Synthroid (Levothyroxine) - Oral

What Is Synthroid?

Synthroid (levothyroxine) is the first-line treatment for an underactive thyroid condition called hypothyroidism. 

In hypothyroidism, the thyroid gland—a small butterfly-shaped organ in the neck—does not make enough of the naturally occurring thyroid hormones thyroxine (T4) and triiodothyronine (T3). When this happens, the pituitary gland in the brain makes more thyroid-stimulating hormone (TSH) to tell the thyroid to produce more T4 and T3. 

However, the thyroid hormones may have trouble maintaining normal levels for long periods without the help of Synthroid—a synthetic version of T4. When this prescription tablet is taken by mouth, T4 is still “asleep” until our bodies wake up T4 into its active version—T3.

Drug Facts

Generic Name: Levothyroxine

Brand Name(s): Synthroid, Levothroid, Levoxyl, Tirosint, Tirosint-Sol, Unithroid

Drug Availability: Prescription

Therapeutic Classification: Hormone, Thyroid supplement

Available Generically: Yes

Controlled Substance: N/A

Administration Route: Oral

Active Ingredient: Levothyroxine

Dosage Form(s): Tablet, capsule, oral solution, injection

What Is Synthroid Used For?

The Food and Drug Administration (FDA) approved Synthroid to treat hypothyroidism and TSH suppression.

Hypothyroidism affects about 5 in every 100 Americans. There are many common causes and risk factors for an underactive thyroid. However, the most common cause of hypothyroidism is Hashimoto’s thyroiditis (an autoimmune thyroid condition).

Regardless of the reason, the results are still low thyroid hormone levels, and the following symptoms are usually experienced:

  • Weight gain
  • Tiredness
  • Depression
  • Brain fog
  • Dry skin
  • Feeling cold
  • Muscle cramps
  • Constipation

Synthroid is the preferred medication to help replace the missing thyroid hormone levels and prevent complications.

How to Take Synthroid

Synthroid is a convenient tablet, but this prescription can negatively interact with other medications and food. 

When taking levothyroxine, try to take the medication every day at one of the following times:

  • In the morning: Many healthcare providers recommend taking Synthroid 60 minutes before your first food, drink, and other medications.
  • At bedtime: Based on a 2010 study, taking Synthroid in the evening is also an option. When taking the drug before bedtime, wait up to four hours after your last meal or snack.

Other considerations include taking Synthroid four hours before or after the following:

  • Foods and medications with calcium or iron: Examples include yogurt and antacids.
  • Certain cholesterol-lowering medications: Examples include Questran (cholestyramine) and Colestid (colestipol).

However, consistency is vital regardless of when you decide to take levothyroxine. Find ways to help you remember to take Synthroid the same way at the same time every day.

Storage

Synthroid, like most medications, can be stored at room temperature (77 degrees). It can be safely kept at temperatures of 59 to 86 degrees. Do not store the medication in your bathroom, the glove compartment of your car, or other damp or hot environments for long periods.

Keep your medications tightly closed and out of the reach of children and pets, ideally locked in a cabinet or closet.

Avoid pouring unused and expired drugs down the drain or in the toilet. Ask your pharmacist or healthcare provider about the best ways to dispose of this medicine. Visit the FDA's website to know where and how to discard all unused and expired drugs. You can also find disposal boxes in your area. Ask your pharmacist or healthcare provider if you have any questions about the best ways to dispose of your medications.

If you travel with Synthroid, get familiar with your final destination's regulations. In general, be sure to make a copy of your Synthroid prescription. Keep your medication in its original container from your pharmacy with your name on the label. Ask your pharmacist or healthcare provider if you have any questions about traveling with your medicine.

Off-Label Uses

Healthcare providers may prescribe Synthroid off-label (for non-FDA-approved uses) for certain conditions that can benefit from the medication. 

