Synthroid (levothyroxine) is a prescription drug that’s used to treat hypothyroidism and thyroid cancer in certain people. The drug comes as a tablet that you swallow. It’s usually taken once per day.
Synthroid is used in adults and children to treat:
- hypothyroidism
- a certain kind of thyroid cancer
The active ingredient in Synthroid is levothyroxine. (An active ingredient is what makes a drug work.)
Synthroid belongs to a group of drugs called thyroid hormones.
This article describes the dosages of Synthroid, as well as its strengths and how to take it. To learn more about Synthroid, see this in-depth article.
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 by prescribing 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 doctor may change your dosage to make sure the drug is helping you meet your 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.
| Condition or age group | Recommended Synthroid starting dosage in adults | How often it’s taken | How often your dosage may be adjusted |
| Primary, secondary, or tertiary hypothyroidism | 1.6 mcg/kg | Once per day | Every 4–6 weeks |
| Newly diagnosed hypothyroidism in a patient who is pregnant and TSH ≥ 10 milli-international unit per liter (miU/L) | 1.6 mcg/kg | Once per day | Every 4 weeks |
| Newly diagnosed hypothyroidism in a patient who is pregnant and TSH < 10 milli-international unit per liter (miU/L) | 1.0 mcg/kg | Once per day | Every 4 weeks |
| Risk of atrial fibrillation or diagnosis of underlying cardiac disease | Lower starting dosage (less than 1.6 mcg/kg) | Once per day | Every 6–8 weeks |
| Older (geriatric) adults | Lower starting dosage (less than 1.6 mcg/kg) | Once per day | Every 6–8 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 primary, secondary, or tertiary hypothyroidism in adults
Synthroid is approved to treat primary, secondary, or tertiary hypothyroidism. 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 thyroid cancer in adults
Synthroid is approved to suppress (decrease) thyroid-stimulating hormone (TSH) levels, as part of treatment for a certain kind of thyroid cancer in adults. For this use, Synthroid’s dosage will depend on several factors, including the stage of the cancer, the symptoms you experience, and your treatment goals.
Your doctor will monitor your thyroid hormone levels during treatment, and they 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?
Synthroid is approved for treating hypothyroidism and a certain kind of thyroid cancer in children.
Dosage chart for hypothyroidism in 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 starting 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 starting daily Synthroid dosage may be: |
| 0–3 months† | 10–15 mcg/kg |
| 3–6 months | 8–10 mcg/kg |
| 6–12 months | 6–8 mcg/kg |
| 1–5 years | 5–6 mcg/kg |
| 6–12 years | 4–5 mcg/kg |
| 13 years or older, and they’re still going through puberty or growing | 2–3 mcg/kg |
| Adolescence, and they’ve completed growth and puberty | 1.6 mcg/kg |
* If your child is at risk for hyperactivity, their doctor may start them on a lower Synthroid dosage. They will then increase the dose once weekly until the full recommended dose is reached.
† If your child (who is 0 to 3 months of age) is at risk for heart failure, their doctor may start them on a lower Synthroid dosage. They will then increase your child’s dose every 4 to 6 weeks based on their body’s response to the drug.
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 for thyroid cancer in children
Synthroid is also approved to suppress (decrease) TSH levels, as part of treatment for a certain kind of thyroid cancer in children. For this use, Synthroid’s dosage depends on several factors, including the child’s age, weight, and the type of thyroid cancer they have.
Your child’s doctor will likely monitor their thyroid hormone levels during treatment and may adjust their Synthroid dosage as needed.
Dosage adjustments
Your doctor may adjust your Synthroid dosage every 4 to 8 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 lower starting dosage of Synthroid. 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.
It’s important for thyroid levels to be monitored throughout Synthroid treatment. Having too much or too little of the drug can cause negative effects in adults and children. 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:
- nervousness or irritability
- diarrhea
- trouble sleeping
- muscle weakness
- hyperactivity
- oversensitivity to heat, excessive sweating, and warm, damp skin
- increased appetite
- increased bowel movements and urination
- fast heart rate, sometimes with palpitations
- trembling hands and shakiness
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 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:
- heart rhythm problems
- heart attack
- muscle spasms (involuntary muscle tightening)
- shortness of breath
- headache
- tremors (uncontrolled shaking)
- muscle weakness
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 America’s Poison Centers, or use its 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?
To learn more about Synthroid, see these articles:
- Synthroid (levothyroxine)
- Side Effects of Synthroid: What You Need to Know
- Synthroid and Cost: What You Need to Know
- Synthroid Interactions: Alcohol, Medications, and Others
To get information on different conditions and tips for improving your health, subscribe to any of Healthline’s newsletters. You may also want to check out the online communities at Bezzy. It’s a place where people with certain conditions can find support and connect with others.
Q:
Why do my Synthroid tablets look different from each other?
AnonymousA:
Synthroid tablets are round, and they come in different colors depending on what dose they are. So you may take tablets of more than one color depending on your dosage. And if your doctor increases or decreases your dosage, you may start taking tablet(s) of a different color. (For a list of the colors and doses of Synthroid, see the drug’s prescribing information.)
It’s important to take the Synthroid dosage prescribed by your doctor. Your pharmacist will talk with you about your dosage when you pick up your prescription. But if you notice an unexpected change in the shape or color of your Synthroid tablets, talk with your doctor or pharmacist.
Also, the generic form of Synthroid may have the same shape and color as some Synthroid tablets, but may not correspond to the same dose. (A generic drug is an exact copy of the active ingredient in a brand-name medication.) But it’s possible that changing drug brands could affect your thyroid function tests during treatment. So it’s important to take the form of medication prescribed by your doctor.
The Healthline Pharmacist TeamAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.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.
