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Thyroid Hormone (T3) Pregnancy: Can be used in pregnancy if required — levothyroxine preferred; close monitoring of TFTs required (requirements increase by ~30% in pregnancy).

Liothyronine Sodium (T3)

Brand names: Tertroxin, Thybon Henning

Adult dose

Dose: Hypothyroidism: 10–20 micrograms TDS initially; maintenance 20–60 micrograms daily in divided doses. Myxoedema coma (IV): 5–20 micrograms IV slowly, then 5 micrograms IV every 12h; increase to 20 micrograms 12-hourly if no improvement after 24h
Route: Oral (routine); IV (myxoedema coma — specialist use only)
Frequency: Two to three times daily (oral); every 12 hours (IV in coma)
Max: 60 micrograms daily (oral); individualised in myxoedema coma — seek endocrinology advice
T3 has rapid onset (hours) vs. levothyroxine T4 (days-weeks). IV form used in myxoedema coma alongside corticosteroids (hydrocortisone 100mg IV 8-hourly). Not recommended as routine replacement for hypothyroidism — levothyroxine (T4) is first-line. Combination T3/T4 therapy may be considered in patients with persistent symptoms on T4 alone (NICE guidance).

Paediatric dose

Route: Oral
Frequency: Two to three times daily
Max: Individualised
Seek specialist paediatric endocrinology opinion. Used in neonatal hypothyroidism alongside levothyroxine under specialist supervision.

Dose adjustments

Renal

No specific dose adjustment — monitor clinically and by TSH levels.

Hepatic

No specific dose adjustment required.

Clinical pearls

  • In myxoedema coma: always give corticosteroids BEFORE or WITH thyroid hormone replacement — occult adrenal insufficiency is common and T3 initiation can precipitate adrenal crisis
  • T3 has 3–4× potency of T4 by weight — conversion: 25 micrograms T3 ≈ 100 micrograms T4
  • Short half-life of T3 (1 day) vs. T4 (7 days) — monitor clinically for over-replacement (palpitations, tremor, anxiety)
  • TSH may remain suppressed for weeks when switching from T3 to T4 — do not use TSH as sole monitor during transition

Contraindications

  • Thyrotoxicosis
  • Adrenal insufficiency (must treat with corticosteroids before commencing thyroid replacement — risk of adrenal crisis)
  • Hypersensitivity to liothyronine

Side effects

  • Palpitations and tachyarrhythmias (particularly AF)
  • Angina precipitation in ischaemic heart disease
  • Tremor, anxiety, insomnia
  • Excessive sweating
  • Heat intolerance
  • Diarrhoea
  • Weight loss
  • Bone loss (long-term excess)

Interactions

  • Warfarin — thyroid hormones enhance anticoagulant effect; monitor INR closely after any dose change
  • Digoxin — hyperthyroid state increases digoxin clearance; dose may need adjustment
  • Antidiabetic drugs — thyroid hormones increase blood glucose; adjust insulin/antidiabetic therapy
  • Calcium/iron supplements — reduce absorption if taken together; separate by 4 hours
  • Colestyramine — reduces absorption; take 4 hours apart

Monitoring

  • TSH and free T3 (4–6 weekly until stable, then 6 monthly)
  • Heart rate and rhythm (ECG if cardiac symptoms)
  • Bone mineral density (long-term — excess T3 causes osteoporosis)

Reference: BNFc; BNF 90; BTA/BTAC Guidelines on Hypothyroidism 2019. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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