Thyroid hormone (T3) Pregnancy: eMC 4.6: Safety during pregnancy is not known; the risk of foetal congenital abnormalities should be weighed against the risk to the foetus of untreated maternal hypothyroidism. Breast-feeding - excreted into breast milk in low concentrations, which may interfere with neonatal screening programmes. No human or animal fertility data.

Liothyronine sodium (T3)

Brand names: Cytomel, Tertroxin

Used in: Thyroid Disorders

The synthetic form of the active thyroid hormone tri-iodothyronine (T3), used for severe or acute hypothyroid states such as myxoedema coma and occasionally as an adjunct in selected patients, where its rapid onset is advantageous over levothyroxine.

Adult dose

Dose: Starting dose 10 or 20 micrograms every 8 hours; usual recommended daily dose 60 micrograms

Route: Oral use only (tablets; a whole tablet may be crushed and dissolved with swirling in a minimum 20 ml of water for 5 minutes, and the entire volume consumed)

Frequency: Starting dose every 8 hours, increased after one week if necessary; usual daily dose of 60 micrograms given in two or three divided doses

eMC SPC 4.2 (Liothyronine Sodium 10 microgram Tablets): 'Adults: Starting dose of 10 or 20 micrograms every 8 hours, increasing after one week, if necessary, to the usual recommended daily dose of 60 micrograms in two or three divided doses.' Other regimens stated in the same section: Myxoedema coma - 60 micrograms given by stomach tube, then 20 micrograms every 8 hours, though 'it is more usual to start treatment with intravenous liothyronine'. Adjunct to carbimazole treatment of thyrotoxicosis - 20 micrograms every 8 hours. Elderly - 5 micrograms daily. Paediatric doses in the SPC are not weight-based: children below 12 years 5 micrograms daily; adolescents 12-17 years initially 10-20 micrograms daily, increased to 60 micrograms daily in 2-3 divided doses - verify against a children's formulary before use. SPC 4.4: monitor TSH during treatment to reduce the risk of over- or under-treatment; baseline ECG is recommended before starting and periodic ECGs during treatment; if metabolism increases too rapidly (diarrhoea, nervousness, rapid pulse, insomnia, tremors, anginal pain), reduce the dose or withhold for 1-2 days and start again at a lower dose; initiate corticosteroid therapy before starting liothyronine in panhypopituitarism or predisposition to adrenal insufficiency.

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Patients with angina of effort or cardiovascular diseases and thyrotoxicosis
Established myocardial ischaemia (SPC 4.4 cross-refers to 4.3; levothyroxine with cautious dose escalation is recommended instead)

Side effects

Cardiac: anginal pain, cardiac arrhythmias, palpitations, tachycardia
Gastrointestinal: diarrhoea, vomiting
Nervous system: headache, tremor
Psychiatric: restlessness, excitability, insomnia
Musculoskeletal: muscle cramps, muscular weakness
General: fever, flushing, heat intolerance, sweating, excessive loss of weight (effects indicative of excessive dosage, usually resolving on dose reduction or withdrawal for a day or two)

Interactions

Bile acid sequestrants (colesevelam, cholestyramine, colestipol) and ion exchange resins (Kayexalate, sevelamer) decrease thyroid hormone absorption - give liothyronine at least 4 hours before these drugs or monitor TSH (US label 7.1; eMC 4.5 was not captured in the source bundle)
Biotin may interfere with thyroid immunoassays based on a biotin/streptavidin interaction, giving falsely decreased or increased results; risk increases with higher biotin doses - inform the laboratory (eMC 4.4)
Insulin and anti-diabetic drugs: liothyronine treatment may increase insulin or anti-diabetic drug requirements (eMC 4.4)

Clinical monograph

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.

Reference: BTA position statement; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways