Synthroid (levothyroxine)

Synthroid is a brand-name prescription drug that’s approved by the Food and Drug Administration (FDA) for certain uses. It’s a lab-made copy of a natural hormone, called T4, that’s made by your thyroid.

Doctors can prescribe Synthroid for the following conditions.

• Hypothyroidism in adults and children of all ages. With hypothyroidism, your thyroid doesn’t make enough thyroid hormones. Synthroid helps increase your thyroid hormones back to normal levels. It can be used for hypothyroidism that’s congenital (present from birth) or acquired (developed later in life). Doctors can prescribe Synthroid to treat the three main forms of hypothyroidism, which are:
  • primary (caused by a problem with your thyroid)
  • secondary (caused by a problem with your pituitary gland)
  • tertiary (caused by a problem with your hypothalamus)
• Thyrotropin-dependent, well-differentiated thyroid cancer in adults. With this form of thyroid cancer, a hormone called thyroid stimulating hormone (TSH) increases the growth of the cancer. Synthroid lowers levels of TSH in your blood. This is called TSH suppression. The drug helps slow the growth of this thyroid cancer. Synthroid is used with surgery and radioactive iodine to treat this form of thyroid cancer.

Synthroid has certain limitation of use. For more details about these conditions and the drug’s limitations, see the “Synthroid: Uses” section below.

The following table provides some key facts about Synthroid.

If you’re interested in taking this drug, search here to find a doctor who might prescribe it.

Synthroid is a brand-name medication. It contains the active drug levothyroxine, which also comes in a generic form. A generic is an identical copy of the active drug found in a brand-name medication.

The Food and Drug Administration (FDA) ensures that generic drugs are as safe and effective as their original drug. Generics tend to be less expensive than brand-name drugs.

If you’d like to learn about using the generic version of Synthroid, ask your doctor or pharmacist. They can tell you if the generic medication comes in forms and strengths recommended for your condition.

As with most drugs, it’s possible to have side effects with Synthroid. These can include some mild side effects, but also some serious ones.

To learn more about Synthroid’s side effects, talk with your doctor or pharmacist. They may also provide information about managing certain side effects of this drug.

Note: The Food and Drug Administration (FDA) tracks side effects of drugs it has approved. If you would like to notify the FDA about a side effect you’ve had with Synthroid, you can do so through MedWatch.

Mild and serious side effects of Synthroid are listed in the table below. However, these side effects typically only occur if your dosage of Synthroid is too high. This table does not include all of Synthroid’s possible side effects.

* This is not a complete list of Synthroid’s mild side effects. To learn about other mild side effects of this drug, ask your doctor or pharmacist. Or you can view the drug’s prescribing information.
† For more information about this side effect, see “Synthroid’s side effects explained” below.
‡ Synthroid has a boxed warning Trusted Source PubMed Central Highly respected database from the National Institutes of Health Go to source stating that it’s not to be used for weight loss or obesity treatment. A boxed warning is the strongest warning required by the FDA. Boxed warnings alert doctors and patients about drug effects that may be dangerous.

Most times, mild side effects of a drug go away within a few days or a couple of weeks. Talk with your doctor or pharmacist if any side effects become severe or don’t go away.

Serious side effects from Synthroid aren’t common, but they are possible. If you have serious side effects, call your doctor right away. However, if you’re having a medical emergency or your symptoms feel life threatening, call 911 or a local emergency number.

In general, side effects of Synthroid in children are similar to those in adults. Examples of these are listed in the table above.

However, additional side effects have been reported in children taking Synthroid. These are:

• pseudotumor cerebri (a condition in which the pressure inside the skull increases, causing headaches and changes in vision)
• slipped capital femoral epiphysis (a hip disorder in which the ball-and-socket joint in the hip slips backward, causing hip pain or limping)
• craniosynostosis (a condition that can affect babies whose Synthroid dosage is too high, in which the bones in the skull join together too early in development, putting pressure on the brain)
• premature closure of the epiphyses (a condition that can affect children whose Synthroid dosage is too high, in which the child’s bones stop growing too early, preventing usual height increases)

If you’re concerned about the risk of side effects in children taking Synthroid, talk with the child’s doctor.

The side effects of Synthroid listed in the table above typically only occur in people taking too much Synthroid. These side effects are symptoms of hyperthyroidism (a condition that causes high thyroid hormone levels).

