Liothyronine sodium (T3)

How it works

Liothyronine is the biologically active thyroid hormone that binds nuclear thyroid receptors to regulate metabolism, but unlike levothyroxine it acts directly without requiring peripheral conversion.

Prescribing in practice

• Its rapid, potent action makes cardiac adverse effects more likely, so start at a low dose and titrate cautiously, especially in the elderly and in those with ischaemic heart disease.
• It has a shorter half-life than levothyroxine, giving a faster onset and offset but more fluctuation in hormone levels.
• Routine substitution of liothyronine for levothyroxine in primary hypothyroidism is not recommended and should be a specialist decision.

Monitoring

Monitor thyroid function and clinical response, together with heart rate and any cardiac symptoms during dose titration.

Counselling the patient

• Take exactly as prescribed and do not change the dose yourself.
• Report palpitations, chest pain, tremor or marked anxiety.
• This is a specialist-initiated treatment and needs regular blood-test review.

Evidence & guidelines

Liothyronine has a defined role in myxoedema coma and specialist scenarios; routine combination or substitution therapy is not supported by NICE/endocrine guidance for most patients with primary hypothyroidism.