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Liothyronine vs Levothyroxine: T3, T4, and Using Both
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Women's Health
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Other Causes of Lingering Symptoms Are Checked First
Fatigue, poor sleep, low mood, and brain fog are common in people taking levothyroxine. However, they are also common in people with no thyroid problem at all. That overlap is why the 2021 consensus statement recommends ruling out other causes first (4).
For women in midlife, perimenopause, the years leading up to menopause, is a common overlap, since it can bring tiredness, broken sleep, and mood changes. If that pattern sounds familiar, menopause care and what HRT involves are worth reading before assuming the thyroid is the whole story.
Both Labels Carry the Same Boxed Warning
The Synthroid® (levothyroxine) and Cytomel® (liothyronine) labels each open with a boxed warning, the most prominent warning a prescription drug label can carry (1,2). The warning states that thyroid hormones, either alone or with other medicines, should not be used for the treatment of obesity or for weight loss (1,2).
The labels give two reasons for it. First, in people whose thyroid levels are normal, doses within the range of daily hormone needs are ineffective for weight reduction (1,2). Second, larger doses may produce serious or even life-threatening toxicity, particularly with stimulant-type drugs (sympathomimetic amines) such as those used to suppress appetite (1,2).
The warning is written about thyroid hormones as a class, so it applies to a compounded capsule containing levothyroxine and liothyronine too. Thyroid hormone is prescribed to treat hypothyroidism.
Thyroid Doses Are Set With the Heart and Bones in Mind
The labels describe most side effects as signs of too much hormone, such as palpitations (a pounding or fluttering heartbeat), a fast heartbeat, heat intolerance, sweating, and trouble sleeping (1). Overtreatment, meaning more hormone than the body needs, can raise the heart rate and may bring on chest pain (angina) or an irregular rhythm (1,2).
Atrial fibrillation, an irregular and often rapid heart rhythm, is the most common rhythm problem seen with levothyroxine overtreatment in older adults (1). The Cytomel® label tells prescribers to start below the full replacement dose in older adults and people with heart disease, because of the risk of cardiac reactions including atrial fibrillation (2).
Bone health has also been studied, mainly through population data. In an observational study, one that tracked 17,684 people on long-term levothyroxine in Scotland without assigning treatments, those with a fully suppressed TSH (a thyroid blood test result pushed very low by too much hormone) had roughly twice the fracture risk of people whose TSH sat in the normal range (7). People with a high TSH, a sign of too little hormone, also had higher risks. The study shows a link rather than proof of cause, and it points toward a balanced dose rather than simply more (7).
Some Health Conditions Call for Extra Care With Thyroid Hormone
Both labels state that levothyroxine and liothyronine are contraindicated, meaning they should not be used, in people with uncorrected adrenal insufficiency, a condition in which the adrenal glands do not make enough of the hormone cortisol (1,2). Starting thyroid hormone first can trigger an adrenal crisis, a medical emergency, so the labels say steroid replacement should come first (2).
The labels also call for caution in several other groups (1,2):
Older adults: a lower starting dose and slower increases are generally used, because hidden heart disease is more likely.
People with heart disease or atrial fibrillation: extra thyroid hormone can worsen chest pain and rhythm problems.
People with diabetes: starting thyroid hormone can worsen blood sugar control, so diabetes treatment may need review.
People who are pregnant or planning pregnancy: thyroid hormone needs often change, so dosing should be reviewed with a prescriber.
A Compounded Capsule Combines T4 and T3 in One Product
Available through Mochi Health with a prescription from an affiliated Mochi Medical provider, this compounded capsule combines levothyroxine and liothyronine, and each hormone has a different job. Levothyroxine supplies T4, the storage form that tissues convert into T3 as they need it (1,3). Liothyronine supplies T3, the active form that switches on activity inside cells (2,3).
Levothyroxine and liothyronine are each FDA approved as standalone tablets, while the capsule is a different product, prepared by a 503A compounding pharmacy for one patient from a prescription. Compounded medications are not approved or evaluated by the FDA for safety, effectiveness, or quality (8). The combination capsule itself is not FDA approved (8).
The FDA also recommends that patients be prescribed an FDA-approved drug when one is available and appropriate (8). Because both hormones exist as approved tablets, it is worth asking a provider whether the capsule or separate tablets fit your plan. The Mochi Health FAQs explain how visits and prescriptions work on the platform.
TSH Blood Tests Guide Every Dose Decision
Thyroid-stimulating hormone, or TSH, is the pituitary gland's signal to the thyroid. It rises when hormone levels run low and falls when they run high. The labels base dose changes on TSH and free T4, another thyroid blood test, alongside how you feel, with periodic checks once a dose is stable (1,2).
How you take the medication matters as well. MedlinePlus advises taking levothyroxine on an empty stomach, 30 to 60 minutes before breakfast (9).
Side by Side, the Options Differ Most in Approval Status
The table below compares the two approved hormones with a compounded combination capsule.
Option | Hormone replaced | FDA status | Form |
|---|---|---|---|
Levothyroxine (Synthroid®) | T4 | FDA approved for hypothyroidism | Prescription tablet |
Liothyronine (Cytomel®) | T3 | FDA approved for hypothyroidism | Prescription tablet |
Compounded levothyroxine and liothyronine | T4 plus T3 | Not FDA approved | Compounded capsule |
All three are covered by the same boxed warning, because it applies to thyroid hormones as a group (1,2).
