Liothyronine Sodium

Thyroid hormone Synthetic thyroid hormone (T3)

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO Initial: 4-6 mcg/kg q8h; Maintenance: 4-6 mcg/kg q8-12h q8-12h Duration: Lifelong for hypothyroidism; adjust based on clinical response and serum T3 levels
Notes: Start at lower end of dose range and titrate upward based on clinical response and serum T3 concentrations. Monitor for signs of hyperthyroidism.
Cat PO Initial: 2-4 mcg/kg q8h; Maintenance: 2-4 mcg/kg q8-12h q8-12h Duration: Lifelong for hypothyroidism; adjust based on clinical response
Notes: Use cautiously in cats with cardiac disease. Monitor serum T3 levels.
Horse PO 0.5-1 mcg/kg q12h q12h Duration: Variable; adjust based on response
Notes: Limited data; use with caution. Monitor thyroid hormone levels.
Cattle PO Not established; use with caution Not established Duration: Not established
Notes: Not commonly used; consult a veterinary endocrinologist.
Small Ruminants PO Not established; use with caution Not established Duration: Not established
Notes: Not commonly used; consult a veterinary endocrinologist.
Rabbit PO Not established; use with caution Not established Duration: Not established
Notes: Not commonly used; consult a veterinary endocrinologist.
Bird/Poultry PO Not established; use with caution Not established Duration: Not established
Notes: Not commonly used; consult a veterinary endocrinologist.
Exotic/Other PO Not established; use with caution Not established Duration: Not established
Notes: Not commonly used; consult a veterinary endocrinologist.

Clinical Indications & Species Uses

General Indications
  • Thyroid hormone replacement therapy for hypothyroidism
  • Myxedema coma (adjunctive therapy)
Dog (Canine)
  • Hypothyroidism (as initial therapy or when rapid correction is needed)
  • Myxedema coma (adjunctive therapy)
  • Thyroid hormone replacement in cases of levothyroxine intolerance
Cat (Feline)
  • Hypothyroidism (rare, usually iatrogenic after treatment for hyperthyroidism)
  • Myxedema coma (adjunctive therapy)
Horse (Equine)
  • Hypothyroidism (rare)
  • Equine metabolic syndrome (investigational)
Cattle (Bovine)
  • Hypothyroidism (rare)
Small Ruminants (Sheep / Goat)
  • Hypothyroidism (rare)
Rabbit & Small Mammals
  • Hypothyroidism (rare)
Avian & Poultry
  • Hypothyroidism (rare)
Exotic & Other Species
  • Hypothyroidism (rare)

Pharmacology & Mechanism of Action

Drug Class: Thyroid hormone | Pharmacological Group: Synthetic thyroid hormone (T3)

Mechanism of Action: Liothyronine is a synthetic form of the naturally occurring thyroid hormone triiodothyronine (T3). It binds to thyroid hormone receptors in the nucleus of cells, modulating gene transcription and leading to increased metabolic rate, protein synthesis, and oxygen consumption. It also enhances the effects of catecholamines and increases cardiac output and heart rate.

Pharmacodynamics: Liothyronine has a rapid onset of action (within hours) and a shorter duration of effect compared to levothyroxine (T4). It increases basal metabolic rate, stimulates thermogenesis, and affects carbohydrate, lipid, and protein metabolism. It also influences growth and development, particularly in the nervous and skeletal systems.

⚡ Pharmacokinetics Summary

Absorption: Rapidly and completely absorbed from the gastrointestinal tract after oral administration. Peak serum concentrations occur within 1-3 hours in most species.
Distribution: Widely distributed throughout the body. It crosses the placenta and is distributed into milk. It is highly protein-bound (approximately 99%) in plasma, mainly to thyroxine-binding globulin, transthyretin, and albumin.
Metabolism: Metabolized primarily in the liver via deiodination and conjugation (glucuronidation and sulfation). It undergoes enterohepatic recirculation.
Excretion: Excreted mainly in the urine as metabolites, with some fecal elimination. The half-life is short, approximately 0.75-1 day in dogs and cats.
Half-Life: Dogs: 0.5-1 day; Cats: 0.5-1 day; Horses: 0.5-1 day; Cattle: 0.5-1 day
Bioavailability: Oral bioavailability is high, approximately 90-100% in most species.
Protein Binding: Approximately 99% bound to plasma proteins (thyroxine-binding globulin, transthyretin, albumin).

