If you have hypothyroidism or thyroid cancer, your doctor might suggest Synthroid (levothyroxine) as a treatment option for you.

Synthroid is a prescription medication that’s used to:

  • treat primary, secondary, or tertiary hypothyroidism in adults and children
  • suppress (decrease) thyroid-stimulating hormone levels in adults, after you’ve had radioactive iodine treatment or surgery for thyroid cancer

This article describes the dosages of Synthroid, including its form, strengths, and how to take the drug. To learn more about Synthroid, including its limitations of usage, see this in-depth article.

Note: This article covers Synthroid’s typical dosages, which are provided by the drug’s manufacturer. But when using Synthroid, always take the dosage that your doctor prescribes.

Below are details about Synthroid’s form, strengths, and typical dosages.

What is Synthroid’s form?

Synthroid comes as a tablet that’s taken by mouth. The tablets come in different colors depending on which strength they are.

What strengths does Synthroid come in?

Synthroid is available in 25-microgram (mcg) strength intervals, as follows: 25 mcg, 50 mcg, 75 mcg, 100 mcg, 125 mcg, 150 mcg, 175 mcg, and 200 mcg. The following strengths are also available:

  • 88 mcg
  • 112 mcg
  • 137 mcg
  • 300 mcg

Note: Synthroid comes in micrograms rather than milligrams (mg). One milligram is equal to 1,000 micrograms.

What are the typical dosages of Synthroid?

Typically, your doctor will start you on a low dosage level. Then they may adjust your dosage as needed during treatment, by amounts of 12.5 mcg to 25 mcg. Your doctor will ultimately prescribe the smallest dosage that provides the desired effect.

Your dosage may be changed to make sure the drug is helping you meet your thyroid-stimulating hormone (TSH) treatment goals. This may also be done to reduce any side effects you’re having. See the “What factors can affect my dosage?” section below.

The information below describes common dosages that are typically used or recommended. However, be sure to take the dosage your doctor prescribes for you. Your doctor will determine the best dosage to fit your needs.

Dosage chart for hypothyroidism

The recommended Synthroid dosage for treating hypothyroidism depends on the cause of your hypothyroidism and how long since you’ve been diagnosed with the condition.

The dosage chart below provides an overview of Synthroid dosage recommendations for adults. These dosages are based on condition and body weight, which is given as micrograms per kilogram (mcg/kg). For more detailed information about each dosage, see the sections below.

ConditionRecommended Synthroid starting dosage in adultsHow often it’s takenHow often your dosage may be adjusted
Newly diagnosed primary hypothyroidism1.6 mcg/kg Once per dayEvery 4–6 weeks
Primary hypothyroidism that’s severe and longstanding12.5 mcg to 25 mcgOnce per dayEvery 2–4 weeks
Secondary or tertiary hypothyroidism1.6 mcg/kgOnce per dayEvery 4–6 weeks

Synthroid is also approved to treat hypothyroidism in children. For this use, recommended dosages vary based on the child’s age and weight. See “What’s the dosage of Synthroid for children?” below for a pediatric dosage chart.

Note: For treating hypothyroidism, a Synthroid dosage that’s higher than 200 mcg in 24 hours is typically considered high. There isn’t a maximum dosage, but needing to take more than 300 mcg per day may suggest that Synthroid isn’t effective for treating your condition.

Dosage for newly diagnosed primary hypothyroidism in adults

Synthroid is approved to treat primary hypothyroidism that’s recently diagnosed. The typical starting dosage for this use in adults is 1.6 micrograms per kilogram of body weight (mcg/kg), once per day.

One kilogram is equal to about 2.2 pounds (lb). For example, an adult weighing 68 kg (about 150 lb) may take 100 mcg to 112 mcg of Synthroid per day as their starting dosage.

Your doctor may adjust your dosage every 4 to 6 weeks as needed during treatment.

Dosage for severe, longstanding primary hypothyroidism in adults

Synthroid is approved to treat primary hypothyroidism that’s severe and longstanding. The typical starting dosage for this use in adults is 12.5 mcg to 25 mcg, once per day.

Your doctor may adjust your dosage every 2 to 4 weeks as needed during treatment.

Dosage for secondary or tertiary hypothyroidism in adults

Synthroid is approved to treat secondary or tertiary hypothyroidism. The typical starting dosage for this use in adults is 1.6 mcg/kg, once per day.

Your doctor may adjust your dosage every 4 to 6 weeks as needed during treatment.

Dosage for TSH suppression in adults

Synthroid is approved to suppress (decrease) thyroid-stimulating hormone (TSH) levels, as part of thyroid cancer treatment in adults. The usual goal for TSH suppression is to keep TSH levels lower than 0.1 international units per liter (IU/L). For this use, the typical Synthroid dosage is at least 2 mcg/kg, once per day.

For example, an adult weighing 68 kg (about 150 lb) may take 137 mcg of Synthroid per day as their starting dosage.

However, your dosage may be higher or lower than 2 mcg/kg depending on your current TSH levels, your treatment goals, and the type of thyroid cancer you have. Your doctor will monitor your thyroid hormone levels during treatment and may adjust your Synthroid dosage as needed.

Is Synthroid used long term?

Yes, Synthroid is typically used as a long-term treatment. If you and your doctor determine that Synthroid is safe and effective for you, it’s likely that you’ll use it long term.

What’s the dosage of Synthroid for children?

For treating hypothyroidism in children, Synthroid’s recommended dosages are based on the child’s age and weight. See the pediatric dosage chart below for details. Recommended dosage ranges are given as micrograms per kilogram of body weight (mcg/kg) and are taken once per day.

