Synthroid is the brand-name form of levothyroxine. They’re both prescribed for certain thyroid conditions. Synthroid and levothyroxine come as oral tablets, while levothyroxine comes in other forms as well.
This article explains the main similarities and differences between Synthroid and levothyroxine. If you’re considering taking one of these drugs, discussing this information with your doctor can help you decide whether one of these treatments may be right for you.
Note: For more comprehensive information about these two drugs, you can refer to our Synthroid and levothyroxine articles.
Key differences between Synthroid and levothyroxine
These are a few of the main differences between Synthroid and levothyroxine:
- Forms: Synthroid comes as an oral tablet. Levothyroxine comes as an oral tablet, oral capsule, and intravenous injection. (See the “Dosages, forms, and administration” section below.)
- Uses: Synthroid and oral forms of levothyroxine are used to treat hypothyroidism and to lower thyroid-stimulating hormone levels. The injectable form of levothyroxine is used to treat myxedema coma. (See the “Uses of Synthroid vs. levothyroxine” section below.)
Below are answers to some common questions about Synthroid and levothyroxine.
How do Synthroid and levothyroxine compare with similar drugs, such as Armour Thyroid?
Synthroid is the brand-name version of levothyroxine. Both drugs belong to the thyroid hormone drug class. Other thyroid hormones are also available. These include:
- other brand-name versions of levothyroxine, such as Unithroid, Euthyrox, and others
- dessicated thyroid (Armour Thyroid, NP Thyroid)
- liothyronine (Cytomel)
These medications are lab-made versions of hormones that are made naturally by your thyroid gland. They’re used for certain thyroid conditions and work by increasing your thyroid hormone levels to a standard range.
Synthroid, levothyroxine, dessicated thyroid, and liothyronine come as oral tablets. Levothyroxine and liothyronine also come in other forms.
To learn more about how Synthroid and levothyroxine compare with similar drugs, talk with your doctor or pharmacist.
Is Synthroid a better treatment option than levothyroxine or vice versa?
Not necessarily. Levothyroxine is a generic version of brand-name Synthroid. A generic drug is an exact copy of the active drug in a brand-name medication. In general, generics are considered to be as effective and safe as the brand-name drug.
Your doctor will give you a blood test to check your thyroid hormone levels during treatment with Synthroid and levothyroxine. They’ll use this test to help decide whether the drug is working. If your thyroid hormone levels don’t fall within the desired range, your doctor will recommend what to do. Synthroid and levothyroxine are narrow therapeutic index drugs, which means that changing from brand to generic, or vice versa, can cause significant changes in blood levels.
If you have other questions about how Synthroid compares with levothyroxine, talk withyour doctor or pharmacist.
Synthroid is the brand-name form of levothyroxine. Both drugs contain the same active drug and likely cause the same side effects. Some of the side effects reported in clinical trials of these drugs are mentioned below.
For more details about side effects of the two drugs, see our side effect articles about Synthroid and levothyroxine. You can also refer to the Synthroid prescribing information and prescribing information for levothyroxine injection, oral tablet, and oral capsule.
Mild side effects
The following table lists some of the more commonly reported mild side effects of Synthroid and levothyroxine. The table may also include mild side effects that are less common but that you might have concerns about in some cases.
| Mild side effects | Synthroid | Levothyroxine |
| feeling irritable or nervous | ✓ | ✓ |
| insomnia | ✓ | ✓ |
| diarrhea | ✓ | ✓ |
| muscle weakness, spasm, or tremor | ✓ | ✓ |
| headache | ✓ | ✓ |
| increased appetite | ✓ | ✓ |
| shortness of breath | ✓ | ✓ |
| irregular periods | ✓ | ✓ |
| weight loss | ✓ | ✓ |
| mild allergic reaction* | ✓ | ✓ |
These side effects may be temporary, lasting a few days to weeks. If the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.
* An allergic reaction is possible after taking Synthroid and levothyroxine. However, this side effect wasn’t reported in clinical trials of Synthroid or levothyroxine injection, oral tablet, or oral capsule.
Serious side effects
The following table lists the reported serious side effects of Synthroid and levothyroxine.
| Serious side effects | Synthroid | Levothyroxine |
| low bone mineral density | ✓ | ✓ |
| heart problems, such as heart attack and irregular heart rhythm | ✓ | ✓ |
| severe allergic reaction* | ✓ | ✓ |
If you have serious side effects while using Synthroid or levothyroxine, call your doctor immediately. If the side effects feel life threatening or you believe you’re having a medical emergency, call 911 or your local emergency number right away.
* An allergic reaction is possible after taking Synthroid and levothyroxine. However, this side effect wasn’t reported in clinical trials of Synthroid or levothyroxine injection, oral tablet, or oral capsule.
