Liothyronine (T3)

Also known as: liothyronine sodium, T3, Cytomel (branded)

Hormone Restoration

In plain English

Liothyronine is the active thyroid hormone (T3). It is sometimes added to or used instead of standard levothyroxine (T4) for an underactive thyroid, though most people do well on T4 alone. Because it acts quickly and wears off fast, doctors monitor it carefully. It is taken by mouth, sometimes in a slow-release form.

The science

Liothyronine is synthetic triiodothyronine (T3), the biologically active thyroid hormone that binds nuclear thyroid receptors directly. It has a short half-life and rapid onset, producing higher peak levels and more potential for cardiac and symptomatic effects than levothyroxine. The ATA guideline (Jonklaas 2014) concluded that combination T4/T3 therapy has not been shown to be consistently superior to levothyroxine monotherapy for health outcomes, though it remains an option for selected patients who do not feel well on T4 alone; sustained-release T3 is used to smooth its pharmacokinetics. FDA-approved liothyronine exists; compounded T3 and fixed T4/T3 capsules are not FDA-approved formulations and can complicate dose consistency. Over-treatment risks (tachyarrhythmia, bone loss) are heightened given T3's potency.

References

• Jonklaas et al., Thyroid 2014 (ATA hypothyroidism treatment guidelines)