Levothyroxine
L-3,5,3',5'-tetraiodothyronine
Also known as: Leventa · Soloxine · Thyforon · l-Thyroxine · T4 · Levothyroxine sodium
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
This dose is based on body surface area (mg/m²). Convert body weight to body surface area before dosing.
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 | 0.02-0.04 mg/kg/day OR 0.5 mg/m2 body surface area daily | PO | sid or divided bid | Lifelong | 🌍 EU |
- Hypothyroidism: Dose given with food. Monitor serum T4 levels pre-dosing and 4-8 hours after dosing.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hypothyroidism | 0.02-0.04 mg/kg/day OR 0.5 mg/m2 body surface area daily | PO | sid or divided bid | Lifelong | 🌍 EU |
- Hypothyroidism: Dose given with food. Monitor serum T4 levels pre-dosing and 4-8 hours after dosing.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Levothyroxine (l-Thyroxine or T4) is a synthetic form of the naturally occurring thyroid hormone thyroxine. It is the drug of choice for the treatment of hypothyroidism in veterinary medicine.
Clinical Warning: Cases with pre-existing cardiac disorders require lower doses initially to prevent excessive cardiac stimulation. It can also unmask Addison's disease in patients with autoimmune polyglandular syndrome.
Mechanism of action
Levothyroxine acts as a replacement for endogenous thyroxine. It enters cells and is converted to the more active T3 (triiodothyronine). T3 then binds to specific intracellular thyroid hormone receptors in the nucleus → alters gene expression → regulates metabolism, growth, and development.
Safety & warnings
Contraindications
- Uncorrected adrenal insufficiency (Addison's disease)
- Acute myocardial infarction (general precaution)
Adverse effects
- Tachycardia
- Excitability
- Nervousness
- Excessive panting
- Unmasking of Addison's disease
Precautions
Use with caution in patients with pre-existing cardiac disorders; lower initial doses are required. Monitor diabetic patients carefully during the initiation of therapy as insulin requirements may change. Many drugs may affect thyroid function tests, which can complicate therapeutic monitoring.
Drug interactions
Actions of catecholamines are enhanced by thyroxine
Actions of sympathomimetics are enhanced by thyroxine
Altered insulin requirements; monitor carefully during initiation of therapy
May increase thyroid requirements by increasing thyroxine-binding globulin
Therapeutic effect may be reduced by thyroid hormones
Therapeutic effect may be reduced by thyroid hormones
Therapeutic effect may be reduced by thyroid hormones
Tachycardia and hypertension may develop when given to patients receiving thyroid hormones
Monitoring
- Serum total T4 levels (pre-pill trough and 4-8 hours post-pill peak)
- Clinical response (resolution of lethargy, weight gain, dermatological signs)
- Signs of thyrotoxicosis (panting, tachycardia, hyperactivity)
- Blood glucose in diabetic patients
Pharmacokinetics
Half-life
Absorption
Variable oral absorption in dogs (typically 10-50%). Bioavailability is significantly influenced by food; therefore, consistent administration with or without food is recommended.
Distribution
Highly protein bound (>99%) to thyroxine-binding globulin, transthyretin, and albumin.
Metabolism
Deiodinated in the liver and peripheral tissues to the active form T3 and inactive reverse T3. Also undergoes hepatic conjugation.
Elimination
Excreted primarily in feces via biliary excretion, with a smaller amount excreted in urine.
Overdose
Clinical signs of overdosage (thyrotoxicosis) include tachycardia, excitability, nervousness, and excessive panting. Treatment involves withholding the medication until signs resolve, then restarting at a lower dose. Acute massive overdose may require gastrointestinal decontamination and supportive care (e.g., beta-blockers for severe tachycardia).
Available products
Formulations
- 0.1 mg tablet
- 0.2 mg tablet
- 0.3 mg tablet
- 0.5 mg tablet
- 0.8 mg tablet
- 1 mg/ml oral solution
Veterinary
- Leventa 1 mg/ml oral solution
- Soloxine tablets (various strengths)
- Thyforon tablets (various strengths)
Human-labeled
- Levothyroxine sodium tablets (various strengths, e.g., 25 mcg, 50 mcg, 100 mcg)
Regulatory status
Approved as Thyro-Tabs Canine and others.
POM-V classification.
POM-V classification.
No regulatory data for: 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store at room temperature in tightly closed containers. Protect from light and moisture.
Client information
Give this medication consistently, ideally at the same time each day and consistently with or without food. Do not change brands without consulting your veterinarian, as different brands may be absorbed differently. Watch for signs of receiving too much medication, such as excessive panting, restlessness, increased thirst, or increased urination. This medication is usually required for the rest of your pet's life, and regular blood tests are necessary to ensure the dose is correct.
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.