Levothyroxine (By mouth)


Levothyroxine (lee-voe-thye-ROX-een)

Treats hypothyroidism. Also treats an enlarged thyroid gland (goiter) and thyroid cancer.

Brand Name(s)

Synthroid, Levoxyl, Levothroid, Tirosint, Unithroid

There may be other brand names for this medicine.

When This Medicine Should Not Be Used

Do not use this medicine if you have had an allergic reaction to any type of thyroid hormone or to a color dye. You should not use this medicine if you have adrenal problems that are not treated or if you have recently had a heart attack.

How to Use This Medicine

Capsule, Tablet

  • Your doctor will tell you how much of this medicine to use and how often. Your dose may need to be changed several times in order to find out what works best for you. Do not use more medicine or use it more often than your doctor tells you to.
  • Take this medicine in the morning on an empty stomach. Wait at least 30 minutes to 1 hour before you eat any food.
  • Swallow the capsule whole. Do not cut or crush it. If you take multiple strengths of this medicine, do not remove the capsules from the blisters in advance. The blisters help identify the strength of each medicine.
  • Swallow the tablet with a full glass (8 ounces) of water. This will help prevent choking or gagging on the tablet. If you still have trouble swallowing the tablet, talk to your doctor or pharmacist.
  • If this medicine is being given to a baby or a small child, you can crush the tablet and mix it in a small amount (1 to 2 teaspoons) of water. This mixture can be given by spoon or dropper. Do not mix the tablet with any other liquid except water, unless your doctor says it is okay. Do not store the mixture. If you do not give the medicine right after it is mixed, throw the medicine away.
  • Some people must take this medicine every day for the rest of their life.

If a dose is missed:

  • If you miss a dose or forget to use your medicine, use it as soon as you can. If it is almost time for your next dose, wait until then to use the medicine and skip the missed dose. Do not use extra medicine to make up for a missed dose.

How to Store and Dispose of This Medicine

  • Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light.
  • Ask your pharmacist, doctor, or health caregiver about the best way to dispose of any outdated medicine or medicine no longer needed.
  • Keep all medicine away from children and never share your medicine with anyone.

Drugs and Foods to Avoid

Ask your doctor or pharmacist before using any other medicine, including over-the-counter medicines, vitamins, and herbal products.

  • There are many other medicines that you should not use together with levothyroxine. This includes over-the-counter medicines. Make sure your doctor knows about all other medicines that you use.
  • Make sure your doctor knows if you use digoxin (Lanoxin®), a blood thinner (such as heparin, warfarin, Coumadin®), insulin, or diabetes medicine that you take by mouth (such as glimepiride, glipizide, glyburide, metformin, repaglinide, tolbutamide, Actos®, Janumet®, Januvia®). Tell your doctor if you use rifampin (Rifadin®, Rimactane®), medicines to treat heart problems (such as amiodarone, furosemide, propranolol, Inderal®, Lasix®), medicines to treat epilepsy or seizures (such as carbamazepine, diazepam, phenobarbital, phenytoin), or pain or arthritis medicine (NSAIDs including aspirin, mefenamic acid, phenylbutazone, Ponstel®).
  • Make sure your doctor knows if you also use medicines to treat depression (such as amitriptyline, doxepin, imipramine, maprotiline, nortriptyline, sertraline, Elavil®, Pamelor®), medicines to treat Parkinson disease (such as bromocriptine, ropinirole, Parlodel®, Requip®), medicines to treat thyroid problems (such as methimazole, PTU, Tapazole®), or growth hormones (such as octreotide, somatrem, somatropin, Nutropin®, Sandostatin®).
  • Make sure your doctor knows if you also use any estrogens or birth control pills that contain estrogen, such as estradiol, estropipate, Activella®, Climara®, Femhrt®, Premarin®, Vivelle-Dot®. Tell your doctor if you also use 5-fluorouracil (Adrucil®), aminoglutethimide (Cytadren®), asparaginase (Elspar®), clofibrate (Atromid-S®), dexamethasone (Decadron®), heroin, iodide, ketamine (Ketalar®), methadone (Dolophine®), mitotane (Lysodren®), tamoxifen (Nolvadex®), theophylline (Theo-Dur®), or slow-release nicotinic acid.
  • If you use antacids (such as Amphojel®, Maalox®, Mylanta®, Tums®), medicines to treat high cholesterol (such as cholestyramine, colesevelam, colestipol, Colestid®, Questran®), orlistat (Xenical®), sevelamer (Renagel®), sodium polystyrene sulfonate (Kayexalate®), sucralfate (Carafate®), or any medicine that contains calcium or iron, take it at least 4 hours before or 4 hours after you take levothyroxine.
  • Cottonseed meal, dietary fiber, soybean flour (infant formula), or walnuts may interfere with the absorption of this medicine. You may need to eat these foods at a different time from when you take this medicine. Talk with your doctor if you have questions.

Warnings While Using This Medicine

  • Make sure your doctor knows if you are pregnant or breastfeeding. Tell your doctor if you are postmenopausal or if you have blood clotting problems, diabetes, heart or blood vessel disease (such as coronary artery disease), heart rhythm problems, osteoporosis, pernicious anemia, pituitary gland problems, or adrenal gland problems. Tell your doctor if you are allergic to any food or medicines.
  • If you have diabetes, it is very important that you keep track of your blood or urine sugar levels as instructed by your doctor. Check with your doctor right away if you notice any changes in your sugar levels.
  • Women who use this medicine for a long time may have some bone loss, which could lead to osteoporosis. Talk with your doctor if you have questions or concerns about this.
  • Call your doctor right away if you have rapid or uneven heartbeats, chest pain, trouble breathing, headaches, nervousness, trouble sleeping, tremors, a change in appetite or weight, diarrhea, sweating, sensitivity to heat, or changes in menstrual periods. These could be symptoms of too much medicine in your body.
  • You may have to take this medicine for 6 to 8 weeks before your symptoms start to get better.
  • Tell any doctor or dentist who treats you that you take this medicine before you have any surgery.
  • Do not stop using this medicine suddenly without asking your doctor. You may need to slowly decrease your dose before stopping it completely.
  • Your doctor will need to check your blood at regular visits while you are using this medicine. Be sure to keep all appointments.

Possible Side Effects While Using This Medicine

Call your doctor right away if you notice any of these side effects:

  • Allergic reaction: Itching or hives, swelling in your face or hands, swelling or tingling in your mouth or throat, chest tightness, trouble breathing
  • Chest pain, fast, uneven, or pounding heartbeat
  • Limp or walk favoring one leg (in children)
  • Nervousness, sensitivity to heat, sweating, leg cramps, trouble sleeping, tremors, weakness
  • Severe headache, blurred or double vision, nausea, vomiting (in children)
  • Severe vomiting, diarrhea, stomach pain
  • Trouble breathing, cold sweats, bluish-colored skin

If you notice these less serious side effects, talk with your doctor:

  • Changes in weight
  • Changes in your menstrual periods
  • Hair loss
  • Mild headache

If you notice other side effects that you think are caused by this medicine, tell your doctor

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088

Version Info

  • Last Reviewed on 06/12/2013

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This page was last updated: June 18, 2013

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