Liothyronine !

Synthetic T3, the active thyroid hormone. It acts within hours and wears off faster than levothyroxine.

Updated 2026-11-01

Class: Thyroid hormones
Receptor: Thyroid hormone receptor β (and α)
ATC code: H03AA02

Also called: T3, L-triiodothyronine

Thyroid hormone receptor β (and α)Liothyronine

The figure shows T3 (liothyronine itself) bound to the ligand-binding domain of human thyroid hormone receptor β.

Target in 3D: Thyroid hormone receptor β (and α). PDB structure 3GWS (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Liothyronine is identical to natural triiodothyronine (T3). It needs no conversion: it enters cells and binds nuclear thyroid hormone receptors, which release corepressors and recruit coactivators to change transcription of genes for metabolism, heat production, heart rate and growth. It binds plasma proteins less than T4, so it acts faster and is cleared faster.

Liothyronine (T3) enters cells without needing conversion
It binds nuclear thyroid hormone receptors
Gene transcription changes: metabolism and heart rate rise

Pharmacokinetics

Absorption: Well absorbed by mouth
Distribution: Less bound to plasma proteins than T4
Half-life: Shorter than T4, so its effects fade faster after stopping
Onset and duration: Within a few hours

Indications

Hypothyroidism, as replacement Hypothyroidism →
TSH suppression in well-differentiated thyroid cancer, with surgery and radioactive iodine Thyroid cancer (papillary) →
Thyroid suppression test

Adverse effects

Common

Palpitations and tachycardia
Tremor, nervousness and insomnia
Heat intolerance and sweating
Weight loss

Contraindications

Uncorrected adrenal insufficiency
Treatment of obesity or weight loss

Key interactions

Bile acid sequestrants and ion-exchange resins: Lower absorption; doses are separated
Warfarin: Stronger anticoagulant effect
Insulin and oral antidiabetic drugs: Requirements may rise
Digoxin: Effect may fall

What is monitored

TSH and free T3 or T4, as ordered
Heart rate and symptoms of excess hormone

Administration and safety

Given by mouth; separated from bile acid sequestrants and resins
When switching from levothyroxine, the effects of each overlap for a time

Nursing care

Check heart rate and ask about palpitations or chest pain
Watch older patients and those with heart disease closely after starting
Teach that it is not a weight-loss drug

Pregnancy and breastfeeding

Thyroid replacement continues in pregnancy when needed, with frequent thyroid tests; untreated hypothyroidism harms the pregnancy.

Test yourself

Think of the answer, then tap the question to see it.

Why does liothyronine act faster than levothyroxine?

It is already T3, the active hormone, and binds less to plasma proteins, so it needs no conversion and reaches tissues quickly.

Why is levothyroxine, not liothyronine, the usual long-term replacement?

T4's long half-life gives steady levels with one daily dose; T3 causes peaks and troughs and more cardiac effects.

Which condition must be corrected before starting liothyronine?

Adrenal insufficiency, because thyroid hormone can trigger an adrenal crisis.

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Levothyroxine

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Liothyronine: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2018); DailyMed - LIOTHYRONINE SODIUM tablet (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025).

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Representative model, for educational purposes only.