Introduction
Liothyronine is a synthetic form of the thyroid hormone triiodothyronine (T3) used primarily for thyroid hormone replacement therapy. As the more biologically active thyroid hormone, liothyronine provides rapid onset of action compared to levothyroxine (T4). It is available in oral tablet form and is typically reserved for specific clinical situations due to its shorter half-life and more potent effects.
Mechanism of Action
Liothyronine exerts its effects by binding to thyroid hormone receptors in various tissues, primarily in the nucleus. This binding activates transcription of specific genes, leading to increased metabolic activity. The drug influences carbohydrate, protein, and fat metabolism, stimulates vitamin metabolism, and increases basal metabolic rate. Unlike levothyroxine (T4), which requires peripheral conversion to T3, liothyronine provides direct T3 activity, bypassing the deiodinase enzyme conversion process.
Indications
- Treatment of hypothyroidism
- Myxedema coma (as part of combination therapy)
- Thyroid suppression therapy for thyroid cancer (as adjunctive treatment)
- Diagnostic agent in thyroid suppression tests
- Adjunct to antithyroid drugs in thyrotoxicosis (in specific cases)
Dosage and Administration
Adults:- Initial dose: 25 mcg daily
- May increase by 12.5-25 mcg every 1-2 weeks
- Maintenance dose: 25-75 mcg daily in divided doses
- Initial dose: 5 mcg daily
- Increase gradually by 5 mcg increments at 2-week intervals
- IV administration: 10-25 mcg every 8-12 hours
- Renal impairment: No dosage adjustment required
- Hepatic impairment: Use with caution; monitor closely
Pharmacokinetics
Absorption: Well absorbed from GI tract (approximately 95%) Distribution: Highly protein-bound (>99%) to thyroid-binding globulin, prealbumin, and albumin Metabolism: Primarily metabolized in liver via deiodination and conjugation Elimination: Half-life approximately 2.5 days; excreted primarily in urine and feces Onset of action: 2-3 hours; peak effect within 2-3 daysContraindications
- Untreated thyrotoxicosis
- Acute myocardial infarction
- Uncorrected adrenal insufficiency
- Hypersensitivity to any component of the formulation
- Un-treated subclinical or overt hyperthyroidism
Warnings and Precautions
- Cardiac effects: May precipitate angina pectoris, arrhythmias, or cardiac decompensation in patients with cardiovascular disease
- Elderly patients: Increased risk of atrial fibrillation and other cardiac complications
- Adrenal insufficiency: May precipitate adrenal crisis in patients with uncontrolled adrenal insufficiency
- Osteoporosis: Long-term overtreatment may contribute to bone loss
- Diabetes mellitus: May worsen glycemic control
- Pregnancy: Category A - requires careful monitoring and dose adjustments
Drug Interactions
- Anticoagulants: Enhanced anticoagulant effect of warfarin
- Antidiabetic agents: May decrease effectiveness of insulin and oral hypoglycemics
- Cholestyramine/colestipol: Decreased absorption of liothyronine
- Estrogens: May increase thyroid-binding globulin levels
- Ketamine: May precipitate hypertension and tachycardia
- TCAs: Increased therapeutic and toxic effects of both drugs
- Catecholamines: Enhanced sympathetic effects
Adverse Effects
Common:- Palpitations
- Tachycardia
- Increased appetite
- Nervousness
- Insomnia
- Tremors
- Headache
- Heat intolerance
- Weight loss
- Cardiac arrhythmias
- Angina pectoris
- Myocardial infarction
- Heart failure
- Thyroid storm
- Adrenal crisis
- Seizures
Monitoring Parameters
- Thyroid function tests (TSH, free T4, free T3) every 6-8 weeks until stable, then every 6-12 months
- Vital signs (especially heart rate and blood pressure)
- ECG in patients with cardiac history
- Bone mineral density with long-term therapy
- Weight changes
- Clinical signs of hyperthyroidism
- Blood glucose in diabetic patients
- INR in patients on anticoagulants
Patient Education
- Take medication at the same time each day, preferably before breakfast
- Do not switch between thyroid products without medical supervision
- Report symptoms of hyperthyroidism (palpitations, nervousness, heat intolerance)
- Regular follow-up with healthcare provider is essential
- Inform all healthcare providers about thyroid medication use
- Do not stop medication abruptly
- Be aware of potential drug interactions
- Pregnancy and breastfeeding require special monitoring
References
1. American Thyroid Association Guidelines for Hypothyroidism in Adults. Thyroid. 2014 2. FDA Prescribing Information: Liothyronine Sodium Tablets 3. Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism. Thyroid. 2014 4. Wiersinga WM, et al. 2012 ETA Guidelines: The Use of L-T4 + L-T3 in the Treatment of Hypothyroidism. Eur Thyroid J. 2012 5. Drugs.com: Liothyronine Professional Monograph 6. UpToDate: Liothyronine drug information 7. Baxter JD, et al. Thyroid hormone receptors and responses. Endocr Rev. 2004 8. Surks MI, et al. Subclinical thyroid disease: scientific review and guidelines for diagnosis and management. JAMA. 2004
This monograph is for educational purposes only and should not replace professional medical advice. Always consult with a qualified healthcare provider for medical guidance.