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Synthroid side effects: A detailed guide

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More common side effectsMild side effectsSerious side effectsSide effects in childrenFAQsCouponSide effects in detailWarningsAsk your doctor
Synthroid is prescribed for hypothyroidism (low thyroid hormone levels) and certain thyroid cancers. It’s a thyroid hormone drug that may cause mild or serious side effects, including hair loss and heart rhythm problems.
Medically reviewed by Alex Brewer, PharmD, MBA
Updated on April 7, 2023

Synthroid is a brand-name medication that comes as a tablet you swallow. It’s available in a generic version called levothyroxine. This article describes possible side effects of Synthroid. You’ll also find information on cost savings and coupon options for Synthroid.

You can explore this overview article to learn more about Synthroid, including the drug’s uses, dosage, ways to save on cost, and more.

Boxed warning: Not to be used for weight loss or to treat obesity

Synthroid has a boxed warning about how it should not be used for weight loss or to treat obesity. Boxed warnings are serious warnings from the Food and Drug Administration (FDA).

For details, see the “Warnings for Synthroid” section below.

More common side effects of Synthroid

Synthroid may cause mild or serious side effects. Below are some of the more common side effects of this medication.

Commonly reported side effects of Synthroid include:

  • diarrhea
  • hair loss*
  • headache
  • nervousness

* To learn more about this side effect, see the “Side effects of Synthroid in detail” section below.

Mild side effects of Synthroid

Synthroid may cause certain mild side effects. Usually, mild side effects are temporary and can be easily managed. Tell your doctor or pharmacist if you have side effects that don’t go away or become bothersome.

Mild side effects of Synthroid that occurred in studies include:

Synthroid may cause more mild side effects than those listed above. For details, see the drug’s prescribing information.

* To learn more about this side effect, see the “Side effects of Synthroid in detail” section below.

Serious side effects of Synthroid

Synthroid may cause certain serious side effects. If you have any serious side effects from Synthroid, call your doctor right away. But if you think you’re having a medical emergency or your side effects feel life threatening, take immediate action. Call 911 or a local emergency number, or go to the nearest emergency room.

After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. Visit MedWatch’s website if you’d like to notify the FDA about a side effect you’ve had with Synthroid.

Serious side effects of Synthroid that occurred in studies include:

* To learn more about this side effect, see the “Side effects of Synthroid in detail” section below.

Side effects of Synthroid in children

Most of the side effects of Synthroid are the same in adults and children. But children and infants taking Synthroid can also have problems with growth and development.

These include:

  • Early closure of an infant’s fontanelle (also called soft spots). This can cause an unevenly shaped skull and pressure inside the skull.
  • Early closure of a child’s bone growth plates (areas of new bone growth). This can cause reduced growth and adult height.

Usually, these side effects occur when taking too much Synthroid. Your child’s doctor will order blood tests to closely monitor Synthroid levels in your child. This can help lower the risk for these side effects.

Other side effects reported in children include:

  • pseudotumor cerebri (a condition that causes symptoms similar to those of a brain tumor), which causes increased pressure inside the skull
  • slipped capital femoral epiphysis, which is a hip joint disorder

If you’d like to know more about Synthroid’s side effects in children, talk with your child’s doctor.

Common questions about side effects of Synthroid

Here are answers to some common questions about side effects of Synthroid. If you have other questions, talk with your doctor or pharmacist.

How do the side effects of Synthroid compare with those of levothyroxine?

Synthroid contains the active drug levothyroxine. An active drug is the ingredient that makes a medication work. So, the side effects of Synthroid and levothyroxine are expected to be the same.

To learn more about levothyroxine, including its possible side effects, talk with your doctor or pharmacist.

Will Synthroid cause certain side effects in females versus males?

Yes, Synthroid can cause different side effects in females compared with males.*

For example, the drug can cause menstrual period changes or irregularities, such as:

Tell your doctor if you have any changes in your period while taking this drug. It could mean that your doctor needs to adjust your Synthroid dosage.

* Sex and gender exist on spectrums. In this article, we use the terms “male” and “female” to refer to a person’s sex assigned at birth.

Can older adults have different side effects compared with younger adults?

No, older adults won’t have different side effects from Synthroid than younger adults.

That said, older adults are more likely to have certain side effects with Synthroid. These include heart rhythm problems and angina (a type of chest pain). This usually results from having too much Synthroid in the body.

Due to this risk, doctors often start with a lower dosage of Synthroid in older adults. Older adults generally refers to people ages 65 years and older.

If you have concerns or questions about taking Synthroid given your age, talk with your doctor or pharmacist.

Does Synthroid cause long-term side effects?

Most of Synthroid’s side effects are caused by having too much of the drug in the body. This can lead to symptoms of hyperthyroidism (high thyroid hormone levels).

Once doctors adjust the drug’s dosage, its side effects usually improve. But some of them can have long-term effects. These include heart problems and osteoporosis (decreased bone mineral density).

