Synthroid (Levothyroxine) for Thyroid Replacement Therapy: What You Should Know - GoodRx (2024)

Levothyroxine is among the most commonly prescribed medications in the U.S. It’s used to treat hypothyroidism (low or underactive thyroid levels) — often for a lifetime. Symptoms of hypothyroidism may include fatigue, weight gain, and depression.

You may also see Synthroid used for treating hypothyroidism. Synthroid’s generic name is levothyroxine. While Synthroid is the brand-name version of levothyroxine, the two are not always interchangeable.

In this article, we’ll talk more about Synthroid and levothyroxine — what they are, how they’re different, and Synthroid dosages. We’ll also discuss potential side effects and medication interactions to consider.

What is Synthroid?

Synthroid is a brand-name medication. It’s a human-made version of a naturally occurring thyroid hormone called thyroxine (T4).

T4 is made by the thyroid gland in your neck. Once it’s made, your body turns this into triiodothyronine hormone (T3) as needed, which is the active version of the hormone. The advantage to taking T4 over T3 is that your body can control how quickly T4 is turned into T3.

What’s the difference between levothyroxine and Synthroid?

Levothyroxine and Synthroid contain the same active (main) ingredient, levothyroxine, which is human-made T4. Levothyroxine is the generic name for Synthroid.

However, there can be small differences in the amount of T4 between Synthroid and some of its generic versions. This might be why you might not experience enough symptom relief with a levothyroxine generic product if you started off by taking Synthroid (or vice versa). There can also be differences in inactive ingredients that can impact your response to treatment.

In general, one version isn’t considered “better” than the other. More on this next.

Is generic levothyroxine as effective as Synthroid?

It depends.

As previously mentioned, one version isn’t technically better than the other. Both generic levothyroxine and Synthroid should work equally well for the treatment of hypothyroidism. It’s just not ideal to switch back and forth between different products, unless you’re directed to do so. It’s preferable to stick with Synthroid or a specific generic version if possible.

If you have questions, reach out to your pharmacist. They can help make sure that your current product is interchangeable with your previous product.

Synthroid vs. levothyroxine for weight loss

Weight loss is a possible side effect of Synthroid and levothyroxine, but typically only when the dose is too high. So, the amount of weight loss depends on the dose — not whether you’re taking the Synthroid brand or a levothyroxine generic product.

That being said, Synthroid and levothyroxine shouldn’t be used for weight loss. In fact, there’s an FDA boxed warning against using these medications for weight loss. If you’re experiencing weight loss with Synthroid or levothyroxine, notify your healthcare provider. They can help adjust your Synthroid or levothyroxine dosage if needed.

Interestingly, older natural thyroid products, like Armour Thyroid (desiccated thyroid), might result in more weight loss than levothyroxine products. But more research is needed to determine which people will more likely benefit from this weight-loss effect. Experts also don’t typically recommend natural thyroid medications as go-to options for hypothyroidism.

Synthroid dosage and how to take it

Synthroid dosages are typically dependent on your TSH blood tests. So, try to keep up with your lab appointments as they’re scheduled. Your healthcare provider may also adjust your Synthroid dosage based on:

• Age

• Body weight changes

• Pregnancy status

• Heart health

• Other medications you take

Most people tend to take Synthroid or levothyroxine in an oral (by mouth) tablet form, which is available in many different dosages, ranging from 25 mcg to 300 mcg. Other available levothyroxine dosage forms include oral capsules, an oral liquid, and an injection given into the vein.

Synthroid should be taken on an empty stomach, ideally 30 minutes to 1 hour before breakfast. You should also separate Synthroid from certain foods and beverages, such as:

• Alcohol

• Coffee

• Fiber

• Grapefruit

• Soy products

• Walnuts

Synthroid maximum daily dose

Synthroid’s daily dose is typically no more than 200 mcg. A high dose of Synthroid is considered more than 300 mcg per day, but doses this high are rare.

