Liothyronine (T3)

Liothyronine (also called LT3 or synthetic T3) is the synthetic form of triiodothyronine (T3) - the more potent, directly active thyroid hormone. Unlike levothyroxine (T4), which must be converted to T3 by the body, liothyronine acts without that conversion step.

Liothyronine is shorter-acting than levothyroxine and typically needs to be taken twice or three times daily to maintain steadier levels. It is not used as a first-line treatment for hypothyroidism in the UK, where levothyroxine monotherapy is standard.

However, some specialist endocrinologists prescribe combination therapy (LT4 + LT3) for people who remain symptomatic despite normal TSH on levothyroxine alone, particularly those with variants affecting T4-to-T3 conversion. NICE guidance for hypothyroidism notes that combination T3 therapy may be considered as a specialist trial in selected patients. Access and prescribing policies vary across NHS areas.

Liothyronine is also used in specific circumstances - for example, short-term before radioactive iodine treatment for thyroid cancer, when a rapid drop in thyroid hormone levels is needed.