Synthroid (levothyroxine) is a prescription brand-name medication. The Food and Drug Administration (FDA) has approved it to treat hypothyroidism in adults and children. It’s also approved to treat a type of thyroid cancer in adults, by lowering thyroid stimulating hormone levels.*
Synthroid comes as tablets that you swallow. The active drug in Synthroid is levothyroxine, which is a type of hormone therapy. Levothyroxine is also available as a generic medication.
For information on the dosage of Synthroid, including its form, strengths, and how to take the drug, keep reading. For a comprehensive look at Synthroid, including details about its uses, see this article.
* For treating thyroid cancer, Synthroid is used along with radioactive iodine treatment and surgery.
This article describes typical dosages for Synthroid provided by the drug’s manufacturer. When taking Synthroid, always follow the dosage prescribed by your doctor.
Synthroid is approved to treat hypothyroidism and a type of thyroid cancer. Below is important information about Synthroid dosage for these uses.
Synthroid form
Synthroid comes as tablets that you swallow.
Synthroid strengths
Synthroid comes as tablets that you swallow. It comes in the following strengths: 25 micrograms (mcg), 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, and 300 mcg.
It’s important to note that Synthroid is dosed in micrograms. This is different from most other medications, which are typically dosed in milligrams (mg).
Typical dosages
The dosage range for Synthroid depends on the condition you’re taking the drug to treat and how effective the drug is for you. Typically, your doctor will start you on a low dosage. Then they’ll adjust it over time to reach the amount that’s right for you. Your doctor will ultimately prescribe the smallest dosage that provides the desired effect.
For example, the average dosage of Synthroid for treating hypothyroidism is 1.6 mcg of drug per kilogram (mcg/kg) of body weight, once per day. Since this dosage varies based on body weight, your “normal” dosage may be different from other people’s dosages.
The following information describes dosages that are commonly used or recommended. But be sure to take the dosage your doctor prescribes for you. Your doctor will determine the best dosage to fit your needs.
Dosage for hypothyroidism in adults
For treating hypothyroidism, Synthroid dosages can differ depending on whether it’s used for hypothyroidism that’s newly diagnosed, or for hypothyroidism that’s severe and long term.
Dosage for hypothyroidism that’s newly diagnosed
For treating hypothyroidism that’s been diagnosed within the past few months, the typical starting Synthroid dosage is 1.6 mcg/kg, once per day. Your doctor may adjust your dosage during treatment if needed.
One kilogram is equal to about 2.2 pounds (lb). For example, if you weigh 80 kg (about 176 lb), your starting Synthroid dosage would be 125 mcg daily. This is rounded down from about 128 mcg. Your doctor will prescribe the closest available strength of Synthroid tablets.
Dosage for long-term, severe hypothyroidism
When used to treat hypothyroidism that’s severe and long term, Synthroid’s recommended starting dosage is 12.5 mcg to 25 mcg, once per day. Your doctor may then adjust your dosage based on your thyroid hormone levels, as well as other factors. See the “Factors affecting your dosage” section below for more information.
Dosage for TSH suppression in adults
Synthroid is approved to treat a type of thyroid cancer. The drug does this by suppressing (decreasing) levels of thyroid stimulating hormone (TSH).
For this use, a Synthroid dosage of 2 mcg/kg is usually needed. Your doctor will adjust your dosage as needed during treatment, depending on your TSH levels.
For example, if you weigh 90 kg (about 200 lb), your Synthroid dosage would be 180 mcg daily.
What’s considered a high dose of Synthroid?
For treating hypothyroidism, dosages above 200 mcg per day are rarely needed, according to Synthroid’s manufacturer. There’s no maximum dosage of Synthroid provided by its manufacturer. So, for example, 88 mcg of Synthroid per day to treat hypothyroidism isn’t considered a high dose.
Long-term use
Synthroid is meant to be used as a long-term treatment. If you and your doctor determine that Synthroid is safe and effective for you, you’ll likely take it long term.
Children’s dosage
Below is a dosage chart for using Synthroid to treat hypothyroidism in children. Like in adults, children’s dosages are given as micrograms per kilogram of body weight (mcg/kg).
| Age | Daily Synthroid dosage |
| 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, but still undergoing puberty or still growing | 2–3 mcg/kg |
| adolescent, and puberty and growth are completed | 1.6 mcg/kg |
For example, the typical Synthroid dosage for a 5-year-old who weighs 25 kg (about 55 lb) would be 125 mcg to 150 mcg per day.
In adolescents who’ve completed puberty and growth, their doctor may adjust their dosage during treatment as needed.
Below are answers to a few important questions about Synthroid dosages.
Is there a dosage calculator for Synthroid?
There is a dosage calculator for Synthroid, which is for use by doctors and other healthcare professionals. A dosage calculator can be helpful because thyroid hormone drugs such as Synthroid require careful monitoring and precise dosages. A dosage calculator helps make sure that the dose your doctor prescribes is safe for you.
If you have questions about how your Synthroid dosage will be calculated, talk with your doctor or pharmacist.
Can lowering the dosage of Synthroid cause side effects?
