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Important Note

The following information is intended to supplement, not substitute for, the expertise and judgment of your physician, pharmacist or other healthcare professional. It should not be construed to indicate that use of the drug is safe, appropriate, or effective for you. Consult your healthcare professional before using this drug.


(lev-oh-thigh-ROX in)

COMMON BRAND NAME(S): Levothroid, Synthroid


This drug is used to replace thyroid hormone, to treat an overactive thyroid and for testing thyroid gland function.

How To Use

This medication is given by injection into a vein or into a muscle and is usually given once a day before breakfast. Do not stop using this medication without consulting with your doctor. Thyroid replacement therapy is usually taken for life. This drug must be mixed immediately before use. Do not mix drug with diluent for later use.

Side Effects

Symptoms of low thyroid levels include fatigue, muscle aches, constipation, dry skin, weight gain, slow heart rate, sensitivity to cold or dry, brittle hair that tends to fall out easily. These symptoms should disappear as your body adjusts to the medication. If these symptoms persist or worsen, inform your doctor. Symptoms of high thyroid levels include headache, chest pain, fast/irregular heartbeat, shortness of breath, trembling, sweating, diarrhea or weight loss. If you experience any of these effects, the dose of your medication may need to be reduced. Consult your doctor. If you notice other effects not listed above, contact your doctor or pharmacist.


Tell your doctor your medical history before using this drug. Avoid using thyroid drugs for weight control. Caution is advised when using this drug in children because overuse may reduce the child's future adult height. Consult your doctor for more details. Current information shows that this drug may be used during pregnancy. Small amounts of this drug are excreted into breast milk. Consult your doctor before breast-feeding.

Drug Interactions

Inform your doctor about all the medicines you use (both prescription and nonprescription) especially of: warfarin, drugs that may increase heart rate or blood pressure such as decongestants or caffeine (decongestants may be found in certain over-the-counter cough-and-cold medicines), digoxin, cholestyramine, oral medicine used for diabetes, birth control pills/estrogen. Do not start or stop any medicine without doctor or pharmacist approval.


If overdose is suspected, contact your local poison control center or emergency room immediately. Symptoms of overdose may include nervousness, fast heart rate, diarrhea, stomach cramps, trouble sleeping, and weight loss.


Lab tests may be done periodically to monitor the effectiveness of this medication.

Missed Dose

If you miss a dose, use it as soon as remembered; do not use if it is almost time for the next dose, instead, skip the missed dose and resume your usual dosing schedule. Do not "double-up" the dose to catch up. Call your doctor if you miss 2 or more doses in a row.


Store at room temperature between 59 and 86 degrees F (15 to 30 degrees C) away from sunlight and moisture.