Should I be on T3 as well as T4?
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2028
Short answer
Levothyroxine alone remains first-line treatment for hypothyroidism. Adding liothyronine, or T3, is a considered trial for a narrow group: people whose symptoms persist despite a normal TSH on adequate levothyroxine, after other causes have been excluded. The joint thyroid association consensus supports a monitored trial rather than routine use.
Levothyroxine monotherapy is still the standard treatment for hypothyroidism, and the American Thyroid Association 2014 guideline recommends it as the preparation of choice. Adding liothyronine is not first-line and is not for everyone with residual symptoms. It is a defined, monitored trial in a specific group, and the position of the major thyroid societies shifted towards permitting that trial rather than dismissing it.
Why some people may not do well on T4 alone
Most T3 in the body is made by converting T4 in peripheral tissues. Levothyroxine restores that supply chain for the large majority of people. A minority report persistent fatigue, low mood and cognitive symptoms despite a normal TSH, and the argument for combination therapy is that levothyroxine alone may not restore tissue T3 exactly as a healthy thyroid would. The evidence that adding T3 fixes this is inconsistent. The American Thyroid Association 2014 guideline found no consistently strong evidence for the superiority of levothyroxine and liothyronine combination therapy over monotherapy.
Who the consensus selects for a trial
The joint American, British and European Thyroid Association consensus document describes the group to consider: "patients with persistent hypothyroid symptoms or inadequate improvement of their hypothyroid symptoms with LT4 therapy should specifically be recruited, once other causes for these symptoms have been excluded." It suggests a baseline levothyroxine dose of at least 1.2 mcg per kg per day with a normal TSH, and excludes pregnancy, cardiac arrhythmias and established cardiovascular disease.
How a proper trial is run
The consensus suggests starting at a levothyroxine to liothyronine ratio of 13:1 to 20:1, which works out at 5 or 10 mcg of liothyronine for someone taking 100 to 200 mcg of levothyroxine, with the levothyroxine dose reduced by 12.5 to 25 mcg at the same time. Twice-daily liothyronine is preferred over once daily because of its short half-life. A minimum trial of 3 to 6 months is suggested, with 9 to 12 months preferred, and monitoring includes pulse regularity, screening for atrial fibrillation and bone densitometry every three years in postmenopausal women.
| Consideration | Levothyroxine alone | Combination with liothyronine |
|---|---|---|
| Guideline status | First-line, standard of care | Trial in selected patients only |
| Dosing | Once daily | Levothyroxine once daily plus liothyronine twice daily |
| Monitoring | TSH | TSH, pulse and rhythm, bone density in postmenopausal women |
| Not suitable in | Rarely a problem | Pregnancy, arrhythmia, established cardiovascular disease |
If you want to try liothyronine, ask for the other causes of your symptoms to be ruled out first, ask for a written trial length and a specific symptom endpoint, and agree in advance what result would end the trial.
The clinical detail
Liothyronine has a serum half-life of roughly 24 hours compared with about 7 days for levothyroxine, producing peaks two to four hours after dosing. TSH interpretation changes on combination therapy: a suppressed TSH with a normal free T4 is common and reflects the T3 peak, so pulse, rhythm and symptoms carry more weight than TSH alone.
The consensus document lists exclusions of pregnancy, cardiac arrhythmias and established cardiovascular disease, and recommends electrocardiographic screening for atrial fibrillation plus bone densitometry every three years in postmenopausal women. Draw thyroid function tests before the morning liothyronine dose. The 2026 American Thyroid Association pregnancy guideline advises avoiding T3-containing therapy in pregnancy, because liothyronine does not cross the placenta in a way that supports fetal brain development and maternal free T4 falls.
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2028
Sources
- Evidence-Based Use of Levothyroxine/Liothyronine Combinations in Treating Hypothyroidism: A Consensus Document. American, British and European Thyroid Associations. PMID 33276704
- Guidelines for the Treatment of Hypothyroidism. American Thyroid Association. PMID 25266247
- American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum. American Thyroid Association. PMID 42219800
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