Thyroid Medication Interactions: What You Need to Know

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If you have a thyroid condition and are taking a prescription treatment, you should know that some medications can interact with your thyroid drugs. Whether you are taking a thyroid hormone replacement drug like levothyroxine or natural desiccated thyroid for hypothyroidism, or an antithyroid drug for hyperthyroidism, learn about the potential interactions.

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Anticoagulants and thyroid treatments

Anticoagulants like warfarin (Coumadin) and apixaban (Eliquis) interact with drugs for both hypothyroidism or hyperthyroidism.

Thyroid hormone replacement drugs like levothyroxine — used to treat hypothyroidism — increase the effect of anticoagulants, which can then increase your risk of bleeding.

Antithyroid medications like methimazole (Tapazole) and propylthiouracil (PTU) — used to treat hyperthyroidism — can reduce the effectiveness of anticoagulants, putting you at risk of blood clots.

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Estrogen drugs and thyroid hormone replacement

Drugs that contain any form of estrogen — birth control pills, patches, and pellets; and hormone replacement pills, creams, patches, and pellets — can make your thyroid hormone replacement less effective. If you are on any treatment that contains estrogen, such as conjugated estrogen or estradiol, be aware that these drugs make your thyroid treatment less effective.

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Diabetes drugs and thyroid hormone replacement

Levothyroxine and thyroid hormone replacement drugs like natural desiccated thyroid can interfere with your blood glucose control by raising blood sugar levels. The interference can reduce the effectiveness of diabetes treatments such as glipizide (Glucotrol), glyburide (DiaBeta), insulin (Lantus, Humalog), metformin (Glucophage), nateglinide (Starlix), and rosiglitazone (Avandia).

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Drugs that block absorption of thyroid hormone

Some drugs reduce the absorption of your thyroid hormone drugs. They should be taken apart from thyroid drugs. Consult your doctor or pharmacist for specific timeframes. The drugs that interfere with thyroid medication absorption include:

  • Prescription calcium supplements
  • Prescription iron supplements, such as Feosol and Slow Fe
  • Cholesterol-lowering drugs such as cholestyramine (Questran), colestipol (Colestid) and colesevelam (Welchol)
  • The heart rhythm drug amiodarone (Cordarone)

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More drugs that block absorption of thyroid hormone

Other drugs that interfere with thyroid medication absorption include:

  • The ulcer drug sucralfate (Carafate)
  • The antibiotic ciprofloxacin (Cipro) and other quinolone antibiotics, such as levofloxacin (Levaquin), lomefloxacin (Maxaquin), moxifloxacin (Avelox), norfloxacin (Noroxin), and ofloxacin (Floxin)
  • The osteoporosis drug Raloxifene (Evista)
  • The kidney drug sevelamer (Renagel, Renvela)

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Drugs that make levothyroxine less effective

Some drugs make your levothyroxine less effective. They include:

  • Antiepileptic drugs, such as carbamazepine (Tegretol), oxcarbazepine (Trileptal), phenobarbital (Luminal, Solfoton), primidone (Mysoline), and phenytoin (Dilantin)
  • Antimicrobial drugs, such as rifampin (Rifadin), efavirenz (Sustiva), nevirapine (Viramune), rifabutin (Mycobutin) and rifapentine (Priftin)
  • Leukemia drug imatinib (Gleevec)
  • Selective serotonin reuptake inhibitor (SSRI) antidepressant sertraline (Zoloft)

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Over-the-counter supplements

Some vitamins and supplements can interfere with your thyroid hormone replacement medication, including:

  • Antidepressant herbal supplement St. John's Wort
  • Iron supplements and multivitamin supplements with iron
  • Calcium supplements and antacids, such as Tums or Rolaids
  • Magnesium supplements, including magnesium hydroxide and magnesium oxide
  • Soy isoflavones
  • Alpha-lipoic acid
  • L-carnitine and acetyl-L-carnitine

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Antithyroid drug interactions

If you are taking the antithyroid drugs methimazole (Tapazole) or propylthiouracil (PTU) for hyperthyroidism, your medication can interact with other drugs, including:

  • Deferiprone (Ferriprox) — this iron overload drug can lower your white blood cell count and increase the risk of dangerous infection
  • Leflunomide (Arava) — this rheumatoid arthritis drug increases your risk of liver problems
  • Clozapine (Clozaril) — Methimazole increases the effects of this antipsychotic drug

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More antithyroid drug interactions

Other antithyroid drug interactions include:

  • Fluphenazine (Prolixin) — this antipsychotic drug increases your risk of agranulocytosis, a rare but serious white blood cell disorder
  • Lomitapide (Juxtapid) — this cholesterol-lowering drug increases your risk of liver problems
  • Mipomersen (Kyamro) — this cholesterol-lowering drug increases your risk of liver problems
  • For more information on methimazole’s interactions, Drugs.com has a complete listing.

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For more information on interactions

You may also want to research drug interactions at the Drugs.com Interaction Checker. If you have concerns about interactions, discuss them with your healthcare provider before making any changes to your treatment. Your pharmacist can also be a helpful resource regarding potential drug and supplement interactions.