On the other hand, taking too little Synthroid may lead to symptoms of hypothyroidism (a condition that causes low thyroid hormone levels). These may include fatigue, constipation, feeling cold, and weight gain. Keep in mind that Synthroid is used to treat hypothyroidism.

During treatment with Synthroid, your doctor will order blood tests every few weeks to check your thyroid hormone levels. Your doctor may increase or decrease your Synthroid dosage based on your test results. They’ll keep adjusting your dosage until you’re taking the right amount of Synthroid to keep your thyroid hormone levels in the desired range.

It’s important to take Synthroid every day, at the same time each day, exactly as prescribed by your doctor. This will keep your thyroid hormones in the desired range, helping you avoid side effects of having too much or too little Synthroid.

Below, you can find detailed information about some of Synthroid’s side effects. To learn more about other side effects of this medication, talk with your doctor.

Some people may have weight loss with Synthroid.

If you have hypothyroidism, which Synthroid is used to treat, weight gain may be one of your symptoms. (Hypothyroidism is a condition that causes low thyroid hormone levels.)

Taking Synthroid increases your thyroid hormone levels, which speeds up your metabolism. And this can lead to weight loss as your thyroid hormones return to normal levels. (Metabolism describes the chemical reactions in your body that control how your body creates and uses energy.)

However, weight loss could also be a sign that your Synthroid dosage is too high.

Synthroid should not be used to aid weight loss or treat obesity. In fact, the drug has a boxed warning Trusted Source PubMed Central Highly respected database from the National Institutes of Health Go to source about this from the Food and Drug Administration (FDA). A boxed warning alerts doctors and patients about drug effects that may be dangerous.

If you have normal thyroid function, taking Synthroid at typical dosages won’t help you lose weight. And taking a high dosage could cause serious or even life threatening side effects. These include fast or irregular heartbeat, heart attack, seizures, and coma.

Taking Synthroid with weight loss drugs, especially those that reduce appetite, can also cause serious or even life threatening side effects.

What to do

You should only take Synthroid as prescribed by your doctor. Don’t take it for weight loss or to treat obesity.

If you have weight loss while taking Synthroid, talk with your doctor. They may need to check your thyroid hormone levels and reduce your Synthroid dosage if the levels are too high.

Synthroid can sometimes cause partial hair loss in the first few months of treatment. However, this typically only occurs if your dosage is too high. And hair loss with Synthroid is usually temporary.

Also, keep in mind that hypothyroidism can cause hair loss. And Synthroid is used to treat this condition. So, your hair loss could be due to your condition rather than the drug.

What to do

If you have hair loss while taking Synthroid, talk with your doctor. They may check your thyroid hormone levels. If they’re too high, your doctor may reduce your Synthroid dosage.

Below, you’ll find dosages that are commonly recommended for Synthroid. However, you should take the dosage your doctor prescribes for you. They’ll recommend the dosage that’s best for your needs.

Most often, doctors start by prescribing a low dosage of Synthroid. Then, they’ll change the dosage over time to an amount that’s right for the condition being treated. Doctors typically prescribe the smallest dosage that gives the desired outcome.

Your doctor will calculate your dosage of Synthroid based on certain factors. The dosage of Synthroid that your doctor prescribes will depend on:

• your age
• your body weight
• any health conditions you have
• other medications you take
• the condition you’re using Synthroid to treat and the severity of the condition

Synthroid is available as follows.

• Form: oral tablet
• Strengths: 25 micrograms (mcg), 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, and 300 mcg

Recommended dosages of Synthroid in adults and children are described below.

Synthroid is prescribed for hypothyroidism and a certain form of thyroid cancer. To learn more about these conditions and how the drug is used, see the “Synthroid: Uses” section below.

Typical dosages for Synthroid in adults are as follows. Your doctor may adjust your dosage as needed, based on your symptoms and certain blood test results.

Typical dosages for Synthroid in children with hypothyroidism are as follows. The child’s doctor may increase their dose every 4 to 6 weeks as needed, based on their thyroid hormone levels and symptoms.

Below are some things to consider about Synthroid’s dosage.

