Liothyronine

Key facts

• Liothyronine is synthetic T3 (triiodothyronine), the active thyroid hormone. It treats an underactive thyroid, sometimes alongside T4 (levothyroxine), and works within hours rather than days.
• It is far more potent and short-acting than levothyroxine, so doses are small and precise; never adjust it on your own.
• Because it is a potent, fast-acting hormone, too much of it stresses the heart and can trigger a fast or irregular heartbeat. It must never be used as an energy, mood, or weight-loss supplement.
• Seek urgent care for chest pain, palpitations, or a racing heartbeat.

What liothyronine treats

Liothyronine treats hypothyroidism, particularly when a fast onset of effect is needed or when the body converts T4 to T3 poorly. It is used in hospital for severe hypothyroidism (myxoedema coma) and occasionally for short-term thyroid hormone withdrawal testing. It is a prescription thyroid medicine, not a supplement for tiredness, low mood, or weight loss in people with a normal thyroid.

How liothyronine works

Liothyronine is chemically identical to the T3 your thyroid makes. Unlike levothyroxine (T4), it does not need to be converted by body tissues; it binds thyroid hormone receptors directly, so it acts faster and clears the body more quickly.

Before you take it

• Tell your prescriber about any heart disease, arrhythmia, or untreated adrenal insufficiency; these need treatment or careful monitoring before you start.
• Older adults and anyone with heart disease usually start on a very low dose, increased gradually.
• Pregnancy and breastfeeding require specialist guidance, since thyroid hormone needs change and dosing must be individualised.
• Anticoagulants, other thyroid medicines, and calcium, iron or antacid supplements taken too close together can all change how liothyronine works.

Side effects

Common effects include tremor, sweating, heat intolerance, headache, and changes in appetite.

Stop and seek urgent medical care for any of these:

• A rapid, pounding, or irregular heartbeat.
• Chest pain or pressure.
• Severe agitation, fever, or profuse sweating (signs of thyroid hormone excess).
• Signs of an allergic reaction: rash, swelling, or difficulty breathing.

Safety essentials

• Liothyronine is a potent, short-acting thyroid hormone with genuine cardiac risk, not a general energy or fatigue remedy; it should only be used for a diagnosed thyroid condition under medical supervision.
• Doses are started low and increased slowly, with regular TSH and T3 blood tests guiding each change, particularly in older adults and anyone with heart disease.
• Take it exactly as prescribed and at consistent times. Because it acts quickly and wears off quickly, missed or extra doses cause noticeable swings in how you feel.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.