Synthroid (levothyroxine) Dosage and Side Effects

    SYNTHROID is a prescription drug used to treat hypothyroidism, when the thyroid gland does not produce enough hormone. It is also used to help decrease the size of enlarged thyroid glands (known as goiter) and to treat thyroid cancer. Buy Synthroid online from a registered pharmacy and save.

    Warnings and Precautions

    Serious Warnings and Precautions

    Thyroid hormones, including SYNTHROID, either alone or with other medicines, should not be used for the treatment of obesity or for weight loss as they could produce serious or even life threatening side effects.

    BEFORE or WHILE you use SYNTHROID tell your doctor or pharmacist if:

    • you are allergic to any foods or medication;
    • you are pregnant or intend to become pregnant, or are breast-feeding. If you become pregnant while taking SYNTHROID, your dose of SYNTHROID will likely have to be increased.
    • you have any heart problem, whether or not you have received treatment for them (especially history of heart attack, heart disease, hardening of the arteries);
    • you have other medical problems, whether or not you have received treatment for them (especially high blood pressure, osteoporosis, blood clotting disorder, or history of thyroid, adrenal and/or pituitary gland problems);
    • you have diabetes. Your dose of insulin or oral antidiabetic agent may need to be changed after starting SYNTHROID. You should monitor your blood and urinary sugar levels as directed by your doctor and report any changes to your doctor immediately.
    • you have signs or symptoms of psychotic disorders;
    • you are taking an oral anticoagulant (blood thinner) such as warfarin. Your dose may need to be changed after starting SYNTHROID.
    • you are taking any other medications (prescription or over-the-counter);
    • you are planning to have any kind of surgery (including dental surgery) or emergency treatment. You should inform your doctor or dentist that you are taking SYNTHROID before having any kind of surgery.
    • your brand of levothyroxine was changed;
    • you are a woman on long-term therapy. You may experience decrease in bone density.
    • you have any allergies to this drug or its ingredients or components of the container;
    • you are taking orlistat.

    Side Effects

    SYNTHROID can cause some side effects. They include:

    • chest pain

    • fast or irregular heart beat

    • weight loss or gain

    • sweating

    • nervousness

    • temporary hair loss

    • decrease of bone mineral density

    • diarrhea

    • vomiting

    Interactions with this medication

    Drugs that may interact with SYNTHROID include but are not limited to the following:

    • digitalis glycosides (e.g., digoxin)
    • anti-diabetic agents (insulin or oral hypoglycemic drugs)
    • glucocorticoids (corticosteroids e.g., dexamethasone, prednisone)
    • antacids containing aluminium and magnesium (e.g., aluminium and magnesium hydroxides, simethicone)
    • calcium carbonate
    • diazepam (e.g., Valium)
    • ferrous sulphate
    • antidepressants
    • lithium
    • iodide
    • anticoagulants (blood thinners e.g., warfarin)
    • appetite suppressants (diet pills)
    • beta blockers (e.g., propranolol, atenolol, metoprolol)
    • cholestyramine (e.g., Olestyr)
    • colestipol (e.g., Colestid)
    • medicines for asthma or other breathing problems
    • medicines for colds, sinus problems, hay fever or other allergies (including nose drops or sprays)
    • orlistat (e.g., Xenical)
    • tyrosine kinase inhibitors (e.g., imatinib, sunitinib)
    • proton pump inhibitors.

    Some medicines may interfere with any blood tests done to determine thyroid hormone levels (thyroid function tests). It is important to inform your doctor of all medicines you are taking before and at the time of blood tests.

    Eating certain foods such as soybean flour, soybean infant formula, cotton seed, walnuts and dietary fiber may decrease absorption of levothyroxine. You may require a change in the dose.

    Proper Use of this medication

    Use SYNTHROID only as prescribed by your doctor. Thyroid hormone replacement is usually taken for life. Do not change the amount you take or how often you take it, unless directed to do so by your doctor. Like all medicines obtained from your doctor, SYNTHROID must be used only by you and for the condition determined appropriate by your doctor. Do not stop taking levothyroxine sodium without talking to your doctor.

    Take SYNTHROID as a single dose, preferably on an empty stomach, one half-to one hour before breakfast. SYNTHROID should be taken with a full glass of water. As food and drink can significantly change the absorption of SYNTHROID, you are advised to take SYNTHROID at the same time every day and be consistent in how you take it with regards to meals [i.e. either always take it on an empty stomach (preferred method) or take it with food]. Levothyroxine sodium absorption is increased on an empty stomach.

    Usual dose:

    The dose of these medicines will be different for different patients. Follow your doctor's orders or the directions on the label.

    Treatment is usually started with lower doses that are increased a little at a time to prevent side effects.

    Adults and children (>12 years): Therapy is usually initiated at the anticipated full replacement dose. The dosage is adjusted by 12.5 to 25 µg increments.

    Patients older than 50 years or patients younger than 50 years with a history of cardiovascular disease: The starting dose should be 25 to 50 µg once daily with adjustments every 6 to 8 weeks as needed.

    Elderly patients with cardiac disease: The recommended starting dose is 12.5 to 25 µg/day, with gradual dose increments at 4 to 6 week intervals.


    If you or someone you know accidentally takes more than the prescribed dose, contact your doctor immediately or go to the nearest hospital with the tablets. Tell your doctor or hospital how much was taken. Treat even small overdoses seriously.

    For management of a suspected drug overdose, contact your regional Poison Control Centre.

    Missed dose:

    If you forget to take one tablet, take another as soon as you remember, unless it is almost time for your next dose. If it is, take the dose you missed with your next dose.

    If you miss 2 or more doses in a row or if you have any questions about this, check with your doctor.

    Other related products

    The above information is an educational aid only. It is not intended as medical advice for individual conditions or treatments. Talk to your doctor, nurse or pharmacist before following any medical regimen to see if it is safe and effective for you.