It’s unlikely that lowering Synthroid’s dosage causes side effects. But lowering your Synthroid dosage could cause or worsen symptoms of the condition you’re taking the drug to treat. This is because if your dosage is lowered, your body may no longer get enough thyroid hormone to help prevent or relieve your symptoms.
If you have other questions about Synthroid dose adjustments and side effects, talk with your doctor or pharmacist.
If you use more Synthroid than your doctor prescribes, you may develop serious side effects.
Synthroid is a synthetic (artificial) thyroid hormone. Your body also naturally makes thyroid hormones. For this reason, Synthroid overdose can cause the same symptoms as hyperthyroidism (high thyroid hormone levels).
It’s important that you don’t use more Synthroid than your doctor recommends.
Symptoms of an overdose
Mild overdose symptoms of Synthroid can include:
- trouble sleeping
- feeling anxious or nervous
- irregular heart rate or heart palpitations (feeling like your heart skips a beat)
- muscle weakness
- unintentional weight loss
A more serious Synthroid overdose may cause additional symptoms, including:
- cerebral embolism (blood clots in your brain)
- coma
- confusion
- seizures
- rarely, death
If you take more than the recommended amount of Synthroid
Call your doctor right away if you believe you’ve taken too much Synthroid. Another option is to call the American Association of Poison Control Centers at 800-222-1222 or use its online tool. If you have severe symptoms, immediately call 911 or your local emergency number, or go to the nearest emergency room.
The Synthroid dosage your doctor prescribes will depend on several factors. These include:
- the type and severity of the condition you’re using Synthroid to treat
- your age and body weight
- any side effects you have
- your thyroid hormone levels
Other medical conditions you have can also affect your Synthroid dosage.
Dosage adjustments
Your doctor may need to adjust your dosage while your condition is being treated with Synthroid.
Dosage adjustments for hypothyroidism
While you’re taking Synthroid for hypothyroidism, your doctor will monitor your thyroid hormone levels with blood tests. If your Synthroid dosage needs to be adjusted, your doctor will likely adjust it in amounts of 12.5 mcg to 25 mcg.
For people with newly diagnosed hypothyroidism, dosage adjustments will likely happen every 4 to 6 weeks. This is the amount of time it can take the drug to have its full effects for treating hypothyroidism. But if you’ve been taking Synthroid for a while, or if you’ve had hypothyroidism for a long time, your doctor may adjust your dosage every 2 to 4 weeks instead.
Your doctor may continue to adjust your Synthroid dosage until your thyroid hormone levels are in a normal range.
Dosage adjustments for TSH suppression
Synthroid is approved to treat a type of thyroid cancer in certain situations. The drug does this by suppressing (decreasing) levels of thyroid stimulating hormone (TSH).
While you’re taking Synthroid for this purpose, your doctor will monitor your TSH levels. They may adjust your Synthroid dosage depending on these levels. Typically, the goal of treatment is for TSH levels to be below 0.1 international units per liter.
Synthroid comes as tablets that you swallow. You should take Synthroid according to your doctor’s instructions.
Synthroid should be taken on an empty stomach, once daily. Specifically, it should be taken 30 minutes to 1 hour before or after eating.
Usually, it’s recommended to take the drug 30 minutes to 1 hour before breakfast. But your doctor may also discuss how to take Synthroid at night. In some cases, your body can absorb the drug better if you take Synthroid at night.
It’s important to note that Synthroid should be taken at least 4 hours before, or 4 hours after, certain medications that it can interact with. For more information about drug interactions with Synthroid, refer to this article.
If you forget a dose of Synthroid, call your doctor’s office. They can recommend the best time for you to take your next dose.
Missing a dose of Synthroid could cause your thyroid hormone levels to decrease. This could lead to side effects. For this reason, it’s important to remember to take your Synthroid dose at about the same time every day.
To help make sure that you don’t miss a dose, try using a medication reminder. This can include setting an alarm or timer on your phone or downloading a reminder app. A kitchen timer can work, too.
Stopping Synthroid doesn’t cause withdrawal. With withdrawal, you have unpleasant symptoms when you stop taking a drug that your body has become dependent upon. But you shouldn’t stop taking Synthroid unless your doctor approves this first. Stopping the drug can cause symptoms of the condition you’re taking it for to come back or get worse.
Stopping Synthroid can also affect other medical conditions you have or other medications you take. For example, the dosage of certain diabetes drugs may need to be adjusted after stopping Synthroid treatment. This is because Synthroid can make certain diabetes drugs less effective.
Talk with your doctor if you have questions about ending your treatment with Synthroid.
The dosages in this article are typical dosages provided by the drug manufacturer. If your doctor recommends Synthroid for you, they will prescribe the dosage that’s right for you. Always follow the dosage that your doctor prescribes for you.
As with any drug, never change your dosage of Synthroid without your doctor’s approval. If you have questions about the dosage of Synthroid that’s right for you, talk with your doctor.
Besides learning about dosage, you may want other information about Synthroid. For information about other aspects of Synthroid, refer to this article.
Disclaimer: Medical News Today 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.