflavones | Timing Sensitive | Levothyroxine | Soy isoflavones (genistein and daidzein) can reduce levothyroxine absorption by binding to the medication in the gastrointestinal tract,... |
| Zinc | Timing Sensitive | Levothyroxine | Zinc supplements may reduce levothyroxine absorption through chelation in the gastrointestinal tract. While the evidence is less robust... |
| Zinc Carnosine | Timing Sensitive | Levothyroxine | Zinc carnosine supplies zinc, a divalent mineral that may interfere with levothyroxine absorption if taken at the same time. Direct... |
| Zinc Picolinate | Timing Sensitive | Levothyroxine | Zinc picolinate may interfere with levothyroxine absorption if taken together. The evidence is weaker than for calcium or iron, but... |
Common thyroid hormones questions.
Quick answers drawn from the tables above.
Do all thyroid hormones interact with supplements the same way?
No. Interactions are documented per drug, and the 2 thyroid hormones in the database differ in how they are absorbed and cleared. The class table above shows which members carry each record; a drug without a record is undocumented, not proven safe.
Is it safe to take Ashwagandha with thyroid hormones?
NutriStack classifies the Ashwagandha and Levothyroxine pairing as caution: Ashwagandha has been associated with increased thyroid hormone levels and rare thyrotoxicosis reports. When combined with levothyroxine, it may contribute to... The interaction is documented for 2 of the 2 thyroid hormones, and severity differs by drug. Always confirm with your prescriber.
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