Synthroid can be used off-label for:

  • Subclinical hypothyroidism: In subclinical hypothyroidism, an early form of hypothyroidism, your body can still compensate for the low-functioning thyroid gland. Therefore, your T4 levels will appear normal. To achieve these normal T4 levels, however, your TSH will be high. Although treatment is not usually recommended, some people might benefit from taking Synthroid in this case.
  • Quality maintenance of donor organs: To improve the quality of transplantable organs, healthcare providers consider Synthroid use with vasopressin (a hormone that helps regulate the amount of water in the body), methylprednisolone (a steroid that treats inflammation), and insulin (a hormone that helps regulate blood sugar, or glucose, levels) in brain-dead donors with unstable blood pressure.

How Long Does Synthroid Take to Work?

You might notice improvement of your symptoms within two weeks of starting Synthroid therapy. However, TSH might require six weeks to return to normal levels.

What Are the Side Effects of Synthroid?

This is not a complete list of side effects, and others may occur. A healthcare provider can advise you on side effects. If you experience other effects, contact your pharmacist or a healthcare provider. You may report side effects to the Food and Drug Administration (FDA) at fda.gov/medwatch or 800-FDA-1088.

At proper doses, Synthroid has very few side effects. The “best” dose varies from person to person. Due to other factors, healthcare providers will regularly order thyroid blood tests to determine the appropriate dose for you.

Common Side Effects

If your dose is too high, you might notice some of the following common side effects:

  • Weight loss
  • Nervousness
  • Feeling hot
  • Tremors
  • Increased appetite
  • Sleep problems
  • Fast heartbeat
  • High blood pressure
  • Skipping heartbeats
  • Diarrhea

These side effects are also very similar to symptoms of an overactive thyroid condition called hyperthyroidism.

Severe Side Effects

If you are experiencing the following severe side effects or conditions, stop taking Synthroid and seek medical attention:

  • Arrhythmias (problem with heart rate or heart rhythm)
  • Brain clot (blood clot in the brain)
  • Confusion (not being able to think clearly)
  • Coma (state of prolonged unconsciousness)
  • Heart attack (blood flow to the heart is blocked)
  • Seizures (sudden and uncontrollable electrical activity in the brain)
  • Shock (sudden decrease of blood and oxygen to your organs or tissues)

Long-Term Side Effects

At proper doses, Synthroid has few side effects and long-term consequences. To prevent inappropriate levothyroxine doses and to determine what’s best for you, your healthcare provider will order thyroid tests. Regular appointments with your healthcare provider will help regulate your dosing and prevent doses of medication that are too high or low for you.

Report Side Effects

Synthroid may cause other side effects. Call your healthcare provider if you have any unusual problems while taking this medication.

If you experience a serious side effect, you or your provider may send a report to the FDA's MedWatch Adverse Event Reporting Program or by phone (800-332-1088).

Dosage: How Much Synthroid Should I Take?

Drug Content Provided and Reviewed by IBM Micromedex®

The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.

The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.