Here’s information about the generic status for Synthroid and levothyroxine, as well as details on their active ingredient.
| Synthroid | Levothyroxine | |
| Generic available | yes (see below) | see below |
| Active ingredient | levothyroxine | levothyroxine |
| Drug class | thyroid hormone | thyroid hormone |
Synthroid is the brand-name form of levothyroxine, which is also available as a generic medication.
Synthroid and levothyroxine have been approved by the Food and Drug Administration (FDA) for the following uses in adults.
- Both Synthroid and oral forms of levothyroxine are FDA-approved for:
- Treating certain types of hypothyroidism. Among others, these include:
- Lowering thyroid-stimulating hormone (TSH) levels in adults with a certain type of thyroid cancer. For this use, the drugs are used after thyroid gland removal surgery and treatment with radioactive iodine.
- Levothyroxine injection is FDA-approved for:
- Treating myxedema coma (a severe form of hypothyroidism). The drug is used as a short-term treatment before switching to an oral form of levothyroxine.
Synthroid or levothyroxine and children
Synthroid and levothyroxine have been approved by the FDA for the following uses in children.
- Both Synthroid and oral forms of levothyroxine are FDA-approved for:
- Treating certain types of hypothyroidism. Among others, these include primary, pituitary, and congenital hypothyroidism. Synthroid and levothyroxine oral tablets are approved for children of any age. Levothyroxine oral capsules are approved for children ages 6 years and older.
- Lowering TSH levels in children with a certain type of thyroid cancer. For this use, the drugs are used after treatment with thyroid gland removal surgery and radioactive iodine. Synthroid and levothyroxine oral tablets are approved for children of any age. Levothyroxine oral capsules are approved for children ages 6 years and older.
Levothyroxine injection is not FDA-approved for children.
Note: Both Synthroid and levothyroxine have a boxed warning about not being used for the treatment of obesity or weight loss. A boxed warning is the most serious warning from the FDA. To learn more, see the “Precautions for Synthroid and levothyroxine” section below. For more information about the drugs’ uses, see our articles about Synthroid and levothyroxine.
Here’s a quick look at the dosage and administration of Synthroid and levothyroxine for the conditions both drugs treat. “Micrograms” is abbreviated as “mcg” and “kilograms” as “kg” in the tables below. One kg is about 2.2 pounds (lb).
Dosage for hypothyroidism in adults
Below are the dosages of Synthroid and levothyroxine for treating hypothyroidism in adults.
| Synthroid for hypothyroidism in adults | Levothyroxine for hypothyroidism in adults | |
| Forms | oral tablet | oral tablet and oral capsule |
| Strengths | • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg • 300 mcg | oral tablet: • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg • 300 mcg oral capsule: • 13 mcg • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg |
| Dose | 1.6 mcg per kg of body weight* but may be adjusted over time | 1.6 mcg per kg of body weight* but may be adjusted over time |
| Frequency | once per day | once per day |
| Given by | self | self |
* For example, a person weighing 70 kg (about 154 lb) would be prescribed 112 mcg.
Your doctor will tell you the exact dose of Synthroid or levothyroxine that’s recommended for you.
Dosage for lowering thyroid-stimulating hormone levels in adults
Below are the dosages of Synthroid and levothyroxine for lowering thyroid-stimulating hormone (TSH) levels in adults with a certain type of thyroid cancer.
| Synthroid for lowering TSH levels in adults | Levothyroxine for lowering TSH levels in adults | |
| Forms | oral tablet | oral tablet and oral capsule |
| Strengths | • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg • 300 mcg | oral tablet: • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg • 300 mcg oral capsule: • 13 mcg • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg |
| Dose | varies depending on your treatment plan | varies depending on your treatment plan |
| Frequency | once per day | once per day |
| Given by | self | self |
Your doctor will tell you the exact dose of Synthroid or levothyroxine that’s recommended for you.
Children’s dosage
Synthroid and levothyroxine oral tablets are approved to treat hypothyroidism in children of any age. In addition, both drugs are used to lower TSH levels in children of any age with a certain type of thyroid cancer.
Levothyroxine oral capsules are approved to treat hypothyroidism and lower TSH levels in children ages 6 years and older.
Dosage for hypothyroidism in children
Below are the dosages of Synthroid and levothyroxine for treating hypothyroidism in children.
| Synthroid for hypothyroidism in children | Levothyroxine for hypothyroidism in children | |
| Forms | oral tablet | oral tablet and oral capsule |
| Strengths | • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg • 300 mcg | oral tablet: • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg • 300 mcg oral capsule: • 13 mcg • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg |
| Dose | varies depending on the child’s age and weight in kg | varies depending on the child’s age and weight in kg |
| Frequency | once per day | once per day |
| Given by | self or caregiver | self or caregiver |
Your child’s doctor will tell you the exact dose of Synthroid or levothyroxine that’s recommended for your child.