• Missing a dose. If you miss a dose of Synthroid, call your doctor’s office to find out what to do. What they advise may depend on certain factors, such as when you last ate and what other medications you take. View these medication reminder options to help avoid missing doses. You could also set an alarm, use a timer, or download a reminder app on your phone.
• What is considered a high dose of Synthroid? More than 200 mcg per day would be considered a high dose of Synthroid.
• Length of treatment. Doctors typically prescribe Synthroid as a long-term treatment. You’ll likely take it long term if you and your doctor feel it’s safe and effective for your condition.

Doctors may prescribe drugs other than Synthroid for your condition. Certain drugs may work better for you than others.

Synthroid is used to treat hypothyroidism. Here’s a summary of other treatments, including drugs, that doctors sometimes prescribe for this condition.

To learn more about some alternatives to Synthroid, view the following articles:

• Synthroid vs. levothyroxine (Levoxyl, Tirosint, Unithroid)
• Synthroid vs. Armour Thyroid

For more information about alternatives to Synthroid, ask your doctor. They can tell you about other medications that could be prescribed for your condition.

Alcohol isn’t known to interact with Synthroid. If you drink alcohol, talk with your doctor about how much is safe to consume with Synthroid.

Synthroid may interact with other medications, certain supplements, and certain foods.

Different interactions can cause different effects. Some interactions can interfere with a drug’s effectiveness. Others can increase a drug’s side effects or cause them to be severe.

If any of the interactions listed below might pertain to you, talk with your doctor. They can tell you what you need to do to avoid the interaction.

• Synthroid and medications that reduce the absorption of Synthroid. Certain medications can reduce the absorption of Synthroid into your body. This can make Synthroid less effective. You should take Synthroid at least 4 hours before or 4 hours after taking these medications. Examples include:
  • antacids for indigestion, such as calcium carbonate (Tums) or magnesium hydroxide (Milk of Magnesia)
  • bile acid sequestrants for high cholesterol, such as colestipol (Colestid), colesevelam (Welchol), or cholestyramine (Prevalite)
  • phosphate binders, such as sevelamer (Renvela) or lanthanum (Fosrenol)
• Synthroid and other medications. Because Synthroid may interact with certain other drugs, your doctor may do extra monitoring or make dose adjustments if you take it with these drugs. Examples include:
  • the heart drug digoxin (Lanoxin)
  • the blood thinner warfarin (Jantoven)
  • the weight loss drug orlistat (Alli, Xenical)
  • the peptic ulcer drug sucralfate (Carafate)
  • the anti-gas drug simethicone (Gas-X)
  • proton pump inhibitors, such as omeprazole (Prilosec), pantoprazole (Prevacid), or esomeprazole (Nexium)
  • certain antidepressants, such as amitriptyline, nortriptyline (Pamelor), or mirtazapine (Remeron)
  • the seizure drug phenobarbital
  • the antibiotic rifampin
  • birth control drugs containing estrogen
• Synthroid and herbs and supplements. Certain mineral supplements can reduce the absorption of Synthroid into your body. This can make Synthroid less effective. You should take Synthroid at least 4 hours before or 4 hours after taking these supplements. Examples include products containing:
  • calcium
  • iron
  • magnesium
• Synthroid and foods. Certain medications interact with foods. Synthroid isn’t absorbed into your body as well if you take it with certain foods. Ask your doctor if there are foods not to eat while you’re taking this drug. Examples of foods and beverages that may interact with Synthroid include:
  • dietary fiber
  • walnuts
  • cottonseed meal
  • soybeans and soybean flour
  • grapefruit and grapefruit juice
  • caffeine-containing drinks, such as coffee

Your doctor will recommend how you should take Synthroid. It’s important that you take the drug exactly as your doctor instructs.

Synthroid comes as oral tablets. You’ll take this drug by swallowing it.

Here’s a list of common questions related to taking Synthroid.

