Key takeaways
- Synthroid (levothyroxine) treats hypothyroidism and some types of thyroid cancer. Your age, weight, and medical condition determine your dosage. Most people start on a low dose that’s adjusted over time.
- Synthroid tablets come in various strengths, measured in micrograms (mcg). Doctors usually adjust doses every 4 to 8 weeks until your thyroid levels are stable.
- Take Synthroid on an empty stomach, at least 30 to 60 minutes before your first meal of the day. Avoid taking it with medications that can interfere with its absorption.
Synthroid is a thyroid hormone replacement. It’s approved for adults and children to treat:
- hypothyroidism
- a particular type of thyroid cancer
The active ingredient in Synthroid is levothyroxine. (An active ingredient is what makes a drug work.)
This article describes the dosages of Synthroid, including its strengths and instructions on how to take it. To learn more about Synthroid, see this in-depth article.
The following information details Synthroid’s form, strengths, and typical dosages.
What are Synthroid forms and strengths?
- Form: Oral tablet
- Strengths: Tablets come in multiple colors and mcg strengths, including:
- Standard intervals: 25, 50, 75, 100, 125, 150, 175, 200 mcg
- Additional strengths: 88, 112, 137, 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?
Your doctor will typically start your treatment with a low dosage. They may adjust this dosage in increments of 12.5 mcg to 25 mcg to ensure the drug meets your treatment goals and reduces side effects. Ultimately, your doctor will prescribe the lowest effective dosage. For more information, refer to the “Factors that can affect your dosage” section.
The following information 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 following dosage chart 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 later.
| 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 to 6 weeks |
| newly diagnosed hypothyroidism in a person 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 person 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 to 8 weeks |
| older (geriatric) adults | lower starting dosage (less than 1.6 mcg/kg) | once per day | every 6 to 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?” later for a pediatric dosage chart.
Note:For treating hypothyroidism, a Synthroid dosage 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 an approved treatment for a specific type of thyroid cancer in adults. It works by decreasing thyroid-stimulating hormone (TSH) levels. The dosage of Synthroid for this purpose is determined by the cancer’s stage, your symptoms, and 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 particular type 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 following pediatric dosage chart 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 to 3 months† | 10 to 15 mcg/kg |
| 3 to 6 months | 8 to 10 mcg/kg |
| 6 to 12 months | 6 to 8 mcg/kg |
| 1 to 5 years | 5 to 6 mcg/kg |
| 6 to 12 years | 4 to 5 mcg/kg |
| 13 years or older, and they’re still going through puberty or growing | 2 to 3 mcg/kg |
| growth and puberty complete | 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 “Factors that can affect your dosage” section later.
Dosage for thyroid cancer in children
Synthroid is approved for the treatment of a specific type of thyroid cancer in children, where it’s used to suppress (decrease) TSH levels. The dosage is determined by various factors such as the child’s age, weight, and the particular type of thyroid cancer.
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 12.5 mcg to 25 mcg. These adjustments may help the drug work more effectively or lower your risk of 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 adverse effects in adults and children. If you have questions about adjustments to your Synthroid dosage, talk with your doctor.
Is there a dosage calculator for Synthroid?
Yes, but only your doctor should use it to set your dose. Your dosage of Synthroid depends on many factors, which are listed in the “Factors that can affect your dosage” section later.
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?
Not always. Too low a dose may cause hypothyroidism symptoms to return. 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.
If I lose weight, will my Synthroid dosage need to be changed?
Possibly. Significant weight changes can affect 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?
You may 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
Contact your doctor if these symptoms develop while taking Synthroid. They can test your thyroid hormone levels and reduce your dosage if needed.
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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
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.
Consider these instructions when taking Synthroid.
- Take once per day, by mouth, on an empty stomach.
- Wait 30 to 60 minutes before your first meal of the day.
- Take it at the same time every day.
- Take it separately from medications (by at least 4 hours) that may interfere with absorption.
See this list of drugs that can interact with Synthroid. Also, it’s important to tell your doctor or pharmacist about all 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, consider 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.
Do not 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.
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:
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 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.