If your child’s age is:Their daily Synthroid dosage may be:
0–3 months10–15 mcg/kg
3–6 months8–10 mcg/kg
6–12 months6–8 mcg/kg
13 months–5 years5–6 mcg/kg
6–12 years4–5 mcg/kg
13 years or older, and they’re still going through puberty or growing2–3 mcg/kg
Adolescence, and they’ve completed growth and puberty1.6 mcg/kg

Adolescents who have completed growth and puberty may have their dosage adjusted during treatment as needed, as do adults. See the “What factors can affect my dosage?” section below.

Dosage adjustments

Your doctor may adjust your Synthroid dosage every 2 to 6 weeks* when you first start treatment, based on your thyroid hormone levels. Your dosage will typically be adjusted by amounts of 12.5 mcg to 25 mcg. These adjustments may help the drug work more effectively or lower your risk for side effects.

If you’re an older adult or you have heart disease, your doctor may prescribe a specific starting dosage of 12.5 mcg to 25 mcg of Synthroid per day. This adjusted dosage helps your doctor monitor you for side effects that may affect your heart. This dosage may be increased gradually, every 6 to 8 weeks as needed.

If you have questions about adjustments to your Synthroid dosage, talk with your doctor.

* See “What are the typical dosages of Synthroid?” above for details about how often your dosage may be adjusted.

Below are answers to some commonly asked questions about Synthroid.

Is there a dosage calculator for Synthroid?

Yes, a dosage calculator is available for your doctor to use when prescribing Synthroid. It’s important to note that your doctor is responsible for calculating your dosage. Your dosage of Synthroid depends on a variety of factors, which are listed in the “What factors can affect my dosage?” section below.

If you have questions about how your dosage is calculated, talk with your doctor.

Does a lower dosage of Synthroid mean my risk for side effects is lower?

Possibly, but it’s important to first determine the correct Synthroid dosage for treating your condition.

If you’re experiencing side effects from Synthroid, your doctor may lower your dosage, and this may reduce your symptoms. However, if your Synthroid dosage is too low, you may experience symptoms of the condition you’re taking it to treat.

Talk with your doctor to learn more about lowering your risk for side effects from Synthroid. And if you have side effects during treatment or questions about lowering your dosage, also talk with your doctor.

If I lose weight, will my Synthroid dosage need to be changed?

It’s possible. Synthroid’s recommended dosages are based on weight. If you have a major change in weight while taking Synthroid, your doctor may adjust your dosage. But minor changes in weight don’t necessarily require a dosage adjustment.

If you have questions about your Synthroid dosage given your weight, talk with your doctor.

What happens if my Synthroid dosage is too high?

If your Synthroid dosage is too high, it can lead to high thyroid hormone levels. These high hormone levels can cause you to develop symptoms of hyperthyroidism, in which you have too much thyroid hormone in your body. This condition is the opposite of hypothyroidism, which is having too little thyroid hormone in your body.

Symptoms of hyperthyroidism can include:

If you develop these symptoms while taking Synthroid, talk with your doctor. They can test your thyroid hormone levels and reduce your dosage if needed.

The dosage of Synthroid that you’re prescribed may depend on several factors. These include:

  • the type and severity of the condition you’re using Synthroid to treat
  • your age
  • your body weight
  • any side effects you experience
  • your thyroid hormone levels
  • any other medications you’re taking
  • other medical conditions you may have (see “Dosage adjustments” under “What is Synthroid’s dosage?” above)

Your doctor will adjust your dosage as needed during your Synthroid treatment. Why this is done can vary from person to person. If you have questions about what your dosage should be, talk with your doctor.

Synthroid comes as tablets that are taken by mouth once per day, without food. It’s usually recommended to take Synthroid 30 minutes to 1 hour before your first meal of the day. In general, the drug should be taken at about the same time each day, on an empty stomach (30 minutes to 1 hour before or after eating). But make sure to take Synthroid exactly as prescribed by your doctor.

You should also take Synthroid at least 4 hours before or after certain drugs that can affect how effective Synthroid is. See the this article for a list of drugs that can interact with Synthroid. And make sure to talk with your doctor or pharmacist about all of the medications you use before starting Synthroid.

If you miss a dose of Synthroid, take it as soon as you remember. But if it’s close to the time of your next dose, skip the missed dose and take your next scheduled dose as usual. If you aren’t sure whether to take the missed dose or skip it, talk with your doctor or pharmacist.

Do not double up on Synthroid doses to make up for a missed dose. Doing this can increase your risk for side effects from the drug.

If you need help remembering to take your dose of Synthroid on time, try using a medication reminder. This can include setting an alarm, downloading a reminder app, or setting a timer on your phone. A kitchen timer can work, too.

Don’t use more Synthroid than your doctor prescribes. Using more than this can lead to serious side effects, likely due to hyperthyroidism (having too much thyroid hormone in your body).

Symptoms of overdose

Symptoms caused by an overdose can include:

What to do in case you take too much Synthroid

Call your doctor right away if you think you’ve taken too much Synthroid. You can also call 800-222-1222 to reach the American Association of Poison Control Centers, or use their online resource. However, if you have severe symptoms, call 911 (or your local emergency number) immediately or go to the nearest emergency room.

The sections above describe the typical dosages provided by the drug manufacturer. If your doctor recommends Synthroid for you, they will prescribe the dosage that’s right for you.

Remember, you shouldn’t change your dosage of Synthroid without your doctor’s approval. Only take Synthroid exactly as prescribed. Talk with your doctor if you have questions or concerns about your current dosage.

Here are some examples of questions you may want to ask your doctor:

  • What drugs should I avoid while I’m taking Synthroid?
  • How will I know which dosage of Synthroid is best for me?
  • Would a different dosage raise or lower my risk for side effects from Synthroid?

Disclaimer: Healthline 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 other 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.