Levothyroxine Drug Interactions: What to Avoid and What to Tell Your Doctor

Levothyroxine has a narrow therapeutic index — even small changes in how much gets absorbed can shift your thyroid levels significantly. Many common medications, supplements, and foods can either reduce levothyroxine absorption or interfere with its metabolism. Here's a comprehensive guide to the most important interactions to know.

Medications That Reduce Levothyroxine Absorption

These medications bind to levothyroxine in the gut and prevent it from being absorbed. Always take levothyroxine at least 4 hours before or after these drugs:

Calcium supplements and antacids (calcium carbonate, calcium citrate, Tums, Rolaids) — extremely common interaction; can significantly reduce absorption if taken together

Iron supplements (ferrous sulfate, ferrous gluconate, iron-containing multivitamins) — iron chelates levothyroxine; separate by 4 hours

Proton pump inhibitors (PPIs) (omeprazole/Prilosec, pantoprazole/Protonix, lansoprazole/Prevacid, esomeprazole/Nexium) — reduce stomach acid, which is needed for levothyroxine dissolution and absorption; separate by 4 hours or consider Tirosint, which absorbs independently of stomach acid

Bile acid sequestrants (cholestyramine/Questran, colestipol/Colestid, colesevelam/Welchol) — dramatically reduce levothyroxine absorption; separate by at least 4 hours

Sevelamer (Renvela) — used in chronic kidney disease; can reduce levothyroxine absorption

Sucralfate (Carafate) — used for ulcers; reduces levothyroxine absorption

Orlistat (Xenical/Alli) — weight-loss medication that reduces fat absorption; also reduces levothyroxine absorption

Medications That Increase Levothyroxine Metabolism (May Need Higher Dose)

These medications increase the speed at which your body breaks down thyroid hormone, which can mean your current levothyroxine dose becomes inadequate:

Rifampin (Rifadin) — antibiotic for tuberculosis; potent inducer of thyroid hormone metabolism

Carbamazepine (Tegretol) — seizure medication; reduces circulating thyroid hormone levels

Phenytoin (Dilantin) — seizure medication; reduces protein binding of T4 and increases metabolism

Estrogens and oral contraceptives — estrogen increases thyroid-binding globulin (TBG), which binds more T4 and can make you functionally hypothyroid at your current dose; patients starting or stopping estrogen should have TSH rechecked

Medications Whose Effects Are Changed by Levothyroxine

Levothyroxine also affects how other medications work in your body:

Warfarin (Coumadin) — MAJOR interaction. Levothyroxine increases the anticoagulant effect of warfarin by increasing warfarin metabolism and sensitivity. Starting, stopping, or changing your levothyroxine dose requires close INR monitoring and possible warfarin dose adjustment.

Digoxin (Lanoxin) — Levothyroxine can reduce the effectiveness of digoxin; serum digoxin levels may decrease when hypothyroidism is corrected

Antidepressants — Sertraline (Zoloft) can increase levothyroxine requirements; tricyclic antidepressants may have increased or more rapid effects when combined with levothyroxine

Diabetes medications — Levothyroxine can alter blood sugar control; patients with diabetes may need adjusted medication doses when thyroid levels are corrected

Food and Drink Interactions

Coffee — Drinking coffee within 15-30 minutes of your levothyroxine dose can reduce absorption by up to 27%. Wait at least 30-60 minutes after your dose before drinking coffee.

Grapefruit juice — Can delay absorption and reduce bioavailability; avoid drinking grapefruit juice close to your levothyroxine dose.

Soy products — Soybean flour (found in infant formula) and soy products can bind levothyroxine and reduce absorption.

Walnuts and high-fiber foods — Can bind levothyroxine and reduce absorption; if you eat these regularly, take your medication well before consuming them.

What to Tell Every Doctor You See

Always tell every healthcare provider — including dentists, pharmacists, urgent care clinicians, and specialists — that you take levothyroxine. Key situations that warrant telling your prescriber:

Any time you start or stop a calcium, iron, or antacid supplement

Any time a new medication is added or stopped, especially PPIs, seizure medications, rifampin, or estrogen

When you become pregnant or start/stop hormone-containing contraceptives

When you switch between brands or manufacturers of levothyroxine

If you start taking warfarin — the interaction is major and requires frequent INR monitoring

For information on side effects that may indicate dose problems, see Levothyroxine Side Effects: What to Expect and When to Call Your Doctor.

If you're having trouble finding your levothyroxine at the pharmacy, medfinder.com helps locate which pharmacies near you have it in stock.