Generic Name: levothyroxine (LEE voe thye ROX een)
Brand Names: Levoxyl, Synthroid, Tirosint, Unithroid
Medically reviewed: December 6, 2017
What is Synthroid?
Synthroid (levothyroxine) is a thyroid medicine that replaces a hormone normally produced by your thyroid gland to regulate the body's energy and metabolism. Levothyroxine is given when the thyroid does not produce enough of this hormone on its own.
Synthroid treats hypothyroidism (low thyroid hormone).
Synthroid is also used to treat or prevent goiter (enlarged thyroid gland), which can be caused by hormone imbalances, radiation treatment, surgery, or cancer.
You may not be able to take Synthroid if you have certain medical conditions. Tell your doctor if you have an untreated or uncontrolled adrenal gland disorder, a thyroid disorder called thyrotoxicosis, or if you have any recent or current symptoms of a heart attack.
Synthroid should not be used to treat obesity or weight problems.
Before taking this medicine
Synthroid should not be used to treat obesity or weight problems. Dangerous side effects or death can occur from the misuse of levothyroxine, especially if you are taking any other weight-loss medications or appetite suppressants.
Since thyroid hormone occurs naturally in the body, almost anyone can take levothyroxine. However, you may not be able to take this medicine if you have certain medical conditions. Tell your doctor if you have:
an untreated or uncontrolled adrenal gland disorder;
a thyroid disorder called thyrotoxicosis; or
symptoms of a heart attack (chest pain or heavy feeling, pain spreading to the jaw or shoulder, nausea, sweating, general ill feeling).
To make sure Synthroid is safe for you, tell your doctor if you have ever had:
heart disease, coronary artery disease, a heart attack, or a blood clot;
diabetes (insulin or oral diabetes medication doses may need to be changed when you start taking levothyroxine);
anemia (lack of red blood cells);
a blood-clotting disorder;
osteoporosis, or low bone mineral density;
problems with your pituitary gland; or
any food or drug allergies.
Tell your doctor if you have recently received radiation therapy with iodine (such as I-131).
Levothyroxine is not expected to harm an unborn baby. If you become pregnant while taking Synthroid, do not stop taking the medicine without your doctor's advice. Having low thyroid hormone levels during pregnancy could harm both mother and baby. Your dose needs may be different during pregnancy.
Levothyroxine can pass into breast milk, but it is not expected to be harmful to a nursing baby. Tell your doctor if you are breast-feeding. Your dose needs may be different while you are nursing.
Do not give this medicine to a child without medical advice. Tirosint is not approved for use by anyone younger than 6 years old.
How should I take Synthroid?
Take Synthroid exactly as prescribed by your doctor. Follow all directions on your prescription label. Your doctor may occasionally change your dose. Do not take this medicine in larger or smaller amounts or for longer than recommended.
Synthroid works best if you take it on an empty stomach, at least 30 minutes before breakfast. Follow your doctor's dosing instructions and try to take the medicine at the same time each day.
If a child is using this medicine, tell your doctor if the child has any changes in weight. Levothyroxine doses are based on weight in children, and any changes may affect your child's dose.
It may take several weeks before your body starts to respond to Synthroid. Keep using this medicine even if you feel well. You may need to use levothyroxine for the rest of your life.
While using Synthroid, you may need frequent medical tests. Tell any doctor, dentist, or surgeon who treats you that you are using levothyroxine.
Store at room temperature away from moisture and heat.
Do not share this medicine with another person, even if they have the same symptoms you have.
See also: Dosage Information (in more detail)
What happens if I miss a dose?
Take the missed dose as soon as you remember. Skip the missed dose if it is almost time for your next scheduled dose. Do not take extra medicine to make up the missed dose.
What happens if I overdose?
Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.
Overdose symptoms may include headache, leg cramps, tremors, feeling nervous or irritable, chest pain, shortness of breath, and fast or pounding heartbeats.
What should I avoid while taking Synthroid?
Certain medicines can make levothyroxine less effective if taken at the same time. If you use any of the following drugs, avoid taking them within 4 hours before or 4 hours after you take Synthroid:
calcium carbonate (Alka-Mints, Calcium Oyster Shell, Caltrate, Os-Cal, Oyster Shell Calcium, Rolaids Soft Chew, Tums, and others);
cholestyramine, colesevelam, colestipol;
ferrous sulfate iron supplement;
sodium polystyrene sulfonate (Kalexate, Kayexalate, Kionex);
stomach acid reducers - esomeprazole, lansoprazole, omeprazole, rabeprazole, Nexium, Prilosec, Prevacid, Protonix, Zegerid, and others; or
antacids that contain aluminum or magnesium - Acid Gone, Gaviscon, Maalox, Milk of Magnesia, Mintox, Mylanta, Pepcid Complete, and others).
Avoid the following food products, which can make your body absorb less levothyroxine: grapefruit juice, infant soy formula, cotton seed meal, walnuts, and high-fiber foods.
Synthroid side effects
Get emergency medical help if you have signs of an allergic reaction to Synthroid: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Call your doctor at once if you have a serious side effect such as:
fast or irregular heart rate;
chest pain, feeling short breath;
fever, hot flashes, sweating;
feeling unusually cold;
weakness, tiredness, sleep problems (insomnia);
memory problems, feeling depressed or irritable;
headache, leg cramps, muscle aches;
feeling nervous or irritable;
dryness of your skin or hair, hair loss;
changes in your menstrual periods; or
vomiting, diarrhea, appetite changes, weight changes.
Certain side effects may be more likely in older adults.
Common Synthroid side effects may include:
skin rash, mild hair loss.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
See also: Side effects (in more detail)
What other drugs will affect Synthroid?
Many other medicines can be affected by your thyroid hormone levels. Certain other medicines may also increase or decrease the effects of levothyroxine.
Many drugs can interact with levothyroxine. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed in this medication guide. Tell your doctor about all your current medicines and any medicine you start or stop using.
Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Synthroid only for the indication prescribed.
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