Dosage for lowering TSH levels in children
Below are the dosages of Synthroid and levothyroxine for lowering TSH levels in children.
| Synthroid for lowering TSH levels in children | Levothyroxine for lowering TSH levels in children | |
| Forms | oral tablet | oral tablet and oral capsule |
| Strengths | • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg • 300 mcg | oral tablet: • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg • 300 mcg oral capsule: • 13 mcg • 25 mcg • 50 mcg • 75 mcg • 88 mcg • 100 mcg • 112 mcg • 125 mcg • 137 mcg • 150 mcg • 175 mcg • 200 mcg |
| Dose | varies depending on the child’s treatment plan | varies depending on the child’s treatment plan |
| Frequency | once per day | once per day |
| Given by | self or caregiver | self or caregiver |
Your child’s doctor will tell you the exact dose of Synthroid or levothyroxine that’s recommended for your child.
Note: For more information about dosage, see our dosage articles about Synthroid and levothyroxine.
This section describes details about the effectiveness of Synthroid and levothyroxine.
Prescribing information. For information about how effective these drugs were in clinical trials, see the prescribing information for Synthroid and levothyroxine injection, oral tablet, and oral capsule. Keep in mind that trial results may not apply to your individual health situation.
Treatment guidelines. Another way to check whether a drug is considered effective is to look at treatment guidelines. When an organization includes certain drugs in treatment guidelines, this means research has shown the drug to be safe and effective.
Synthroid and levothyroxine are recommended treatment options for people with hypothyroidism in guidelines from the American Thyroid Association (ATA). Both drugs are also recommended as a treatment option for lowering thyroid-stimulating hormone levels in ATA guidelines for thyroid cancer.
How much Synthroid or levothyroxine costs depends on the treatment plan your doctor prescribes, your insurance plan, and your pharmacy. You can visit Optum Perks* for price estimates of Synthroid and levothyroxine.
Synthroid is a brand-name version of levothyroxine, which is a generic drug. Brand-name medications are often more expensive than generics. Talk with your doctor or pharmacist to learn about the brand and generic forms of this drug.
* Optum Perks is a sister site of Medical News Today.
Synthroid and levothyroxine may not be right for you if you have certain medical conditions or other factors that affect your health. These drugs share some of the same precautions, but they also have different ones. Some of these precautions are mentioned below.
Boxed warning: Not for treatment of obesity or weight loss
Synthroid and levothyroxine have a boxed warning about not being prescribed for the treatment of obesity or weight loss. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). A boxed warning alerts doctors and patients about drug effects that may be dangerous.
Synthroid and levothyroxine are not prescribed for the treatment of obesity or weight loss. Taking either drug for these purposes can lead to harmful effects, such as heart palpitations and chest pain. These effects may be life threatening in extreme cases.
Due to this risk, doctors will not prescribe Synthroid or levothyroxine for obesity or weight loss. If you’re interested in options for weight management, talk with your doctor.
Precautions
In addition to boxed warnings, Synthroid and levothyroxine have other warnings.
If any of the following medical conditions or other health factors are relevant to you, talk with your doctor before using Synthroid or levothyroxine.
| Precaution | Synthroid | Levothyroxine oral forms | Levothyroxine injection |
| if you’ve had an allergic reaction to the drug or any of its ingredients | ✓ | ✓ | ✓ |
| if you’re pregnant or thinking about becoming pregnant | ✓ | ✓ | ✓ |
| if you’re breastfeeding or thinking about breastfeeding | ✓ | ✓ | ✓ |
| if you have type 1 or type 2 diabetes | ✓ | ✓ | ✓ |
| if you have myxedema coma (a severe form of hypothyroidism) | ✓ | ✓ | |
| if you’ve been through menopause | ✓ | ✓ | |
| if you have cardiovascular disease | ✓ | ✓ | ✓ |
| if you have adrenal insufficiency | ✓ | ✓ | ✓ |
Note: For more comprehensive information about these two drugs, you can refer to our Synthroid and levothyroxine articles.
Switching between Synthroid and levothyroxine is possible.
If you’d like to know more about switching between Synthroid and levothyroxine, talk with your doctor. They can give you additional details and help determine the best course of action for your personal situation. Even if two drugs treat the same condition or are in the same drug class, your body can still respond differently.
It’s important that you do not stop, start, or switch any of your drug treatments without your doctor’s recommendation.
Some key points to remember when comparing Synthroid and levothyroxine include:
- Synthroid is the brand-name form of levothyroxine.
- Synthroid only comes as an oral tablet. Levothyroxine comes as an oral tablet, an oral capsule, and an intravenous injection.
- Synthroid, levothyroxine oral tablets, and levothyroxine oral capsules treat hypothyroidism and lower thyroid-stimulating hormone levels. Levothyroxine injection is only used for the short-term treatment of myxedema coma.
If you’d like to learn more about Synthroid or levothyroxine, talk with your doctor or pharmacist. They can help answer any questions you have about similarities and differences between the drugs. They can also help determine whether one drug or the other might work well for you.
Note: For more information about hypothyroidism, see our list of hypothyroid articles.
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 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.