Available Formulations & Strengths

Oral Tablet 5 mcg, 25 mcg, 50 mcg (PO)
Injectable Solution 10 mcg/mL (for IV use) (IV)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to liothyronine or any component of the formulation
  • Untreated adrenal insufficiency (Addison's disease)
  • Untreated thyrotoxicosis (hyperthyroidism)
  • Acute myocardial infarction or unstable cardiac disease (unless under close supervision)
Warnings & Clinical Precautions:
  • Use with extreme caution in animals with cardiovascular disease (e.g., heart failure, arrhythmias, hypertension).
  • Use with caution in animals with diabetes mellitus; thyroid hormones may increase blood glucose levels.
  • Use with caution in animals with adrenal insufficiency; may precipitate an adrenal crisis.
  • Monitor serum thyroid hormone levels and clinical signs regularly.
  • Do not use to treat obesity or for weight loss in euthyroid animals.
  • In food animals, observe withdrawal times; extra-label use is prohibited in some countries.

Adverse Effects & Reactions

Common:

  • Increased appetite
  • Weight loss
  • Nervousness
  • Tremors
  • Increased heart rate
  • Panting
  • Diarrhea

Serious / Severe:

  • Cardiac arrhythmias
  • Hypertension
  • Tachycardia
  • Thyrotoxic crisis (overdose)
  • Seizures (rare)

Rare:

  • Allergic reactions (urticaria, angioedema)
  • Hepatotoxicity (rare)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Ketamine May increase risk of hypertension and tachycardia. Moderate
Tricyclic antidepressants (e.g., amitriptyline) May potentiate effects of both drugs, leading to cardiac arrhythmias. High
Digoxin May reduce serum digoxin levels; monitor digoxin concentrations. Moderate
Warfarin May increase anticoagulant effect; monitor coagulation parameters. High
Corticosteroids May reduce thyroid hormone levels; monitor thyroid status. Mild
Phenobarbital May increase metabolism of thyroid hormones, reducing efficacy. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Vomiting
  • Diarrhea
  • Hyperactivity
  • Tremors
  • Tachycardia
  • Arrhythmias
  • Hyperthermia
  • Seizures
  • Cardiac arrest

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce vomiting if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide supportive care including IV fluids, cooling measures for hyperthermia, and antiarrhythmic drugs (e.g., beta-blockers) as needed. Monitor cardiac function and electrolyte balance. In severe cases, consider administration of cholestyramine to enhance elimination.

Food Animal Withdrawal Times

Liothyronine is not approved for use in food animals in many countries. If used in food animals, withdrawal times must be established based on regulatory guidelines; consult a veterinarian. In the US, extra-label use in food animals is prohibited if an approved drug is available.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C or 68-77°F). Protect from light and moisture.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Keep in a tight, light-resistant container. Do not freeze.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use.

Extra-Label (Off-Label) Use: In the US, liothyronine is not FDA-approved for veterinary use; however, it may be used extra-label under the Animal Medicinal Drug Use Clarification Act (AMDUCA) when a valid veterinarian-client-patient relationship exists. Extra-label use in food animals is prohibited if an approved drug is available.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Liothyronine (T3) is a potent thyroid hormone with a rapid onset and short duration of action. It is primarily used in veterinary medicine for the treatment of hypothyroidism when a rapid response is needed (e.g., myxedema coma) or when animals are intolerant to levothyroxine. Because of its short half-life, it requires more frequent dosing (q8-12h) compared to levothyroxine (q12-24h). Monitoring of serum T3 levels is essential to avoid iatrogenic hyperthyroidism. In dogs, it is not the first-line treatment for hypothyroidism; levothyroxine is preferred. Use with caution in animals with cardiac disease, diabetes mellitus, or adrenal insufficiency. Always consider the potential for drug interactions and adjust dosages accordingly. For food animals, extra-label use is highly restricted; consult regulatory authorities before use.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)