In this drug factsheet:
What side effects are possible with this medication?
Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent. The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.
The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.
Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.
- hair loss (temporary; particularly in children during the first month of treatment)
- increased appetite
- menstrual cycle changes
- nervousness or irritability
- sensitivity to heat
- stomach cramps or upset stomach
- tremor (shaking)
- weight loss
Although most of these side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.
Stop taking the medication and seek immediate medical attention if any of the following occur:
- chest pain, irregular heartbeat, or shortness of breath
- severe allergic reaction (e.g., hives; difficulty breathing; wheezing; or swelling of the eyes, mouth, or lips)
Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.
Are there any other precautions or warnings for this medication?
Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.
Bone density: Levothyroxine can cause bones to lose thickness. People at an increased risk for osteoporosis (bone thinning) or who are taking medications that reduce bone thickness (e.g., prednisone or antiseizure medications) should discuss this with their doctor before starting treatment. Your doctor may monitor your bone thickness while you are taking this medication.
Diabetes: Levothyroxine raises blood sugar levels. For people with diabetes, this may result in an increase in the requirements for insulin or antidiabetes medications. Monitor your blood sugar more closely when starting or changing doses of this medication.
Heart disease: When starting levothyroxine, people with heart disease may be started on a lower dosage as it may cause the heart to work harder than it has been used to.
Signs of getting too much or too little medication: Contact your doctor as soon as possible if you notice any signs of getting too much medication (such as chest pain, confusion, fast or irregular heartbeat, mood swings, muscle weakness, psychosis, extreme restlessness, yellow eyes or skin, or shortness of breath) or signs of not getting enough medication (such as clumsiness, coldness, constipation, dry, puffy skin, listlessness, muscle aches, sleepiness, tiredness, weakness, or weight gain).
Weight loss: Levothyroxine should not be used for weight loss. Larger doses could cause serious side effects especially when taken together with other medications for weight loss.
Breast-feeding: The use of appropriate amounts of this medication by breast-feeding women has not been shown to cause harm for breast-fed babies.
Pregnancy: When this medication is used to regulate the levels of thyroid hormone, treatment is considered necessary for pregnant women.
Seniors: Seniors may be more sensitive to the effects of levothyroxine.
What other drugs could interact with this medication?
There may be an interaction between levothyroxine and any of the following:
- antacids that contain aluminum
- antidiabetes medications (e.g., insulin)
- anticonvulsants (e.g., carbamazepine, phenytoin, phenobarbital)
- beta-blockers (e.g., metoprolol, propranolol)
- birth control pills containing estrogen
- calcium carbonate
- calcium polystyrene sulfonate
- certain statins (e.g., lovastatin, simvastatin)
- diet pills
- ferrous sulfate (iron)
- orlistat (only interacts with oral levothyroxine, not injectable levothyroxine)
- selective serotonin reuptake inhibitors (e.g., fluoxetine)
- sodium polystyrene sulfonate
- sympathomimetic medications (e.g., amphetamines)
- theophyllines (e.g., aminophylline, oxtriphylline, theophylline)
- thiazide diuretics (e.g., hydrochlorothiazide)
- tricyclic antidepressants (e.g., amitriptyline, imipramine)
If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:
- stop taking one of the medications,
- change one of the medications to another,
- change how you are taking one or both of the medications, or
- leave everything as is.
An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.
Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications that you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.