Medically reviewed by Philip Thornton, DipPharm. Last updated on May 4, 2021.
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.
Synthroid is a prescription medicine used to treat hypothyroidism (low thyroid hormone). Levothyroxine is given when your thyroid does not produce enough of this hormone on its own.
Synthroid is also used in patients who need surgery and radioiodine therapy to manage a type of thyroid cancer called well-differentiated thyroid 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.
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:
a thyroid nodule;
heart disease, a blood clot, or a blood-clotting disorder;
diabetes (insulin or oral diabetes medication doses may need to be changed when you start taking Synthroid);
anemia (lack of red blood cells);
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).
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.
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.
How should I take Synthroid?
Take Synthroid exactly as it was prescribed for you. Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose.
Synthroid works best if you take it on an empty stomach, 30 to 60 minutes before breakfast. Follow your doctor's dosing instructions and try to take the medicine at the same time each day.
Synthroid can be given to infants and children who cannot swallow whole tablets by crushing the tablet and mixing the freshly crushed tablet in a small amount (5 to 10 mL or 1 to 2 teaspoonfuls) of water and immediately giving the misture by spoon or dropper. Do not store the crushed tablet/water mixture and do not administer it mixed with foods that decrease absorption of levothyroxine, such as soybean-based infant formula.
Levothyroxine doses are based on weight in children. Your child's dose needs may change if the child gains or loses weight.
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 take Synthroid for the rest of your life.
You may need frequent medical tests. Tell any doctor, dentist, or surgeon who treats you that you are using Synthroid.
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.
What happens if I miss a dose?
Take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not take two doses at one time.
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 to avoid
Avoid the following food products, which can make your body absorb less levothyroxine: grapefruit juice, infant soy formula, soybean flour, 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:
fast or irregular heartbeats;
chest pain, pain spreading to your jaw or shoulder;
shortness of breath;
fever, hot flashes, sweating;
tremors, or if you feel 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;
irregular menstrual periods; or
Certain side effects may be more likely in older adults.
Common Synthroid side effects may include:
chest pain, irregular heartbeats;
shortness of breath;
headache, leg cramps, muscle pain or weakness;
tremors, feeling nervous or irritable, trouble sleeping;
changes in your menstrual periods;
skin rash, partial 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.
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 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:
ferrous sulfate iron supplement;
Many drugs can interact with levothyroxine. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed here. Tell your doctor about all your current medicines and any medicine you start or stop using.
Although the American Thyroid Association guidelines do not recommend taking Cytomel and Synthroid together, many people who are under the supervision of an internist/endocrinologist do take this combination treatment, prefer it, and have good results on it. Never take this combination without your doctor's advice because it may result in excessive levels of thyroid hormones which can be dangerous. Continue reading
In most cases, levothyroxine causes some weight loss. According to the American Thyroid Association, when this medication is started, you may lose up to 10% of your weight. This weight is mainly water weight, since being hypothyroid makes you retain water. Continue reading
- Should you take your thyroid medication at night?
- What happens if you stop taking levothyroxine?
- Can I lay down after I take levothyroxine?
More about Synthroid (levothyroxine)
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- Drug class: thyroid drugs
Related treatment guides
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.
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.
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