Liothyronine (Cytomel): how T3 is dosed, monitored and combined with levothyroxine. | Reddit

What liothyronine is

Your thyroid makes two hormones: T4, the storage form, and T3, the form your cells use. About 80% of the T3 in your blood is made by converting T4 in the body, according to the Cytomel and Synthroid labels. Liothyronine supplies T3 directly, so it does not depend on that conversion.

Levothyroxine (synthetic T4) is the standard treatment for hypothyroidism. The American Thyroid Association calls it “the preparation of choice,” and FDA reported that about 94% of the 24 million Americans on thyroid hormone replacement in 2024 took it. Liothyronine is used alongside levothyroxine for some patients and on its own for others, and it is also labeled for two specialist uses: thyroid cancer care and a diagnostic suppression test.

The boxed warning on the label says thyroid hormones, including Cytomel, “should not be used for the treatment of obesity or for weight loss.” In people with normal thyroid function, replacement doses do not cause weight loss, and larger doses can be toxic, especially with stimulant drugs. The American Thyroid Association makes a strong recommendation against using T3 to treat obesity in people with normal thyroid function.

Liothyronine dosing on the label

Cytomel tablets come in 5, 25 and 50 mcg. Generic liothyronine has been FDA-approved since at least 2009, and DailyMed lists labels from many manufacturers.

The label says to take Cytomel once a day and lists food among the factors that set the dose, without requiring an empty stomach. Levothyroxine is different: its label says to take it on an empty stomach 30 to 60 minutes before breakfast. Cholestyramine, colesevelam and other bile acid binders reduce thyroid hormone absorption, so the label says to take Cytomel at least 4 hours before them.

T3 vs T4: how the two compare

The strength ratio matters when a dose is converted. The 2021 consensus statement from the American, British and European thyroid associations puts the equivalence at “approximately a 1:3 ratio,” so 25 mcg of T3 does roughly the work of 75 mcg of T4. An older Cytomel label used a 1:4 ratio, and you will still see that figure quoted.

Adding T3 to levothyroxine: what the guidelines say

Most people do well on levothyroxine alone. The European Thyroid Association estimates that 5% to 10% of people on levothyroxine with a normal TSH still have symptoms, and that group is where adding T3 comes up.

• American Thyroid Association, 2014. Recommends against routine combination therapy, citing conflicting trial results and few long-term data, and found insufficient evidence for trying it outside a formal or N-of-1 trial.
• European Thyroid Association, 2012. Treats combination therapy as an experimental option for people who take levothyroxine reliably and still have symptoms with a normal TSH. It suggests starting at a levothyroxine-to-T3 ratio between 13:1 and 20:1 by weight, splitting the T3 into two doses with the larger one at bedtime, testing blood before the morning dose, and stopping after 3 months if nothing improves. It advises against combination therapy in pregnancy and in people with heart rhythm problems.
• British Thyroid Association and Society for Endocrinology, 2023. Optimize levothyroxine first, aiming for a TSH of 0.3 to 2.0 mU/L for 3 to 6 months. After that, for some people with confirmed hypothyroidism and persistent symptoms, “a trial of liothyronine/levothyroxine combined therapy may be warranted.”

Here is the European method worked through for someone on 100 mcg of levothyroxine a day. The T3 dose is the levothyroxine dose divided by 17, about 6 mcg. The new levothyroxine dose is the old dose minus 3 times the T3 dose: 100 minus 18, about 82 mcg. Your clinician adjusts from there using your labs.

The trial evidence is mixed. A 2006 meta-analysis of 11 randomized trials with 1,216 patients and a 2021 review of 18 studies found no difference between combination therapy and levothyroxine alone in symptoms, quality of life, depression or fatigue. Patients still leaned toward T3: across studies, 43% preferred the combination, 23% preferred levothyroxine and 30% had no preference. In a 2021 trial, the most symptomatic third of patients strongly preferred treatment that contained T3. Genetic testing for the DIO2 gene, sometimes suggested to predict who benefits, is not recommended by the American Thyroid Association.

Sustained-release T3

Standard T3 rises after a dose and then falls. The 2021 consensus statement notes that once-daily T3 added to levothyroxine raises blood T3 up to 40% above its low point, and it calls slow-release T3 the way to reach steady, physiological levels. It also states: “no approved slow-release T3 therapies are available at this time.”