• When should I take Synthroid? You should take Synthroid once every day, typically 30 minutes to 1 hour before you have breakfast. As well, you should take it at least 4 hours before or 4 hours after taking certain medications that interact with it. See “Synthroid: Interactions” above for details. Taking Synthroid around the same time of day helps keep a steady level of it in your body. This helps the medication work effectively. View these medication reminder options to help avoid missing doses of Synthroid. You could also set an alarm, use a timer, or download a reminder app on your phone.
• Do I need to take Synthroid with food, or should it be taken on an empty stomach? You should take Synthroid without food, on an empty stomach. Take it at least 30 minutes before eating or drinking anything other than water.
• How long after eating can I take Synthroid? If you’ve eaten, call your doctor or pharmacist to find out how long you should wait before taking Synthroid. Depending on the type of food you’ve eaten, you may need to wait 4 hours before taking Synthroid.
• Can Synthroid be chewed, split, or crushed? Yes, you can chew, split, or crush Synthroid tablets. For example, you can crush Synthroid to give it to a child who can’t swallow tablets. To do this, mix the crushed tablets with 1 to 2 teaspoons (5 to 10 milliliters) of water. Give this to the child right away using a spoon or dropper. Don’t mix crushed tablets with food, milk, or infant formula. And don’t save the mixture for later administration.
• Is there a best time of day to take Synthroid? Should I take Synthroid at night? The best time to take Synthroid is usually 30 minutes to 1 hour before eating breakfast. However, in some cases, your doctor may recommend taking it at night.

Prescription drugs, such as Synthroid, are approved by the Food and Drug Administration (FDA) to treat certain conditions.

Synthroid is FDA-approved to treat hypothyroidism in adults and children of all ages. Hypothyroidism is also known as an underactive thyroid gland. With this condition, your thyroid doesn’t make enough thyroid hormones, called T3 and T4.

Thyroid hormones are responsible for controlling your metabolism. Metabolism describes chemical reactions in your body that regulate how your body creates and uses energy. Your metabolism affects every process in your body, from digestion to cell growth and repair.

Hypothyroidism leads to a slow metabolism. It can cause many symptoms, including:

• fatigue
• weakness
• feeling cold
• weight gain
• constipation
• dry skin
• thin or brittle hair and nails

Synthroid is a lab-made copy of the T4 made naturally by your thyroid. Taking Synthroid increases the level of T4 in your blood. Your body then uses T4 to make T3. So, Synthroid helps increase your thyroid hormones back to normal levels.

Doctors prescribe Synthroid for hypothyroidism that’s congenital (present from birth) or acquired (developed later in life). They prescribe it to treat the three main forms of hypothyroidism, which are described below.

Synthroid is FDA-approved to lower levels of thyroid stimulating hormone (TSH) in people with a certain form of thyroid cancer. This is called TSH suppression.

Synthroid is approved for this use in adults with thyrotropin-dependent, well-differentiated thyroid cancer. With this form of thyroid cancer, the cancer cells are similar to healthy thyroid cells. As with healthy thyroid cells, the growth of these cancer cells is increased by TSH. (Another name for TSH is thyrotropin.)

Symptoms of thyrotropin-dependent, well-differentiated thyroid cancer may include:

• a lump in your throat
• nodules or swelling in your neck
• neck pain
• trouble swallowing
• hoarseness or voice changes that don’t go away

Normally, TSH is produced by your pituitary gland when you have low levels of thyroid hormones in your blood. TSH tells your thyroid to make more thyroid hormones. It also increases the growth of cells in your thyroid, including cancer cells.

Synthroid is a lab-made copy of a thyroid hormone called T4. Doctors prescribe the drug to stop the level of thyroid hormones in your blood from falling too low. This stops your pituitary gland producing TSH, helping slow the growth of this form of cancer.

Doctors prescribe Synthroid in combination with surgery and radioactive iodine to treat this type of thyroid cancer.

The manufacturer of Synthroid has stated certain limitations for the drug’s use. These are situations in which doctors will likely not prescribe the drug.

Limitations for Synthroid’s use state that the drug isn’t FDA-approved for:

• Treating hypothyroidism in people recovering from subacute thyroiditis (inflammation of the thyroid that causes hyperthyroidism at first, followed by hypothyroidism).*
• Suppressing benign (noncancerous) thyroid nodules and nontoxic diffuse goiter (enlarged thyroid) in people who have enough iodine in their body. Using Synthroid for these conditions isn’t beneficial and may lead to hyperthyroidism in some cases.

* Hyperthyroidism is a condition that causes high thyroid hormone levels. Hypothyroidism, on the other hand, causes low thyroid hormone levels.

When used for thyrotropin-dependent, well-differentiated thyroid cancer, Synthroid is prescribed with surgery and radioactive iodine treatment. To read more about this condition, see above.

Surgery is used to remove cancerous tissue. Radioactive iodine delivers radiation therapy to the thyroid. (The thyroid uses iodine to make thyroid hormones.)

