Levothyroxine: veterinary dose and monograph

At a glance

Drug class

Veterinary drug

Also known as

Thyro-Tabs, Soloxine, Eltroxin, L-thyroxine

Species with doses

Dogs, Cats, Horses and Reptiles

General dose range

0.02 mg/kg PO q12–24h — check the species tables for the indication

Routes

PO, IV

Last reviewed

1 October 2026

Indications

• Canine hypothyroidism

Levothyroxine dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Hypothyroidism: Papich 5e: 18–26 µg/kg/DAY PO (average start 22 µg/kg once daily ≈ 0.1 mg per 10 lb), once daily or divided q12h; food reduces absorption by up to 50% — keep timing consistent. Plumb’s p.725: 20 µg/kg (0.02 mg/kg) PO twice daily to start, maximum 0.8 mg per dose (Scott-Moncrieff); 10–22 µg/kg q12–24h (Daminet 2010); or 0.5 mg/m² for large breeds. Post-pill T4 4–6 h after dosing at 4–8 weeks, then adjust.
• Myxoedema coma (IV): Papich 5e: 4–5 µg/kg IV once or q12h for acute treatment. Plumb’s p.725: 5 µg/kg IV q12h initially, as oral absorption may be poor.

Cats

• Hypothyroidism (incl. iatrogenic): Papich 5e: 10–20 µg/kg/day PO, adjusted by monitoring. Plumb’s p.725: 0.05–0.1 mg per cat once daily; after thyroidectomy 0.1–0.2 mg per cat daily from 24–48 h post-op for weeks to months (Scherk 2009). Reassess at 4–6 weeks with pre- and 6–8 h post-pill T4.

Horses

• Hypothyroidism: Papich 5e: 10–60 µg/kg q24h or 5–30 µg/kg q12h PO (oral powder: 1 level teaspoon = 12 mg). Plumb’s p.725: 10 mg in corn syrup once daily with T4 monitoring after a week (Chen 1987).
• Equine metabolic syndrome / weight loss (adjunct): Plumb’s p.725 (Frank 2010): about 0.1 mg/kg/day — start 48 mg/day, increase to 72 mg/day after 3 months if still obese, with caloric restriction and no free pasture; taper 24 mg/day for 2 weeks then 12 mg/day for 2 weeks. Papich 5e: 0.1 mg/kg (48 mg per horse) per day.

Reptiles

• Hypothyroidism (tortoises): Plumb’s p.725 (Gauvin 1993): tortoises 0.02 mg/kg PO every other day.

Contraindications

Untreated hypoadrenocorticism (correct cortisol first). Caution in cardiac disease, diabetes mellitus — start low and increase gradually. Acute thyrotoxicosis if over-dosed.

Adverse effects

• Signs of thyrotoxicosis if over-dosed: panting, tachycardia, restlessness, polyphagia, weight loss, PU/PD
• Generally none at correct dose

Drug interactions

• Sucralfate, antacids, calcium / iron, high-soy diets: reduce absorption — separate dosing by several hours
• Phenobarbital / rifampin: increase T4 clearance — may require a higher dose
• Insulin / oral hypoglycaemics: thyroid replacement can raise insulin requirements
• Sympathomimetics / ketamine: enhanced cardiovascular response if over-replaced

Monitoring

• Post-pill total T4 (4–6 h after dosing) 4–6 weeks after starting / dose change — target upper half of reference range
• Clinical response: weight, coat regrowth, activity (lags weeks behind biochemistry)
• Watch for thyrotoxicosis signs in cardiac / diabetic patients

Toxicity and overdose

Over-replacement or accidental tablet ingestion causes thyrotoxicosis — tachycardia, panting, hyperactivity, polyphagia with weight loss, PU/PD. Dogs tolerate acute overdose far better than humans owing to rapid clearance, but cardiac patients are at higher risk.

Management: Acute ingestion in dogs usually needs only monitoring (rapid T4 clearance). For symptomatic thyrotoxicosis: withhold doses, supportive care, and a beta-blocker (e.g. atenolol / propranolol) for tachycardia; resume at the correct dose once stable.

Clinical pearls

• Dose to the post-pill T4 and the dog, not to the pre-pill number alone
• Coat and energy improvements take 4–8 weeks — reassure owners not to expect overnight change
• In a dog with concurrent Addison’s, replace glucocorticoid before thyroxine to avoid precipitating a crisis

Mechanism of action

Synthetic levorotatory thyroxine (T4) identical to the endogenous hormone. Absorbed and partly deiodinated to active T3; restores normal metabolic rate, protein synthesis, thermogenesis and cardiac function in hypothyroid dogs, with negative feedback normalising TSH as euthyroidism returns.

Formulations and products

• Thyro-Tabs 0.1 mg tablet
• Thyro-Tabs 0.3 mg tablet
• Thyro-Tabs 0.5 mg / 0.8 mg tablet
• Thyro-Tabs (Dechra) — 0.1 – 0.8 mg, Tablet
• Leventa (MSD Animal Health) — 1 mg/mL, Oral solution
• Eltroxin — 25 – 100 µg, Tablet (human-label)
• Thyronorm — 12.5 – 100 µg, Tablet (human-label)
• Thyrox — 25 – 100 µg, Tablet (human-label)

Storage

Store at 15–25 °C, protected from light and moisture.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

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Frequently asked questions