  • For oral dosage form (capsules):
    • For hypothyroidism:
      • Adults and children older than 12 years of age (growth and puberty complete)—Dose is based on body weight and must be determined by your doctor. The dose is usually 1.7 microgram (mcg) per kilogram (kg) of body weight per day but may be less in older adults.
      • Children older than 12 years of age (growth and puberty incomplete)—Dose is based on body weight and must be determined by your doctor. The dose is usually 2 to 3 mcg per kg of body weight per day.
      • Children 6 to 12 years of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 4 to 5 mcg per kg of body weight per day.
      • Children younger than 6 years of age—Use is not recommended as your child might not be able to swallow the capsules.
    • For thyroid cancer:
      • Adults—Dose is based on body weight and must be determined by your doctor. The dose is usually more than 2 microgram (mcg) per kilogram (kg) of body weight per day.
      • Children—Use and dose must be determined by your doctor.
  • For oral dosage form (solution):
    • For hypothyroidism:
      • Adults and children older than 12 years of age (growth and puberty complete)—Dose is based on body weight and must be determined by your doctor. The starting dose is usually 1.7 microgram (mcg) per kilogram (kg) of body weight per day. Your doctor may adjust your dose as needed.
      • Children older than 12 years of age (growth and puberty incomplete)—Dose is based on body weight and must be determined by your doctor. The dose is usually 2 to 3 mcg per kg of body weight per day.
      • Children 6 to 12 years of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 4 to 5 mcg per kg of body weight per day.
      • Children 1 to 5 years of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 5 to 6 mcg per kg of body weight per day.
      • Children 6 to 12 months of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 6 to 8 mcg per kg of body weight per day.
      • Children 3 to 6 months of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 8 to 10 mcg per kg of body weight per day.
      • Children 0 to 3 months of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 10 to 15 mcg per kg of body weight per day.
    • For thyroid cancer:
      • Adults—Dose is based on body weight and must be determined by your doctor. The dose is usually more than 2 microgram (mcg) per kilogram (kg) of body weight per day.
      • Children—Use and dose must be determined by your doctor.
  • For oral dosage form (tablet):
    • For hypothyroidism:
      • Adults and children older than 12 years of age (growth and puberty complete)—Dose is based on body weight and must be determined by your doctor. The dose is usually 1.6 microgram (mcg) per kilogram (kg) of body weight per day but may be less in older adults.
      • Children older than 12 years of age (growth and puberty incomplete)—Dose is based on body weight and must be determined by your doctor. The dose is usually 2 to 3 mcg per kg of body weight per day.
      • Children 6 to 12 years of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 4 to 5 mcg per kg of body weight per day.
      • Children 1 to 5 years of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 5 to 6 mcg per kg of body weight per day.
      • Children 6 to 12 months of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 6 to 8 mcg per kg of body weight per day.
      • Children 3 to 6 months of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 8 to 10 mcg per kg of body weight per day.
      • Children 0 to 3 months of age—Dose is based on body weight and must be determined by your doctor. The dose is usually 10 to 15 mcg per kg of body weight per day.

Modifications

The best Synthroid dose varies from person to person. Your healthcare provider may modify your dose based on age or other situations. 

Children: In children, normal thyroid hormone levels are essential for intellectual and physical development. Therefore, healthcare providers tend to monitor levels to avoid undertreatment or overtreatment. Also, healthcare providers might recommend liquid versions of levothyroxine for children. These include Tirosint-SOL6 and Thyquidity. Parents can mix the crushed tablet in 5–10 milliliters of water if necessary to encourage kids to take the medication.

Pregnancy: The thyroid makes more thyroid hormones during pregnancy to support fetal development. Therefore, your healthcare provider usually will increase your levothyroxine dose by 40%–50% if you’re pregnant. 

Adults over 65: Older adults might require less levothyroxine than young adults. In older adults, the starting Synthroid dose is usually lower—between 25–50 micrograms. Then, if necessary, your healthcare provider will slowly increase the medication dose. 

Other considerations: Synthroid is available as other brands, including Levoxyl, Unithroid, Tirosint, and Euthyrox. Levothyroxine is an intravenous (IV, within a vein) injection. However, the injectable form is only used for the FDA-approved treatment of myxedema coma.

Myxedema coma can result if your Synthroid dose is too low, which can be fatal. Alert your healthcare provider if you suspect your current daily dose is too low.

Missed Dose

If you accidentally forgot your Synthroid dose, take it as soon as you remember. If it's already close to your next scheduled dose, skip the missed dose and take the following dose at your next scheduled dosing time. Don't try to double up to make up for the missed dose.

Try to find ways to help yourself remember to keep your appointments and take your medication routinely. If you miss too many doses, Synthroid might be less effective at treating your condition.

Overdose: What Happens If I Take Too Much Synthroid?