Synthroid is FDA-approved for use in children of all ages with hypothyroidism. To read more about this condition, see above.

If you’re interested in taking Synthroid, you can find a doctor who may prescribe it by searching here. To prepare for your appointment, you may find it helpful to visit our appointment guides for hypothyroidism or cancer.

Here are some common questions about Synthroid and brief answers to them. If you’d like to know more about these topics, ask your doctor.

TSH stands for thyroid stimulating hormone (TSH). It’s released by your pituitary gland if your thyroid hormone levels are low. This hormone tells your thyroid to make more thyroid hormones.

If your TSH is high, this means your thyroid hormones are too low. If blood tests show that your TSH level is higher than the desired range, your doctor will typically increase your Synthroid dosage.

You won’t have withdrawal symptoms if you stop taking Synthroid. However, symptoms of your condition will likely come back.

Be sure to talk with your doctor before stopping Synthroid or any other prescribed medications.

T3 and T4 are thyroid hormones made naturally by your thyroid. Synthroid is a lab-made copy of natural T4.

To learn how this drug works, see the “Synthroid: How it works” section below.

No, that’s not likely. However, weight gain can be a symptom of hypothyroidism, which Synthroid is used to treat. (Hypothyroidism is a condition that causes low thyroid hormone levels.)

If you have weight gain while taking Synthroid, it may be because your dosage is too low. Talk with your doctor if you have weight gain while taking this drug. They can recommend what to do.

Synthroid is a prescription drug approved by the Food and Drug Administration (FDA). It contains levothyroxine, which is a lab-made copy of the natural thyroid hormone T4.

NP thyroid is a prescription drug that hasn’t been approved by the FDA. It contains natural levothyroxine (T4) and liothyronine (T3) obtained from pig thyroids.

Your body makes T3 from T4. So, when treating hypothyroidism, most doctors prescribe a drug that only contains T4. (Hypothyroidism is a condition that causes low thyroid hormone levels.)

If you’re interested in taking NP thyroid instead of Synthroid, talk with your doctor. They can recommend the treatment option that’s best for you.

Yes, in general, you can take Synthroid and vitamin D together. Doctors sometimes prescribe these drugs together if blood tests show low vitamin D levels as well as low thyroid hormone levels.

If you’d like to know if you should take vitamin D with Synthroid, ask your doctor.

Like other medications, prices for Synthroid may vary. The drug’s price will depend on factors such as:

• the pharmacy you use
• your insurance coverage

Here’s a list of things to consider when looking into the cost of Synthroid.

• Option for a 90-day supply. For some drugs, it’s possible to get a 90-day supply. If this option is approved by your insurance company, it can help lower the cost of the drug. It can also help you avoid frequent trips to your pharmacy. If you’d like to learn more about this option, ask your doctor, pharmacist, or insurance company.
• Need for prior authorization. Before insurance coverage for Synthroid is approved, your insurance company may require prior authorization. In this case, your doctor and insurance company will communicate about your prescription for Synthroid. Then, the insurance company will decide if the drug will be covered. To find out if you need prior authorization for Synthroid, contact your insurance company.
• Possible cost assistance options. Financial assistance to help lower the cost of Synthroid is available. A copay card for Synthroid may help reduce the drug’s cost. To learn more and see if you’re eligible for support, visit the manufacturer’s website. Also, check out this article to learn about ways to save on prescription drugs.
• Use of a mail-order pharmacy. Synthroid may be dispensed through mail-order pharmacies. Getting your prescription through a mail-order pharmacy could lower its cost. It can also allow you to get the drug without leaving home. To find out more about this option, check with your doctor, pharmacist, or insurance company.
• Availability of a generic form. Synthroid comes in a generic form called levothyroxine. A generic drug is an exact copy of the active drug in a brand-name medication. Generics are typically less expensive than brand-name drugs. If your doctor prescribes Synthroid but you want to learn about using levothyroxine, talk with your doctor about which option might be better for you. Also, check your insurance plan because it might cover just one form or the other.

Doctors prescribe Synthroid to treat:

• hypothyroidism (a condition that causes low thyroid hormone levels)
• a certain form of thyroid cancer

To learn about these conditions and how Synthroid is used for them, see the “Synthroid: Uses” section above.

With hypothyroidism, your thyroid doesn’t make enough of the thyroid hormones T3 and T4. Because of this, you have low levels of these hormones in your blood.

