Liothyronine Sodium
3,5,3'-Tri-iodo-L-thyronine sodium
Also known as: Cytomel · Triostat · Tertroxin · T3 thyronine sodium · sodium L-triiodothyronine · liothyroninum natricum · sodium liothyronine · l-tri-iodothyronine sodium
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hypothyroidism (unresponsive to T4) | 4-6 micrograms/kg PO q8h | PO | q8h | — | 🌎 NA |
| Hypothyroidism (refractory to T4) | 4-6 μg/kg | PO | q8h | — | 🌍 EU |
- Hypothyroidism (unresponsive to T4): Some dogs may require less frequent dosing. Only indicated when T4 supplementation has failed.
- Hypothyroidism (refractory to T4): Not recommended as first-line therapy due to short half-life.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hypothyroidism | 4.4 micrograms/kg PO 2-3 times a day | PO | q8h-q12h | — | 🌎 NA |
| T3 suppression test (diagnostic for hyperthyroidism) | 25 μg | PO | q8h | 7 doses total | 🌍 EU |
- T3 suppression test (diagnostic for hyperthyroidism): Administer 7 doses, then measure T4 and T3 on the morning of the 3rd day, 2-4 hours after the 7th dose.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
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 of action
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.
Safety & warnings
Contraindications
- Acute myocardial infarction
- Thyrotoxicosis
- Untreated adrenal insufficiency (Addison's disease)
Adverse effects
- Tachycardia
- Polyphagia
- Polyuria/Polydipsia (PU/PD)
- Excitability
- Excessive panting
- Apathy or listlessness (seen in about 10% of cats)
- Nervousness/Anxiety
- Weight loss
Precautions
Use with extreme caution and at a lower initial dosage in patients with concurrent hypoadrenocorticism (must be treated first), cardiac disease, diabetes mellitus, or in elderly patients. Combination products containing both T3 and T4 should generally be avoided due to the risk of iatrogenic thyrotoxicosis.
Drug interactions
Increased risk for CNS stimulation and cardiac arrhythmias.
Thyroid hormones may increase the requirements for insulin or oral antidiabetic agents.
May reduce liothyronine absorption; separate doses by at least 4 hours.
Thyroid hormones can potentiate the cardiovascular effects of sympathomimetics.
Thyroid hormones increase the catabolism of vitamin K-dependent clotting factors, potentially increasing anticoagulation effects.
Potential for reduced digoxin levels and decreased therapeutic effect.
Concurrent use may cause tachycardia and hypertension.
Increased risk of cardiac arrhythmias and coronary insufficiency
May alter glucose control, requiring dosage adjustments
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
- Heart rate and rhythm
Pharmacokinetics
Half-life
Absorption
Nearly completely absorbed in dogs after oral administration. Unlike levothyroxine, absorption is not significantly affected by stomach contents or intestinal flora changes.
Distribution
Widely distributed to peripheral tissues. Highly protein-bound in plasma.
Metabolism
Metabolized primarily in the liver and peripheral tissues via deiodination.
Elimination
Excreted primarily via feces and urine.
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.
Available products
Formulations
- Injection
- 20 μg tablets
Human-labeled
- Liothyronine Sodium Tablets: 5 mcg, 25 mcg & 50 mcg (Cytomel, generic)
- Liothyronine Sodium Injection Solution: 10 micrograms/mL in 1 mL vials (Triostat, generic)
- Tertroxin 20 μg tablets
- Cytomel tablets
Regulatory status
POM (Prescription Only Medicine)
POM (Prescription Only Medicine)
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets should be stored at room temperature (15-30°C) in tight containers. The injection should be stored refrigerated (2-8°C).
Client information
Important: Give this medication exactly as prescribed by your veterinarian. Do not change the dose or stop the medication without consulting them.
- What it is: A synthetic thyroid hormone used to treat underactive thyroid glands (hypothyroidism) when standard treatments don't work.
- Dosing Schedule: Because it doesn't last long in the body, it usually needs to be given 2 to 3 times a day. Consistency is key.
- What to watch for: Contact your veterinarian if you notice signs that the dose might be too high. These include excessive panting, pacing, nervousness, increased thirst and urination, a ravenous appetite, or a racing heart rate. In cats, watch for unusual lethargy or loss of appetite.
- Follow-up: Your pet will need periodic blood tests to ensure the dose is correct. The timing of the blood draw relative to when you give the pill is very important, so follow your clinic's instructions carefully.
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.