If your Synthroid dose is too high, you might experience an overdose, which increases your risk of heart attack, stroke (interrupted blood supply to your brain), and thyroid storm (a rare but life-threatening condition). If you suspect that your side effects are due to a high dose or if you accidentally took multiple Synthroid doses in one day, please contact your healthcare provider.

What Happens If I Overdose on Synthroid?

If you think you or someone else may have overdosed on Synthroid, call a healthcare provider or the Poison Control Center (800-222-1222).

If someone collapses or isn't breathing after taking Synthroid, call 911 immediately.

Precautions

Drug Content Provided and Reviewed by IBM Micromedex®

If you will be taking this medicine for a long time, it is very important that your doctor check the progress of you or your child at regular visits. This will allow your doctor to see if the medicine is working properly and to decide if you should continue to take it. Blood tests will be needed to check for unwanted effects.

Levothyroxine should not be used for the treatment of obesity or for the purpose of losing weight. This medicine is not effective for weight reduction. If taken in large amounts, levothyroxine may cause serious unwanted effects.

Hypothyroidism can sometimes cause infertility in men and women. Levothyroxine should not be used for the treatment of infertility unless it is caused by hypothyroidism.

For patients with diabetes: It is very important that you keep track of your blood or urine sugar levels as instructed by your doctor. Check with your doctor right away if you notice any changes in your sugar levels.

If you think you have become pregnant while using this medicine, tell your doctor right away. You may need a larger dose of levothyroxine while you are pregnant.

Women who are post-menopausal or who use this medicine for a long time may have some bone loss, which could lead to osteoporosis. Talk with your doctor if you have questions or concerns about this.

Call your doctor right away if you or your child start to have rapid or irregular heartbeats, chest pain, leg cramps, headaches, nervousness, irritability, sleeplessness, tremors, a change in appetite, weight gain or loss, vomiting, diarrhea, excessive sweating, heat intolerance, a fever, changes in menstrual periods, hives, or a skin rash. These could be symptoms of too much medicine in your body.

Do not suddenly stop taking this medicine without first checking with your doctor. Your doctor may want you or your child to gradually reduce the amount you are using before stopping completely.

Make sure any doctor or dentist who treats you knows that you or your child are using this medicine. You or your child may need to stop using this medicine several days before having surgery or medical tests.

A temporary loss of hair may occur during the first few months of levothyroxine therapy. Ask your doctor about this if you have any concerns.

Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.

What Are Reasons I Shouldn’t Take Synthroid?

  • Myxedema coma: Myxedema coma is a life-threatening emergency due to long-term hypothyroidism. As a result, the medical condition requires immediate attention with IV levothyroxine, which is more easily absorbed than the oral version of Synthroid.
  • Adrenal insufficiency: Adrenal insufficiency, or Addison’s disease, is also life-threatening. However, it is due to low cortisol—a naturally occurring steroid. Synthroid can worsen this condition by breaking down and clearing out more cortisol from the body. Therefore, healthcare providers will focus on replacing cortisol before giving Synthroid to people with adrenal insufficiency and hypothyroidism.

What Other Medications Interact With Synthroid?

Synthroid has many interactions with multiple medications. Healthcare providers usually recommend taking Synthroid either in the morning on an empty stomach or four hours after your last meal of the day to avoid drug and food interactions.

However, the following specific drug interactions deserve special attention. 

Try to separate Synthroid by at least four hours from these medications:

  • Medications containing calcium or iron: Examples include antacids, such as Tums or Mylanta.
  • Certain cholesterol-lowering medications include Prevalite (cholestyramine) and Colestid (colestipol).

If you take supplements, please keep the following in mind:

  • Biotin: If you take 5,000 micrograms or more of biotin every day, these high doses might cause inaccurate thyroid blood tests. Stop taking biotin for at least three days before your scheduled thyroid tests.
  • Iodine: Limit your intake of iodine or kelp supplements. The thyroid is very sensitive to iodine. Too much iodine might worsen your medical condition.