The human data on slow-release T3 come from investigational products. In a 2022 phase 1 study in 12 healthy volunteers, a poly-zinc form of liothyronine produced a peak about 30% lower and an hour later than standard T3, held a plateau for up to 6 hours, and kept T3 above half its peak at 24 hours. Compounded sustained-release capsules have been tested in the lab for how they release T3; we found no published study of them in people. The American Thyroid Association adds a caution: because T3 itself is short-lived once it is in the blood, even a slow-release product may still need more than one dose a day.

What compounding adds is choice of form and dose. Cytomel tablets come in three strengths. PepScribe’s pharmacy lists sustained-release T3 capsules in 16 strengths from 5 to 26 mcg, labeled for once-daily use in the morning, so a dose can move in steps of 0.5 to 2.5 mcg.

Compounded T3: what to know before you choose it

Compounded medicines are prepared for one patient on a prescription, and FDA does not review them for safety, effectiveness or quality before they are sold. Thyroid hormone has a narrow therapeutic range, so guidelines set a high bar.

• The American Thyroid Association recommends against routine use of compounded thyroid hormones, citing safety, potency and a lack of evidence that they work better. It makes an exception for an allergy to an ingredient that cannot be avoided by switching brands; Cytomel’s inactive ingredients include sucrose, gelatin and corn starch.
• In 2021, FDA staff described 12 US cases of thyroid over- or under-replacement linked to compounded thyroid products, including products tested at 10 to more than 1,000 times their labeled strength, and wrote that approved products should be used unless a compounded one is clinically necessary.
• The same FDA staff letter notes there is no standard conversion between approved and compounded thyroid products, so a switch in either direction calls for a recheck of your labs.
• Desiccated (animal) thyroid cannot be compounded, because FDA regulates it as a biological product, so any compounded T3/T4 combination is synthetic.

In practice, compounded T3 is chosen for something approved tablets do not offer: a sustained-release form, a dose between the tablet strengths, or a formula without an ingredient you react to. PepScribe’s T3 is compounded in the USA by a licensed 503A pharmacy, and your clinician rechecks your labs after you start or switch.

When to draw labs on T3

• During the first weeks. The Cytomel label prefers total T3 for checking early response, before TSH has had time to change.
• On a stable dose. TSH periodically after any change, then a full review every 6 to 12 months.
• Timing on the day. European guidelines test blood drawn before the morning dose, so the result reflects your low point. The American Thyroid Association suggests a fasting TSH before the dose, and if a peak is wanted, a T3 level about 2 hours after the dose; another review suggests 2 to 4 hours.
• Before any thyroid test. Stop biotin supplements for at least 2 days, because biotin can distort TSH, T4 and T3 results.
• Central hypothyroidism. When the problem is in the pituitary or hypothalamus, TSH is not reliable, and the label uses T3 levels instead.

Tell your clinician when you took your last dose before the draw, and keep that timing the same from one test to the next. Blood T3 also has its own daily rhythm, varying by 14% to 35% in healthy adults.

Side effects and over-replacement

Most side effects of liothyronine are signs of too much thyroid hormone. The label lists palpitations, a fast or irregular heartbeat, nervousness, anxiety, insomnia, heat intolerance, sweating, tremor, diarrhea, weight loss and menstrual changes.

• Heart rhythm. In the Framingham study, people aged 60 and older with a TSH of 0.1 or lower had about 3 times the 10-year risk of atrial fibrillation. Atrial fibrillation is the most common rhythm problem from overtreatment in older adults, which is why the label starts them low.
• Bones. Over-replacement can speed bone loss, especially after menopause, and the label advises the lowest dose that does the job.
• What the observational data show. A 2025 meta-analysis found that adverse events tied to T3 came from unregulated T3 or compounding errors, and in cohorts of 630,254 people, regulated T3 users had no increase in atrial fibrillation compared with levothyroxine users.
• Who should not start T3. People with untreated adrenal insufficiency. People with diabetes may need their medicines adjusted, and blood thinners can become stronger.
• Pregnancy. The label says not to stop thyroid hormone in pregnancy, and needs often rise. European guidelines advise against T4/T3 combination therapy in pregnancy.

How to get T3 through PepScribe

1. Start with the free online visit from the thyroid treatment page. Have your recent TSH, free T3 and free T4 results ready; lab fees may apply depending on your state.
2. A licensed clinician reviews your labs, symptoms and current medicine, and decides with you whether T3, a T3/T4 combination or your current treatment fits best.
3. If T3 is prescribed, it is compounded in the USA by a licensed 503A pharmacy and shipped to you, with follow-up labs to set the dose.

Frequently asked questions