Synthroid's Dosage: What to Know

Medically Reviewed By Alex Brewer, PharmD, MBA

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Synthroid should not be used to treat obesity or for weight loss. If your thyroid works properly, a normal dose of Synthroid will not cause you to lose weight. Larger doses may cause toxicity that can be severe and even life threatening. Due to this risk, doctors typically will not prescribe Synthroid for weight loss or obesity.

Synthroid: Introduction

Synthroid is a brand-name prescription medication that contains the active drug levothyroxine. It is a lab-made copy of the natural thyroid hormone known as T4. Synthroid is available in a generic form.

This medication has been approved by the Food and Drug Administration (FDA) to treat the following conditions:

In this article, you’ll find additional information about Synthroid’s dosage and details on how to take the drug. For a comprehensive overview of Synthroid, view this article.

Finding a healthcare professional

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

This article describes typical recommended dosages for Synthroid. These dosages are provided by the drug’s manufacturer.

Always follow the dosage of Synthroid that your doctor prescribes.

Synthroid: Dosage

Commonly recommended dosages for Synthroid are shown below.

You should take the dosage your doctor prescribes for you. They’ll recommend the dosage that’s best for your condition and thyroid hormone level.

Synthroid’s form and strengths

Synthroid is an oral tablet that is taken by mouth.

It’s available in 12 strengths:

  • 25 micrograms (mcg)
  • 50 mcg
  • 75 mcg
  • 88 mcg
  • 100 mcg
  • 112 mcg
  • 125 mcg
  • 137 mcg
  • 150 mcg
  • 175 mcg
  • 200 mcg
  • 300 mcg

Typical recommended dosages

Usually, doctors will prescribe a low dosage of Synthroid at the start of treatment. Then, they’ll adjust the dosage over time if needed. Ultimately, your doctor will prescribe the smallest dosage of Synthroid that gives the desired effect.

Your starting dosage of Synthroid will depend on several factors. To view a list of these, see the “Synthroid: Dosage considerations” section below. Your doctor will prescribe a starting dosage and continue to monitor your thyroid hormone level. Your doctor will change your dosage as needed to get your thyroid hormone level into a desired range.

Your maintenance dosage will keep your thyroid hormone level in the desired range. The maintenance dosage can change over time. As such, your doctor will continue to monitor and adjust your dosage as needed.

If you are taking Synthroid for hypothyroidism, it may take 4 to 6 weeks to determine if your current dosage will be your maintenance dosage. If your doctor needs to adjust your dosage, it could take an additional 4 to 6 weeks to test each new dosage. This process will continue until you and your doctor find the best dosage for you.

Synthroid dosage chart for adults

Below is a dosage chart for Synthroid in adults. It includes the starting dosage for most adults with hypothyroidism. Some adults will need a lower starting dosage. Your dosage will depend on your symptoms and thyroid hormone level.

Condition Starting dosage Maintenance dosage Maximum dosage
hypothyroidism 1.6 mcg per kilogram (kg) of body weight once per day depends on your symptoms and thyroid hormone level 300 mcg per day
thyroid cancer depends on the level of thyroid hormone suppression needed depends on your thyroid hormone level 300 mcg per day*

* Your doctor may prescribe a higher dosage if you have thyroid cancer. They’ll recommend the lowest dose for the shortest time possible as part of your treatment plan.

Children’s dosage

The chart below shows starting dosage recommendations of Synthroid for hypothyroidism in children. The dosage depends on age and body weight. Your child’s doctor will adjust the dosage, as necessary, until your child’s thyroid hormone levels are within a desired range. The dosage that achieves this will be your child’s maintenance dosage.

Age Starting dosage
0 to 3 months 10 mcg to 15 mcg per kilogram (kg) of body weight once per day
3 to 6 months 8 mcg to 10 mcg per kg of body weight once per day
6 to 12 months 6 mcg to 8 mcg per kg of body weight once per day
1 to 5 years 5 mcg to 6 mcg per kg of body weight once per day
6 to 12 years 4 mcg to 5 mcg per kg of body weight once per day
older than 12 years but growth and puberty incomplete 2 mcg to 3 mcg per kg of body weight once per day
older than 12 years with growth and puberty complete 1.6 mcg per kg of body weight once per day

The recommended dosage for thyroid cancer in children depends on the level of thyroid hormone suppression needed. Your child’s doctor will recommend the dosage that’s best for them.

Length of treatment

Doctors typically prescribe Synthroid as a long-term treatment for hypothyroidism. You’ll likely take it for the rest of your life if you and your doctor feel it’s safe and effective for your condition.

The length of treatment for thyroid cancer will depend on how well the drug is managing the cancer. The length of treatment may vary between people.

Synthroid: Common questions about dosage

Below are some common questions related to Synthroid’s dosage.

Can my Synthroid dose be calculated by weight?

Yes, your doctor will prescribe a Synthroid dosage that is based on your weight and other specific factors. See the “Synthroid: Dosage considerations” section below to learn more.

Talk with your doctor if you have questions or concerns about your Synthroid dosage.

What doses of Synthroid are available?

Synthroid is available is several strengths. These range from the lowest dose of 25 micrograms (mcg) to the highest dose of 300 mcg. The Synthroid dosage your doctor prescribes depends on your weight, thyroid hormone levels, and symptoms.

If you’d like to know more about recommended dosages of Synthroid, talk with your doctor.

Is there a normal dose of Synthroid?

