Liothyronine Sodium
Liothyronine sodium is a synthetic form of **T3 (triiodothyronine)**, the active cellular form of thyroid hormone. While levothyroxine (T4) is the standard first-line treatment for hypothyroidism, liothyronine is reserved for specific cases where animals fail to respond to T4 supplementation. This lack of response may be due to impaired gastrointestinal absorption of T4 or, theoretically, a defect in the peripheral conversion of T4 to T3 via deiodinase enzymes. **Clinical Pearls:** * Liothyronine has a much **shorter duration of effect** and half-life compared to levothyroxine, necessitating more frequent dosing (often TID). * It is **not recommended for initial therapy** because it only normalizes serum T3 concentrations while leaving T4 levels low, which disrupts the normal hypothalamic-pituitary-thyroid feedback axis. * Because it bypasses the body's natural regulatory conversion step (T4 → T3), patients are at a higher risk of iatrogenic thyrotoxicosis.
Mechanism: Thyroid hormones act primarily at the cellular level to regulate metabolism. * **Mechanism:** T3 enters the cell and binds to **nuclear thyroid hormone receptors (TRs)** → the receptor-hormone complex binds to DNA → stimulates or inhibits the transcription of specific mRNA → alters protein synthesis. * **Physiologic Effects:** Increases the metabolic rate of tissues, enhances oxygen consumption, increases body temperature, stimulates gluconeogenesis, and promotes the mobilization and utilization of glycogen stores. * **Cardiovascular Effects:** Increases heart rate, cardiac output, and blood volume by upregulating beta-adrenergic receptors. * **Development:** Crucial for the adequate development of the central nervous system and skeletal growth.
Dosing by species
- Hypothyroidism · 4.4 micrograms/kg PO 2-3 times a day · PO · q8h-q12h
- T3 suppression test (diagnostic for hyperthyroidism) · 25 μg · PO · q8h · 7 doses total · Administer 7 doses, then measure T4 and T3 on the morning of the 3rd day, 2-4 hours after the 7th dose.
- Hypothyroidism (unresponsive to T4) · 4-6 micrograms/kg PO q8h · PO · q8h · Some dogs may require less frequent dosing. Only indicated when T4 supplementation has failed.
- Hypothyroidism (refractory to T4) · 4-6 μg/kg · PO · q8h · Not recommended as first-line therapy due to short half-life.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Acute myocardial infarction
- Thyrotoxicosis
- Untreated adrenal insufficiency
- Untreated adrenal insufficiency (Addison's disease)
Adverse effects
- Tachycardia
- Polyphagia
- Polyuria/Polydipsia (PU/PD)
- Excitability
- Nervousness
- Excessive panting
- Apathy or listlessness (seen in about 10% of cats)
- PU/PD (Polyuria/Polydipsia)
- Nervousness/Anxiety
- Panting
- Weight loss
Drug interactions
- Tricyclic/Tetracyclic Antidepressants · Increased risk for CNS stimulation and cardiac arrhythmias.
- Antidiabetic Agents (insulin, oral agents) · Thyroid hormones may increase the requirements for insulin or oral antidiabetic agents.
- Cholestyramine · May reduce liothyronine absorption; separate doses by at least 4 hours.
- Sympathomimetic Agents (epinephrine, norepinephrine) · Thyroid hormones can potentiate the cardiovascular effects of sympathomimetics.
- Warfarin · Thyroid hormones increase the catabolism of vitamin K-dependent clotting factors, potentially increasing anticoagulation effects.
- Digoxin · Potential for reduced digoxin levels and decreased therapeutic effect.
- Ketamine · Concurrent use may cause tachycardia and hypertension. · moderate
- Sympathomimetics · Increased risk of cardiac arrhythmias and coronary insufficiency · moderate
- Insulin · May alter glucose control, requiring dosage adjustments · minor
Monitoring
- Serum T3 levels (draw just prior to dosing and 2-4 hours post-pill)
- Clinical signs of hyperthyroidism/thyrotoxicosis
- Note: Serum T4 levels will remain low during therapy
- Serum T3 levels
- Clinical signs of hyperthyroidism
- Heart rate and rhythm
Overdose
**Chronic Overdosage:** Produces signs of iatrogenic hyperthyroidism, including tachycardia, polyphagia, PU/PD, excitability, nervousness, and excessive panting. Some cats (approx. 10%) may exhibit 'apathetic' hyperthyroidism (listlessness, anorexia). **Treatment:** Reduce dosage or temporarily withhold until signs subside. **Acute Massive Overdosage:** Can produce signs resembling a **thyroid storm**. * **Treatment:** If oral ingestion is recent, reduce absorption using standard protocols (emetics, gastric lavage, activated charcoal, cathartics) unless contraindicated by the patient's clinical status. * **Supportive Care:** Oxygen, artificial ventilation, beta-blockers (e.g., propranolol) for tachycardia/arrhythmias, IV fluids, dextrose, and antipyretic agents as needed.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.