Key takeaways:
Liothyronine (Cytomel) and levothyroxine (Synthroid, Levoxyl) are both medications for hypothyroidism. But there are important differences between them.
Levothyroxine contains T4 and is a first-choice treatment option for low thyroid hormone levels. Liothyronine contains T3 and is typically only prescribed if levothyroxine isn’t effective or well tolerated on its own.
GoodRx can help you access brand-name Cytomel or Levoxyl at exclusive cash prices. And you may be able to save over 80% off the average retail price of the generic versions with a free GoodRx discount.
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Hypothyroidism is a condition that can affect your body in many ways, from your heartbeat to your energy levels. It happens when thyroid hormone levels are too low to support your body’s needs.
Liothyronine (Cytomel) and levothyroxine (Synthroid, Levoxyl) are two common medications for hypothyroidism. They both contain thyroid hormones to help keep your levels within a healthy range. But which one works better? And is one safer? When comparing liothyronine versus levothyroxine, here are seven things to keep in mind.
1. Liothyronine and levothyroxine contain different forms of thyroid hormone
Your thyroid gland normally makes two types of thyroid hormone: triiodothyronine (T3) and thyroxine (T4). T4 is the main form of thyroid hormone in the body. It’s converted into T3 by the liver and kidneys. T3 is considered the “active” form of thyroid hormone. It's the hormone that does the final work — managing body temperature, mood, and energy levels, among having various other effects.
Levothyroxine contains a synthetic (lab-made) version of T4. Your body converts levothyroxine into T3 the same way it converts T4 produced by your thyroid gland. Liothyronine, on the other hand, contains a lab-made form of T3.
2. Liothyronine starts working faster, but levothyroxine keeps hormone levels more stable
It takes about 4 to 6 weeks for levothyroxine to be fully effective. That’s because the T4 has to be converted into T3 by the body before it becomes active. But a dose of levothyroxine can last in your body for a few weeks. This long-lasting effect helps keep your thyroid hormone levels stable over time.
In contrast, liothyronine can start working within a few hours, since it contains the active T3 hormone. It also takes less time — about 1 to 2 weeks — for liothyronine to be fully effective. But it doesn’t last as long in your body, so it can’t keep T3 levels as stable as levothyroxine does.
3. Levothyroxine is considered more effective than liothyronine for hypothyroidism
The American Thyroid Association recommends levothyroxine as a first-choice treatment for hypothyroidism. The main reasons for this are:
Levothyroxine is easily absorbed by the body and provides stable thyroid hormone levels when taken once a day.
Levothyroxine is easily converted into T3 by the body.
Levothyroxine provides a thyroid hormone ratio of T4 to T3 that mimics what the body naturally produces.
But liothyronine still has a role in managing hypothyroidism for some people. Sometimes, levothyroxine can’t return thyroid hormone levels to normal on its own. In this case, liothyronine is sometimes prescribed in combination with levothyroxine. Your prescriber may also recommend liothyronine if you don’t absorb levothyroxine well or have a negative reaction to it.
Some people still feel symptoms of hypothyroidism while taking levothyroxine, even if their thyroid hormone levels are within their goal range. When this happens, some prescribers may add liothyronine to see if it helps resolve the symptoms. This approach is gaining popularity, but there isn’t a lot of evidence behind it. And the American Thyroid Association doesn’t recommend it.
4. Liothyronine and levothyroxine have similar side effects to consider
Levothyroxine and liothyronine share similar side effects and risks. The most common problems happen because the dosage is too high. This can cause hyperthyroid symptoms, such as:
Nervousness
Tremors
Thin skin
Problems sleeping
Unintended weight loss
Sweating and feeling hot all the time
To avoid these symptoms, your prescriber will likely start you with a low dosage and slowly increase it based on how your thyroid hormone levels respond. If you notice any of the above symptoms during your treatment, report them to your healthcare team right away.
5. Levothyroxine and liothyronine aren’t safe to take for weight loss
You shouldn’t take levothyroxine or liothyronine for weight loss. Both medications have a boxed warning — the most serious type of FDA warning for medication — saying typical doses aren’t effective for weight loss and larger doses can be life-threatening.
If you’re interested in taking medication for weight loss, talk to your healthcare team about safe and effective options. Thyroid medications aren’t safe to take for weight loss.
6. You should take levothyroxine and liothyronine on an empty stomach
It’s important to take your thyroid hormone medication correctly to get the most from it. In most cases, people take levothyroxine and liothyronine once a day. It’s best to take them at the same time every day on an empty stomach.
It’s recommended to take thyroid hormone medications in the morning, about 30 minutes to 1 hour before breakfast. If this doesn’t work for your schedule, bedtime doses may be just as effective — and easier for you to remember. But you’ll need to take your doses at least 4 hours after your last meal or snack of the day.
You should also separate your thyroid hormone medication from any other medications you take. Typically, people separate thyroid hormone medications from other pills by at least 4 hours. This can help determine what time of day works best to take the medications.
The main goal is to take your thyroid hormone medication the same way every day. This means taking it at the same time of day and with the same amount of time between other medications and meals. Being consistent with your doses will help your levels stay where they should be.