Synthroid is a lab-made copy of natural T4. Doctors prescribe it to replace your natural T4. Once it’s in your blood, your body uses T4 to make T3. So, Synthroid helps your thyroid hormones return to normal levels.

Doctors prescribe Synthroid to people with thyrotropin-dependent, well-differentiated thyroid cancer. With this form of thyroid cancer, the growth of the cancer cells is increased by a hormone called thyroid stimulating hormone (TSH).

Your pituitary gland produces TSH when you have low levels of thyroid hormones in your blood. TSH tells your thyroid to make more thyroid hormones.

Taking Synthroid stops the level of thyroid hormones in your blood from falling too low. This stops your pituitary gland from producing TSH. This is called TSH suppression. It helps slow the growth of this type of thyroid cancer.

Synthroid typically takes a few weeks to start working. Your doctor will likely order blood tests after about 4 to 6 weeks of treatment to check your thyroid hormone levels.

You may notice your symptoms start to improve during this time. However, based on the results of your blood tests, your doctor may need to adjust your Synthroid dosage to make sure the drug is working properly for you.

Synthroid has a half-life of about 6 to 7 days in people with normal thyroid function. This timeframe increases to about 9 to 10 days in people with underactive thyroid function.

A drug’s half-life describes how long it takes your body to remove half of a dose of the drug.

It takes about five half-lives for your body to fully remove a drug from your system. So, if your thyroid function is normal, Synthroid stays in your system for about 4 to 5 weeks after you take a dose. If your thyroid function is underactive, it stays in your system for about 6 to 7 weeks.

It’s important to try and keep levels of Synthroid steady in your body while you’re taking the drug. To do this, you shouldn’t skip doses of Synthroid. This is because your thyroid hormone levels will start to fall, even if it occurs slowly.

Serious effects can occur if you use more than the recommended dosage of Synthroid. Do not use more Synthroid than your doctor recommends.

Symptoms that an overdose could cause are due to high thyroid hormone levels in your blood. These include:

• headache
• nervousness
• insomnia
• muscle spasm or tremor
• diarrhea
• feeling hot
• fast or irregular heartbeat
• heart attack
• confusion
• disorientation
• seizures
• coma

Call your doctor if you think you’ve taken too much of this drug. Also, you can call the American Association of Poison Control Centers at 800-222-1222 or use its online tool. However, if your symptoms are severe, call 911 or your local emergency number. Or go to the nearest emergency room right away.

Doctors generally consider Synthroid safe for use during pregnancy. Clinical studies haven’t found any increased risk of health problems when levothyroxine is taken during pregnancy. (Levothyroxine is the active drug in Synthroid.) This includes pregnancy loss and congenital anomalies (commonly called birth defects).

If you have hypothyroidism and become pregnant or plan to, it’s important to continue taking thyroid hormones as prescribed by your doctor. (Hypothyroidism is one of Synthroid’s approved uses. It’s a condition that causes low thyroid hormone levels.)

If hypothyroidism isn’t treated during pregnancy, the risk of certain complications is increased. These include pregnancy loss, high blood pressure, and premature delivery.

Your doctor will order more blood tests than usual to check your thyroid hormone levels during pregnancy and after delivery. And they’ll adjust your dosage of Synthroid as needed to make sure your thyroid hormones stay in the desired range.

Talk with your doctor about your birth control needs while taking Synthroid if you’re sexually active and you or your partner can become pregnant. Your doctor can determine if you should use birth control with this medication.

It’s not known if it’s safe to breastfeed while taking Synthroid. Levothyroxine can pass into breast milk, but it’s not known if it can affect a child who’s breastfed. (Levothyroxine is the active drug in Synthroid.)

Talk with your doctor about the risks and benefits of breastfeeding while taking Synthroid.

This drug comes with several precautions.

This drug has a boxed warning Trusted Source PubMed Central Highly respected database from the National Institutes of Health Go to source , the most serious warning from the Food and Drug Administration (FDA). A boxed warning alerts doctors and patients about drug effects that may be dangerous.

Synthroid should not be used to aid weight loss or treat obesity. It is approved for treating certain thyroid conditions.

If you have normal thyroid function, taking Synthroid at typical prescribed dosages won’t help with weight loss. And taking a high dosage could cause serious or even life threatening side effects. These include fast or irregular heartbeat, heart attack, seizures, and coma.