Steps to Take Before Asking About T3
None of these steps replaces a talk with your prescriber, although they make that talk more useful and more focused.
Write down the symptoms that persist on levothyroxine and how long each has lasted, because a clear list helps a provider look for other causes.
Ask for your recent TSH and free T4 results with their dates, so the provider can see a trend rather than one number.
List every supplement and medication you take, including calcium and iron, since several can change absorption.
Ask your provider whether combination therapy is reasonable for you, and what monitoring it would involve.
Talk to your prescriber before changing when or how you take your thyroid medication.
Symptoms That Need Prompt Medical Care
Contact a provider the same day for chest pain, a racing or irregular heartbeat, fainting, or severe shortness of breath while taking thyroid hormone (1,2). Call 911 for chest pain that does not settle, sudden weakness, or new confusion.
Severe tiredness, confusion, feeling very cold, and swelling can point to the opposite problem, a dangerously underactive thyroid (myxedema coma), which also needs emergency care (1). In either case, contact your prescriber about whether to continue your medication rather than deciding alone.
The Bottom Line
For most people, the liothyronine and levothyroxine decision starts with symptoms, labs, and other causes before anyone adds T3. Your provider will determine whether T3 is added and which form fits. Separate FDA-approved tablets and a compounded capsule, which is not FDA approved, are both available through Mochi Health. Regular TSH checks guide the dose (1,2). Mochi Labs bloodwork explains how testing works, and the Mochi Health medications list shows current options.
FAQs
Is Synthroid the Same as Liothyronine?
Synthroid is not the same as liothyronine. Synthroid® is a brand of levothyroxine, which is synthetic T4, while liothyronine is synthetic T3 sold under the brand name Cytomel® (1,2). Both are prescription tablets for hypothyroidism, but they are dosed differently.
What Is Another Name for Liothyronine?
Another name for liothyronine is T3, short for triiodothyronine, and the tablets are labeled as liothyronine sodium (2). The best-known brand name is Cytomel®, and generic tablets are also available by prescription.
Can Levothyroxine Cause Diarrhea?
Levothyroxine can cause diarrhea, mainly when the dose supplies more hormone than your body needs. The prescribing information lists diarrhea among the signs of too much thyroid hormone, alongside palpitations and heat intolerance (1). Persistent diarrhea is worth reporting to your prescriber, who can check your TSH and assess whether treatment needs adjusting.
Can You Take Liothyronine and Levothyroxine Together?
You can take liothyronine and levothyroxine together when a prescriber decides the combination suits you, and many patients do (4). Major guidance still treats levothyroxine alone as the standard, so combination therapy is usually considered only when symptoms persist and other causes have been ruled out (3,4).
References
AbbVie Inc. Synthroid (levothyroxine sodium) tablets, prescribing information. DailyMed, National Library of Medicine. Synthroid prescribing information
Pfizer Inc. Cytomel (liothyronine sodium) tablets, prescribing information. DailyMed, National Library of Medicine. dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=51452b31-ff68-4e0c-b982-c15502ebf1d3
Jonklaas J, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid, 2014. ATA hypothyroidism guideline
Jonklaas J, et al. Evidence-based use of levothyroxine/liothyronine combinations in treating hypothyroidism: a consensus document. Thyroid, 2021. pmc.ncbi.nlm.nih.gov/articles/PMC7983670/
Walsh JP, et al. Combined thyroxine/liothyronine treatment does not improve well-being, quality of life, or cognitive function compared to thyroxine alone: a randomized controlled trial in patients with primary hypothyroidism. Journal of Clinical Endocrinology and Metabolism, 2003. pubmed.ncbi.nlm.nih.gov/14557419/
Millan-Alanis JM, et al. Benefits and harms of levothyroxine/L-triiodothyronine versus levothyroxine monotherapy for adult patients with hypothyroidism: systematic review and meta-analysis. Thyroid, 2021. pubmed.ncbi.nlm.nih.gov/34340589/
Flynn RW, et al. Serum thyroid-stimulating hormone concentration and morbidity from cardiovascular disease and fractures in patients on long-term thyroxine therapy. Journal of Clinical Endocrinology and Metabolism, 2010. pubmed.ncbi.nlm.nih.gov/19906785/
U.S. Food and Drug Administration. Compounding and the FDA: Questions and Answers. FDA compounding questions
MedlinePlus. Levothyroxine: MedlinePlus Drug Information. National Library of Medicine. medlineplus.gov/druginfo/meds/a682461.html
This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider before starting, stopping, or changing any medication or treatment plan. Levothyroxine, liothyronine, and compounded levothyroxine and liothyronine capsules require a written prescription from a licensed provider. Speak with a provider to determine if levothyroxine, liothyronine, or a compounded combination capsule is appropriate for you. Levothyroxine and liothyronine are each FDA approved to treat hypothyroidism, a compounded capsule combining them is not FDA approved, and no thyroid hormone is approved for the treatment of obesity or for weight loss. Compounded medications are not approved or evaluated by the FDA for safety, effectiveness, or quality. Individual results are not guaranteed and may vary from person to person. All professional medical services are provided by licensed physicians and clinicians affiliated with independently owned and operated professional practices. Mochi Health Corp. provides administrative and technology services to affiliated medical practices it supports, and does not provide any professional medical services itself.
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