No, there is not a “normal” or average dose of Synthroid. Dosing is specific to each person. Your dosage depends on several factors, including your weight and thyroid hormone levels. Your doctor will adjust your dosage over time based on your blood test results and other factors.

Ask your doctor about how they determine your Synthroid dosage.

If I’m experiencing side effects of Synthroid, will my doctor lower my dosage?

Your doctor may lower your Synthroid dosage if you’re experiencing side effects. Most side effects are due to a dosage that’s too high for you. This can lead to hyperthyroidism, which occurs when thyroid hormone levels are too high.

Tell your doctor as soon as you experience possible side effects of Synthroid or other symptoms. Your doctor will adjust your dosage to find what works best for you.

To learn more about Synthroid’s side effects, see this article. You can also talk with your doctor about the risks of overmedication, which are described in this article.

Synthroid: Dosage considerations

Your doctor will consider several factors when prescribing your dosage of Synthroid. These factors include:

  • your age
  • your weight
  • the condition you’re taking Synthroid to treat
  • the severity of your condition
  • other medical and health conditions, including pregnancy
  • other medications you take
  • presence of side effects

Recommended adjustments in dosage

Your doctor will adjust your dosage as necessary until they find the dosage that is best for you. They may either increase or decrease your dosage to do this. Your maintenance dosage will be the dosage that keeps your thyroid hormone level in a desired range without causing side effects.

You will have regular blood tests to measure your thyroid hormone level. Your doctor will monitor your thyroid hormone level while you take Synthroid. Your maintenance dose could change over time based on your test results.

Synthroid: How to take

Synthroid comes as an oral tablet. You’ll take it by mouth once per day.

Your doctor may recommend that you take Synthroid around the same time of day. Usually, this is when you first wake up, 30 to 60 minutes before eating your first meal or drinking something other than water. This practice helps maintain a consistent amount of the drug in your body. And this can help Synthroid work effectively.

You may need to take Synthroid 4 hours before or after taking certain other medications. If you take other medications, talk with your doctor about the best time to take Synthroid.

Certain foods may interfere with the effects of Synthroid. You may need to take your dose at a different time than when you consume:

  • soy products
  • walnuts
  • dietary fiber*
  • grapefruit juice

* Dietary fiber may reduce the amount of Synthroid your body can absorb. Dietary fiber is also an important part of a healthy diet. Talk with your doctor about how to balance your dietary fiber needs while taking Synthroid.

If you have difficulty swallowing tablets, view this article. It provides suggestions on how to take this form of medication. Also, talk with your doctor or pharmacist about taking your medication.

Below are tips for giving Synthroid to children who cannot swallow tablets:

  • Crush the tablet and add it to a small amount (5 milliliters [mL] to 10 mL) of water.
  • Immediately give the mixture to the child by using a spoon or dropper.
  • Do not store the mixture.
  • Do not give your child Synthroid in soy-based formula.

If you need help, ask your doctor or pharmacist. They can show you how to create this mixture and give it to your child.


If you find it challenging to read the label on your prescription, tell your doctor or pharmacist. Certain pharmacies provide medication labels with large print or braille. They may also offer labels containing a scannable code that your smartphone can convert from text to speech. If your pharmacy doesn’t provide these choices, your doctor or pharmacist may be able to recommend a pharmacy that does.

If it’s difficult for you to open medication bottles, ask your pharmacist if they can dispense Synthroid in an easy-open container. They also may recommend ways to help make it easier to open the bottles.

Synthroid: Missing a dose

If you miss a dose of Synthroid, call your doctor’s office. They can recommend whether you should take the missed dose. It may depend on when you took your last dose and the last time you ate. See the “Synthroid: How to take” section above for details.

If you missed your Synthroid dose for 2 to 3 days or more, call your doctor. They may want to check your thyroid hormone levels. They may also ask you about possible symptoms due to low thyroid hormone levels.

View these medication reminder options to help avoid missing doses. You could also set an alarm or download a reminder app on your phone.

Synthroid: Overdose

If you take more than the recommended dosage of Synthroid, serious effects can occur. Do not take more Synthroid than your doctor prescribes. For details about dosages, see the “Synthroid: Dosage” section above.

Symptoms of overdose

Symptoms that an overdose of Synthroid could cause include:

Synthroid overdose can also cause death.

What to do in case of overdose

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 a local emergency number. Or go to the nearest emergency room right away.

Synthroid: What to discuss with your doctor

Talk with your doctor or pharmacist for additional information about Synthroid’s dosage. Keep in mind that the dosages presented in this article are typical dosages provided by the drug’s manufacturer. Your doctor will recommend the dosage of Synthroid that’s right for you and your condition. Always follow the dosage that your doctor prescribes for you.

As with any medications you’re taking, do not change your Synthroid dosage without a recommendation from your doctor.

In addition to discussing Synthroid with your doctor, you may find the following articles helpful in learning more.

  • Overview of Synthroid. For comprehensive details on Synthroid, see this article.
  • Drug comparisons. To learn how Synthroid compares with levothyroxine, read this article.
  • Information on side effects. If you’d like to know about possible side effects of Synthroid, view this article.
  • Details about Synthroid’s uses. To learn more about the conditions Synthroid is used to treat, see these articles on thyroid cancer and hypothyroidism.

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.

Medical Reviewer: Alex Brewer, PharmD, MBA
Last Review Date: 2022 Nov 18
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