Taking Synthroid with weight loss drugs, especially those that reduce appetite, can also cause serious or even life threatening side effects.

Only take Synthroid as prescribed by your doctor. To learn more, see the “Synthroid: Side effects” section above.

Tell your doctor about your health history before starting treatment with Synthroid. Your doctor may not recommend this medication if you have certain factors affecting your health or specific medical conditions.

These factors and conditions include those listed below.

• Allergic reaction. Your doctor will likely not prescribe Synthroid if you’ve had an allergic reaction to it or any of its ingredients. To find out about other treatment options, talk with your doctor.
• Myxedema coma. Myxedema coma is a life threatening complication of severe hypothyroidism. (And Synthroid is used to treat hypothyroidism.) Doctors don’t prescribe Synthroid to treat myxedema coma. This condition is a medical emergency that’s typically treated with thyroid hormones given by IV injection.
• Adrenal insufficiency. With adrenal insufficiency, your adrenal glands don’t make enough adrenal hormones, such as cortisol. Synthroid can also lower levels of these hormones. If you have unmanaged adrenal insufficiency, taking Synthroid can cause an adrenal crisis. With this condition, your adrenal hormone levels fall too low. Unmanaged adrenal insufficiency is a contraindication for Synthroid. (A contraindication is a factor or condition that could prevent your doctor from prescribing a drug due to risk of harm.) Before starting Synthroid, your doctor will prescribe adrenal hormones to make sure your adrenal hormones are in the desired range.
• Heart problems. Synthroid can sometimes cause heart problems, such as fast or irregular heartbeat, chest pain, or heart attack. If you already have a heart problem, such as atrial fibrillation or coronary artery disease, you may have an increased risk of these side effects. Your doctor will likely prescribe a starting dosage of Synthroid that’s lower than usual. And they’ll likely monitor your heart more often than usual to make sure your condition isn’t worsening.
• Diabetes. If you have diabetes, Synthroid may increase your blood sugar levels and make your diabetes harder to manage. Your doctor may ask you to check your blood sugar level more often than usual if you take Synthroid. And they may increase the dosage of your diabetes medication.
• Pregnancy. Synthroid is considered safe to take during pregnancy, but your doctor will likely need to adjust your dosage of the drug. If you’d like to learn more about taking Synthroid while pregnant, view the “Synthroid: Taking while pregnant” section above.
• Breastfeeding. Synthroid passes into breast milk. If you’d like to learn more about taking Synthroid while breastfeeding, view the “Synthroid: Taking while breastfeeding” section above.

To learn more about effects of Synthroid that could be harmful, see the “Synthroid: Side effects” section above.

Here’s some information about Synthroid’s expiration date, as well as how to store and dispose of the drug.

• Expiration. Your pharmacist will add an expiration date to the label on Synthroid’s bottle. This date is usually 1 year from the date the medication was dispensed to you. Expiration dates help ensure that a medication is effective during a period of time. The Food and Drug Administration (FDA) advises Trusted Source Food and Drug Administration (FDA) Governmental authority Go to source that you avoid using expired drugs. If you have an unused medication and it’s past its expiration date, talk with your pharmacist. They can let you know whether you might still be able to use the medication.
• Storage. Many factors determine how long a medication remains good to use. These factors include how and where you store the drug. Synthroid tablets should be stored at room temperature. Avoid storing the drug in areas where it could get damp or wet, such as bathrooms. The medication should be kept away from light in a tightly sealed container.
• Disposal. It’s important to safely dispose of Synthroid if you no longer need to take it and have unused medication. Doing so helps prevent others, including children and pets, from accidentally taking the drug. It also helps avoid causing harm to the environment. Ask your pharmacist for information about disposing of Synthroid. Also, check out this page for several tips on safe medication disposal.

If you have questions about Synthroid, talk with your doctor. They can help advise you on whether Synthroid could be a good treatment option for you.

Here’s a list of questions you may want to ask your doctor.

• How often will I need to have blood tests while I’m taking Synthroid?
• Will my Synthroid dosage need to be changed if I become ill?
• Will Synthroid affect my current birth control method?

Your doctor may also tell you about other treatment options for your condition. You may find this article helpful in learning about alternative treatments for thyroid conditions. And check out our selection of videos on thyroid disease.

Disclaimer: Healthgrades has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.