BLACK BOX WARNING
- thyroid hormones should not be used for weight reduction in euthyroid individuals; larger doses may produce serious or life-threatening adverse events including cardiac toxicity
liothyronine sodium
Brand: Cytomel, Triostat (IV)
⚠ BBW Beers Criteria TDM Required Prototype: levothyroxine
Drug Class: thyroid hormone
Drug Family: thyroid agent
Subclass: synthetic triiodothyronine (T3)
Organ Systems: endocrine
Mechanism of Action
Synthetic T3 (triiodothyronine) that directly activates thyroid hormone receptors without requiring peripheral conversion from T4. Produces more rapid onset of action than levothyroxine; binds nuclear TRs with 3–5 times greater affinity than T4, regulating transcription of metabolic genes.
thyroid hormone receptor (TR-alpha, TR-beta)
Indications
- hypothyroidism (short-term use or in patients who cannot convert T4 to T3)
- myxedema coma (IV)
- thyroid suppression testing
- T3 preparation for radioiodine scanning
Contraindications
- untreated adrenal insufficiency
- thyrotoxicosis
Adverse Effects
Common
- palpitations
- tremor
- insomnia
- heat intolerance
- diaphoresis
Serious
- cardiac arrhythmias
- angina
- myocardial infarction
- thyroid storm (overdose)
Pharmacokinetics (ADME)
| Absorption | oral bioavailability ~95% |
| Distribution | binds plasma proteins (TBG, albumin, transthyretin); highly lipophilic; enters cells |
| Metabolism | deiodination in peripheral tissues |
| Excretion | fecal (conjugated forms) |
| Half-life | 2.5 days (T3; much shorter than T4) |
| Onset | 24–72 hours |
| Peak | days |
| Duration | days |
| Protein Binding | 99% (less than T4) |
| Vd | moderate |
Drug Interactions
| Drug / Agent | Mechanism | Severity |
|---|---|---|
| warfarin | thyroid hormones increase catabolism of vitamin K-dependent clotting factors; dose reduction needed | major |
| sympathomimetics | additive cardiovascular effects (palpitations, hypertension) | moderate |
Nursing Considerations
- For myxedema coma: IV liothyronine given; monitor cardiac rhythm continuously as arrhythmias are common.
- Administer oral liothyronine once or twice daily; shorter half-life than levothyroxine produces more fluctuating T3 levels.
- Monitor for signs of hyperthyroidism: tachycardia, palpitations, excessive weight loss, tremor.
- Do not use for weight loss — no benefit in euthyroid individuals and significant cardiovascular risk.
Clinical Pearls
- Liothyronine's short half-life (2.5 days vs. levothyroxine's 7 days) produces more variable thyroid hormone levels and is generally not recommended for long-term hypothyroidism management except in patients with documented T4-to-T3 conversion impairment.
- Some thyroid cancer protocols use liothyronine to prepare for radioiodine scanning: T3 suppression is faster to achieve and reverse than T4 suppression.
Safety Profile
Pregnancy use-with-caution
Lactation use-with-caution
Renal Adjustment Not required
Hepatic Adjustment Not required
TDM Required
Concordance Terms
Cross-referenced clinical concepts — click any term to see all content where it appears.