If you need to take more than 300 mcg of Synthroid per day, your healthcare provider will likely assess you for the following:

• Absorption problems

• Medication interactions

• Multiple missed doses

• Combination of these reasons

  See Also

  Synthroid and Dosage: Strengths, Form, When to Use, and More

TSH blood tests

Your healthcare provider determines and adjusts your Synthroid dosage with a blood test — specifically, a TSH blood test.

TSH stands for thyroid stimulating hormone. TSH is a hormone from your brain that tells your thyroid gland how much T4 and T3 to make. In hypothyroidism, your body doesn’t have enough T4 and T3. So, you’ll likely have a high level of TSH that’s trying to encourage your thyroid gland to make more T4 and T3.

Once you start taking Synthroid, the medication will need several weeks to take effect. You’ll likely need a TSH blood test 6 to 8 weeks after starting Synthroid. This is to make sure that the new or recently changed Synthroid dosage is working and stabilizing your TSH. Children will need this test in 2 weeks after first starting Synthroid.

How often do I need TSH blood tests?

When your TSH blood test is within normal range, your child’s healthcare provider (pediatrician) may ask for a repeat TSH blood test every 3 to 12 months. Adults, on the other hand, may need to get a TSH blood test every 6 to 12 months.

You or your child might get more frequent TSH blood tests depending on a number of factors, such as your symptoms. You may also want a TSH blood test if your levothyroxine manufacturer changed or if you switched between a brand and generic product.

What are normal TSH blood test results?

Normal TSH blood test results are typically within the 0.5 to 5 mIU/L range.

But your healthcare provider may try to aim for a range of 0.4 to 4 mIU/L. Based on a 2019 study, this range isn’t linked to any health issues.

While the 0.4 to 4 mIU/L range works well for most people, the “best” TSH range for you might be lower or higher depending on other factors, like other medical conditions you may have. A healthcare provider can help you target specific levels based on your individual situation.

Synthroid side effects

Side effects of Synthroid and levothyroxine are very similar. The following is a list of possible side effects:

• Abnormal heart beat

• Abnormal menstrual periods

• Appetite changes

• Bowel movement changes

• Headache

• Mood changes

• Heat intolerance

• Weight changes

But most people taking Synthroid or levothyroxine actually feel normal. These side effects are rare but may happen when the dose is too high or low. During the first few months of taking Synthroid or levothyroxine, you may also notice some hair loss. But this side effect will usually go away.

This isn’t a complete list of possible side effects with Synthroid or levothyroxine. If you have any questions, reach out to your pharmacist or healthcare provider.

Synthroid drug interactions

Synthroid has many drug interactions. Several medications may lower Synthroid’s effectiveness by affecting its absorption into the bloodstream.

The following are some medications that will likely interact with Synthroid:

• Antacids like calcium carbonate (Tums) for heartburn

• Bile acid sequestrants like cholestyramine (Prevalite) for high cholesterol

• Proton pump inhibitors (PPIs) like esomeprazole (Nexium) and omeprazole (Prilosec) for gastroesophageal reflux disease (GERD)

• Certain supplements (e.g., calcium, iron, magnesium)

This isn’t a complete list of possible Synthroid interactions. It’s best to talk with your pharmacist or healthcare provider. Be sure to share a full list of medications and supplements with them so they can guide you in safely taking your medication.

Therapy with additional liothyronine (T3)

Some people with hypothyroidism may take liothyronine along with levothyroxine. Liothyronine (Cytomel) is the human-made version of T3. This combination of T4 and T3 seems to better relieve low thyroid symptoms in some people. If you’re giving this combo a try, it might take 3 to 6 months to notice any difference. But more research on this combination therapy is still needed.

There are also older natural thyroid products — like Armour Thyroid — that contain a combination of both T4 and T3 in one tablet. While this may sound convenient, these older natural medications have T4 to T3 ratios that are not typically found in humans. They’re not always consistent, either. So, there’s a higher chance of experiencing side effects. And, as previously mentioned, experts don’t usually recommend these older natural thyroid products.