Liothyronine Sodium L-Triiodothyronine (T3) 💊
Related Subjects:
|L-Thyroxine (T4)
|Hypothyroidism
💊 Liothyronine (T3) is the active thyroid hormone. It acts faster and is shorter-lived than levothyroxine (T4). It is mainly used in myxoedema coma and in specialist situations. NICE NG145 advises that it is not routinely offered for primary hypothyroidism, alone or combined with levothyroxine. NHS England guidance restricts new starts to NHS consultant endocrinologists.
📖 About
- T4 (levothyroxine) is converted peripherally to T3 by deiodinases; most patients do well on T4 alone.
- Liothyronine is much more expensive than levothyroxine, and doses are in micrograms - take care with prescribing.
- Always check the BNF for current guidance.
⚙️ Mode of Action
- Binds nuclear thyroid hormone receptors → alters gene transcription → ↑ basal metabolic rate, protein synthesis, cardiac output and sensitivity to catecholamines.
- Rapid onset (within hours); half-life about 1 day (T4: about 7 days), so levels fluctuate more.
🎯 Indications
- Myxoedema coma (IV, with hydrocortisone first if adrenal insufficiency is possible).
- Specialist use: patients with persistent symptoms despite optimised levothyroxine (trial under an endocrinologist), short-term T3 before radioiodine whole-body scanning in thyroid cancer (allows a short withdrawal).
- Liothyronine suppression test (rare).
💊 Dose (adult)
| Indication | Dose |
|---|
| Hypothyroidism (specialist) | Initially 10–20 micrograms daily, increased gradually to 60 micrograms daily in 2–3 divided doses |
| Older people / cardiac disease | Start 5 micrograms daily; increase very slowly |
| Myxoedema coma (IV) | 5–20 micrograms by slow IV injection, repeated every 12 hours (can be as often as every 4 hours), under specialist supervision |
- Approximate equivalence: 20–25 micrograms liothyronine ≈ 100 micrograms levothyroxine.
🧪 Monitoring
- TSH and clinical response; free T3 measured at a consistent time after the dose (levels fluctuate).
- ECG and close cardiac monitoring in myxoedema coma, older people and ischaemic heart disease.
🔄 Interactions
- Warfarin: ↑ anticoagulant effect when thyroid status rises - monitor INR.
- Antidiabetic drugs/insulin: requirements may rise.
- Digoxin: levels may fall as thyroid status normalises.
- Absorption ↓ by calcium, iron, PPIs and colestyramine - separate doses.
- Enzyme inducers (carbamazepine, phenytoin, rifampicin) ↑ clearance.
⚠️ Cautions
- Cardiovascular disease (angina, arrhythmia, heart failure): start low, go slow.
- Adrenal insufficiency: correct with corticosteroid first (thyroid hormone can precipitate adrenal crisis).
- Diabetes; long-standing hypothyroidism; older people.
- Over-replacement → AF and osteoporosis.
🚫 Contraindications
- Thyrotoxicosis.
- Untreated adrenal insufficiency.
💥 Side Effects
- Usually due to excess dose: palpitations, tachycardia, arrhythmias (AF), angina, tremor, restlessness, insomnia, sweating, heat intolerance, diarrhoea, weight loss, headache, flushing.
- Long-term over-replacement: ↓ bone mineral density.
📚 References