Tell your doctor if you have any side effects that aren’t going away or become bothersome or serious. It may be necessary for your doctor to adjust your dosage.

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Side effects of Synthroid in detail

Below you’ll find details on some of the side effects of Synthroid.

Hair loss

In some people, Synthroid can cause hair loss. But it isn’t common. When it occurs, the hair loss is usually partial. This means it isn’t a complete loss of all hair.

Hair loss usually occurs in the first few months of Synthroid treatment and is temporary. It should get better, even if you continue taking Synthroid.

Hair loss can also be a symptom of too much or too little thyroid hormone. Keep in mind, Synthroid is prescribed to treat hypothyroidism (low thyroid hormone levels).

Ways to manage

If you have hair loss while taking Synthroid, tell your doctor. They can order tests to make sure your thyroid hormone levels are in the right range. If the levels are too high or too low, your doctor will adjust your Synthroid dosage.

If your hair loss is due to Synthroid, know that it’s usually temporary. Your doctor can suggest ways to manage it while you wait for hair to grow back.

Heart rhythm problems

Heart rhythm problems are possible side effects of Synthroid. These include increased heart rate or heart arrhythmia. Usually, these problems are caused by having too much Synthroid in the body.

Symptoms of heart rhythm problems can include:

Ways to manage

If you think you’re having heart rhythm problems, call your doctor right away. But if your symptoms feel life threatening, seek immediate medical care. Call 911 or a local emergency number, or go to the nearest emergency room.

If you develop heart rhythm problems while taking Synthroid, your doctor may stop your treatment for a while. Then, they’ll likely have you restart Synthroid at a lower dosage.

Trouble sleeping

Sleep problems are possible side effects of Synthroid. These can include insomnia, which is trouble falling asleep or staying asleep.

As with most other side effects of the drug, sleep problems are usually caused by having too much Synthroid in the body.

Ways to manage

Tell your doctor if you’re having trouble sleeping. It could mean the Synthroid level in your body is too high. Your doctor can order tests to check your Synthroid level. If it’s too high, your doctor may need to adjust your dosage.

Allergic reaction

For some people, taking Synthroid can cause an allergic reaction. The allergic reaction is thought to be to the inactive ingredients in the drug and not to the drug itself. An inactive ingredient is any ingredient that isn’t meant to have an effect in the body. An example is dyes.

Symptoms can be mild or serious, and they may include:

  • itchiness
  • skin rash
  • swelling under the skin, usually affecting eyelids, lips, hands, or feet
  • swelling of the mouth, tongue, or throat, which can make it hard to breathe
  • flushing (temporary warmth, redness, or deepening of skin color)

Ways to manage

For mild allergic reaction symptoms, such as a mild rash, call your doctor right away. They may recommend treatments to help manage your symptoms. They’ll also let you know whether you should keep taking the medication.

For severe allergic reaction symptoms, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms require immediate medical care because they can become life threatening. If you’ve had a serious allergic reaction to Synthroid, your doctor may recommend taking a different medication.

Warnings for Synthroid

Below are some factors you may want to discuss with your doctor before taking Synthroid.

Boxed warning: Not to be used for weight loss or to treat obesity

Synthroid has a boxed warning about how it should not be used for weight loss or to treat obesity. Boxed warnings are serious warnings from the Food and Drug Administration (FDA).

In people with healthy thyroid function, taking the usual dosages of Synthroid won’t lead to weight loss.

Taking higher than usual dosages in an attempt to lose weight can cause serious life threatening side effects. These include:

The risk of these side effects increases when Synthroid is taken with other weight loss medications.

Due to these risks, you should only take Synthroid exactly as your doctor prescribes it.

Other warnings

It may not be safe for you to take Synthroid if you have specific health conditions. These are sometimes called drug-condition interactions. Other things may also affect whether Synthroid is a safe treatment option for you.

Before taking Synthroid, tell your doctor about your health and medications you take. Things to consider include:

Synthroid and pregnancy

In general, Synthroid is considered safe to take during pregnancy. That said, talk with your doctor if you are pregnant or can become pregnant. They may need to adjust your dosage of Synthroid during pregnancy.

Synthroid and breastfeeding

In general, Synthroid is considered safe to take while breastfeeding. That said, ask your doctor whether they feel it’s safe for you specifically.

Synthroid and alcohol

Synthroid isn’t known to interact with alcohol.

If you drink alcohol, talk with your doctor about whether it’s safe to consume it during your treatment with Synthroid.

What to ask your doctor

This article describes common and serious side effects of Synthroid. If your doctor prescribes this medication, they can discuss possible side effects with you. Talk with your doctor if you have concerns or questions about treatment with this medication.

Here’s a list of some possible questions you may want to ask your doctor:

  • Do I take any other medications that could cause side effects with Synthroid?
  • Am I at a higher risk of any side effects from Synthroid?
  • How often will you check my Synthroid levels to see if they are too high or too low?

Disclaimer: Optum Perks 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.

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