What Medications Are Similar?

Levothyroxine—the manufactured version of T4—is the first-choice medication for an underactive thyroid.

However, other treatment options include:

  • Cytomel (liothyronine): This medication is the synthetic version of the T3 hormone. The available brand names of liothyronine are Cytomel and Triostat.
  • Natural desiccated thyroid (NDT): NDT is available under many names, including Armour Thyroid, Nature-Throid, NP Thyroid, Westhroid, and WP Thyroid. NDT is a tablet made of dried thyroid glands from pigs. NDT contains both T4 and T3. The T4 to T3 ratio, however, varies for each manufactured batch. Additionally, the T4 to T3 ratio in pigs differs from those in humans. 
  • Thyrolar (liotrix): Liotrix is available under the brand name Thyrolar. Like NDT, liotrix also has T4 and T3. Although the T4 to T3 ratio is consistently 4-to-1, liotrix contains more T3 than the usual amounts in humans. Therefore, unlike NDT, liotrix does not require multiple doses.

Although many thyroid products treat underactive thyroid, our bodies can convert T4 to T3 as needed. Therefore, many healthcare providers prefer levothyroxine or synthetic T4 over T3. Compared to T3 medication selections, T4 therapy is less likely to lead to symptoms associated with hyperthyroidism.

However, for some people, Synthroid is not enough, and even when using the drug, they continue to have symptoms of an underactive thyroid. Although research is ongoing, studies are looking into whether these individuals may have a genetic variation that causes a difference in Synthroid response. Therefore, some healthcare providers will be open to combination therapy of T4 and T3 for a trial period of three months to six months.

Do not take these thyroid medications together. Contact your healthcare provider if you have any questions.

Frequently Asked Questions

  • Should I only take brand-name Synthroid, or should I take generic levothyroxine?

    In general, generic medications are equally safe as brand-name prescriptions. Although there might be variations in the inactive or filler ingredients, generics have the same active ingredients at the same doses as their brand-name counterparts. 


    Compared to other medications, some people can notice drastic differences between the various levothyroxine generics due to changes in the fillers. 


    If this is true for you, ask your healthcare provider or pharmacist about the following options:

    • Continue to take brand-name Synthroid.
    • Continue to take the same generic version of levothyroxine as always.
    • Switch to the Mylan levothyroxine generic, which is therapeutically equivalent to brand-name Synthroid and all other levothyroxine generics.
  • Is Synthroid a lifelong medication?

    For many people, Synthroid is a lifelong medication. Your dose might lower as you age, but regularly taking levothyroxine will still be necessary to avoid complications, such as myxedema coma.

  • What if changing my morning routine is hard?

    If changing your morning routine is very difficult, talk with your healthcare provider about Tirosint. However, remember that Tirosint is a brand-name medication, which can be more expensive than a generic.

How Can I Stay Healthy While Taking Synthroid?

After receiving a diagnosis of hypothyroidism from your healthcare provider, it is understandable to feel overwhelmed. As you continue to manage your chronic underactive thyroid condition, it is normal to have feelings of anger, frustration, or sadness.

Fortunately, there are many ways for you to feel great and stay healthy while taking Synthroid, such as:

  • Exercising regularly
  • Eating a nutritious diet
  • Having a solid support network
  • Partnering with your healthcare providers

Medical Disclaimer

Verywell Health's drug information is meant for educational purposes only and is not intended as a replacement for medical advice, diagnosis, or treatment from a healthcare provider. Consult your healthcare provider before taking any new medication(s). IBM Watson Micromedex provides some of the drug content, as indicated on the page.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Ross Phan, PharmD, BCACP, BCGP, BCPS
Ross is a writer for Verywell with years of experience practicing pharmacy in various settings. She is also a board-certified clinical pharmacist and the founder of Off Script Consults.