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Can I take Cytomel and Synthroid together?

Medically reviewed by Carmen Pope, BPharm. Last updated on Aug 14, 2023.

Official answer


Although the American Thyroid Association guidelines do not recommend taking Cytomel and Synthroid together, many people who are under the supervision of an internist/endocrinologist do take this combination treatment, prefer it, and have good results on it. Never take this combination without your doctor's advice because it may result in excessive levels of thyroid hormones which can be dangerous.

Cytomel contains a thyroid hormone called liothyronine (also called LT3) and Synthroid contains a thyroid hormone called levothyroxine (also called LT4). In our bodies, our thyroid gland primarily makes T4 which is then converted by tissues into the active hormone T3. T4 lasts a lot longer in the body than T3 (half-life of 5 to 7 days for T4 versus approximately 1 day for T3), although T3 is 4 times more active than T4. The reason the American Thyroid Association prefers LT4 (Synthroid) is because it can be administered orally, effectively relieves the symptoms of hypothyroidism in most patients, and its long half-life allows once-daily dosing. Clinical trials have not shown any benefit in adding LT3 (Cytomel) to LT4 (Synthroid) which is why the American Thyroid Association discourages it.

International guidelines are reluctant to recommend it either, but some suggest it may be worth considering in certain circumstances with the European Thyroid Association stating “combination therapy should be considered solely as an experimental treatment overseen by accredited internists/endocrinologists and discontinued if no improvement is seen after three months”. The Italian Thyroid Association says, “combination therapy is generally not recommended but a trial may be considered to improve adherence to treatment or patient well-being”, and outlines a protocol for combination therapy.

If combination therapy is trialed, it is important that regular laboratory monitoring of both T4 and T3 levels are done and dosing is based on the residual thyroid function (RTF). Well-designed, randomized clinical trials are needed to resolve the controversy over combing Cytomel and Synthroid

Related questions

  • Seaborg E. A Controversy Continues: Combination Treatment for Hypothyroidism Endocrine News April 2017.
  • Jonklaas J, Bianco AC, Bauer AJ, Burman KD, Cappola AR, Celi FS, Cooper DS, Kim BW, Peeters RP, Rosenthal MS, Sawka AM; American Thyroid Association Task Force on Thyroid Hormone Replacement. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014 Dec;24(12):1670-751. doi: 10.1089/thy.2014.0028. PMID: 25266247; PMCID: PMC4267409.
  • DiStefano J III and Jonklaas J (2019) Predicting Optimal Combination LT4 + LT3 Therapy for Hypothyroidism Based on Residual Thyroid Function. Front. Endocrinol. 10:746. doi: 10.3389/fendo.2019.00746

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