If you make changes to how or when you take your medication, be sure to let your prescriber know. They may want to run some tests to see if a dosage change is needed.
7. You should stay consistent with the form of thyroid hormone you take
Both levothyroxine and liothyronine are available as brand-name medications as well as lower-cost generics. The generics are considered similarly effective to the brand-name versions. But once you find the thyroid medication that works for you, it’s important to stick with it. You shouldn’t switch from a brand-name option to a generic (or vice versa) or even from one generic manufacturer to another.
Even though brand-name and generic thyroid medications share the same active ingredients, they aren’t made exactly the same way. A small difference in ingredients between manufacturers can change how your body absorbs a thyroid medication — which affects your thyroid hormone levels.
In some cases, your pharmacy may start carrying a different manufacturer’s medication, especially for a generic version of a thyroid medication. You may notice that your tablets look slightly different or have different markings on them than usual if this happens. Ask if there’s a way to get the type of medication you normally take. If it’s not available, let your prescriber know. They may want to perform lab tests to make sure your thyroid hormone levels stay stable on the new form of your medication.
How to save on levothyroxine and liothyronine
As mentioned, levothyroxine and liothyronine are available as brand-name medications as well as lower-cost generics. GoodRx can help you find ways to save on your prescription:
Save with GoodRx. Anyone with a valid prescription, regardless of insurance status, can access brand-name Cytomel or brand-name Levoxyl at an exclusive cash price through GoodRx. You could also pay as little as $4.00 for a 30-day supply of levothyroxine or $13.27 for a 30-day supply of liothyronine with a free GoodRx discount.
Save with patient assistance programs. If you’re uninsured or underinsured, you may be eligible for patient assistance programs. There are currently programs that offer brand-name Synthroid and Cytomel at no charge.
Save with a copay savings card. If you have commercial insurance and meet eligibility requirements, you could get brand-name Synthroid for as little as $25 with the manufacturer’s copay savings card. Manufacturer savings are also available for those without insurance coverage.
Frequently asked questions
There are special instructions for giving levothyroxine to an infant or young child who’s unable to swallow pills. For each dose, you’ll need to crush the tablet and mix it with a small amount of water (5 mL to 10 mL). Then, immediately give the mixed dose using a spoon or medication dropper.
Do not mix the tablets in food or formula, as this can decrease absorption. Your pediatrician can also provide guidance that’s specific to your child’s needs.
Switching from brand-name Synthroid to generic levothyroxine may cause a minor change in your thyroid hormone levels. That’s because small differences between the two forms can affect how your body absorbs them.
You shouldn’t switch from one form of thyroid hormone to another unless your prescriber recommends it. If you do need to make a change, they’ll likely run some extra blood tests after the switch to ensure your thyroid hormone levels stay within your goal range.
You may take levothyroxine and liothyronine together for a couple of reasons. If levothyroxine isn’t able to return your thyroid hormone levels to normal on its own, your prescriber may add liothyronine to your regimen. Some people also continue to experience hypothyroid symptoms while taking levothyroxine, even if their levels are within the normal range. In this case, your prescriber may add liothyronine to see if it helps resolve your symptoms.
There are special instructions for giving levothyroxine to an infant or young child who’s unable to swallow pills. For each dose, you’ll need to crush the tablet and mix it with a small amount of water (5 mL to 10 mL). Then, immediately give the mixed dose using a spoon or medication dropper.
Do not mix the tablets in food or formula, as this can decrease absorption. Your pediatrician can also provide guidance that’s specific to your child’s needs.
Switching from brand-name Synthroid to generic levothyroxine may cause a minor change in your thyroid hormone levels. That’s because small differences between the two forms can affect how your body absorbs them.
You shouldn’t switch from one form of thyroid hormone to another unless your prescriber recommends it. If you do need to make a change, they’ll likely run some extra blood tests after the switch to ensure your thyroid hormone levels stay within your goal range.
You may take levothyroxine and liothyronine together for a couple of reasons. If levothyroxine isn’t able to return your thyroid hormone levels to normal on its own, your prescriber may add liothyronine to your regimen. Some people also continue to experience hypothyroid symptoms while taking levothyroxine, even if their levels are within the normal range. In this case, your prescriber may add liothyronine to see if it helps resolve your symptoms.
The bottom line
Liothyronine (Cytomel) and levothyroxine (Synthroid, Levoxyl) are both medications that treat hypothyroidism, but they’re not the same. Levothyroxine contains T4 and is a first-choice treatment option for low thyroid hormone levels. Liothyronine contains T3 and is typically only prescribed if levothyroxine isn’t effective on its own or well tolerated.
You should take your thyroid hormone medication on an empty stomach and at least 4 hours apart from other medications. This helps the body absorb it better.
Whether you take levothyroxine, liothyronine, or a brand-name version, it’s best to stick with the same form and manufacturer every month. This helps you avoid minor differences between products, which helps keep your thyroid hormone levels stable.
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References
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De Mello, R. B., et al. (2022). Evaluation of bedtime vs. morning levothyroxine intake to control hypothyroidism in older patients: A pragmatic crossover randomized clinical trial. Frontiers in Medicine.
Jonklaas, J., et al. (2014). Guidelines for the treatment of hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid.
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