Levothyroxine

Levothyroxine is synthetic thyroxine, or T4, used to replace deficient thyroid hormone. Its principal veterinary application is hypothyroidism in dogs, with additional use in selected cats and other species. Tissue conversion to triiodothyronine supplies much of its biologic activity. Oral products are used for maintenance, while parenteral treatment has a specialist emergency role.

Group

Thyroid hormone replacement

Mechanism of Action

Levothyroxine replenishes the circulating T4 pool and is converted by tissue deiodinases to active T3. T3 binds nuclear thyroid-hormone receptors and alters gene transcription controlling energy metabolism, thermogenesis, growth and organ function. Restoring adequate hormone availability reverses consequences of thyroid deficiency and re-establishes feedback signaling within the hypothalamic–pituitary–thyroid axis.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Hypothyroidism	18–26 mcg/kg	PO	Every 24 h	Average starting amount 22 mcg/kg/day; adjust using thyroid testing.
Hypothyroidism	9–13 mcg/kg	PO	Every 12 h	Exact half of the stated total daily 18-26 mcg/kg dose, divided into two treatments.
Myxoedema coma	4–5 mcg/kg	IV	Every 12 h	Acute treatment, repeated only according to response.
Hypothyroidism	20 mcg/kg	PO	Every 12 h	Maximum 0.8 mg/dose; Distinct initial twice-daily recommendation; maximum 0.8 mg per dose. Reassess clinical response and post-pill T4 after 4-8 weeks.
Hypothyroidism	10–22 mcg/kg	PO	Every 12 h	Maximum 0.8 mg/dose; Formulation-dependent starting alternative; titrate to response.
Hypothyroidism	10–22 mcg/kg	PO	Every 24 h	Maximum 0.8 mg/dose; Formulation-dependent starting alternative; titrate to response.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Hypothyroidism	10–20 mcg/kg	PO	Every 24 h
Hypothyroidism	0.05–0.1 mg/animal	PO	Every 24 h	Reassess after 4-6 weeks; measure trough and post-dose T4.
Post-thyroidectomy replacement	0.1–0.2 mg/animal	PO	Every 24 h	Begin 24-48 hours after surgery; continue only while thyroid testing supports replacement.

Horses

Use / stage	Dose	Route	Schedule	Instructions
Hypothyroidism	10–60 mcg/kg	PO	Every 24 h
Hypothyroidism	5–30 mcg/kg	PO	Every 12 h	Divided-dose alternative.
Historical PPID-associated use	Reference only	PO		0.1 mg/kg/day is listed for horses with pituitary pars intermedia dysfunction, with a 48 mg/horse example. Thyroid replacement is not a complete treatment for PPID and the example is patient-dependent.
Equine metabolic syndrome adjunct	Reference only	PO		Approximate 0.1 mg/kg/day with calorie restriction and exercise. A fixed example starts 48 mg/day, increasing to 72 mg/day after 3 months if still obese, then after a typical 6-month course tapers to 24 mg/day for 2 weeks and 12 mg/day for 2 weeks. Patient size and response determine dosing; unrestricted pasture undermines weight loss.

Reptiles

Use / stage	Dose	Route	Schedule	Instructions
Tortoise hypothyroidism	0.02 mg/kg	PO	Every 48 h	Tortoises only.

Birds

Use / stage	Dose	Route	Schedule	Instructions
Historical thyroid supplementation in water	Reference only	PO		One 0.1 mg tablet in 30-120 mL water, stirred and offered for 15 minutes daily; intake and species determine exposure, so no fixed individual dose is derived.

Indications

Hormone replacement for confirmed hypothyroidism in dogs.
Treatment of selected congenital or acquired hypothyroidism in cats and other veterinary species.
Emergency thyroid-hormone replacement in hypothyroid patients with myxedema coma, within a complete critical-care plan.

Contraindications

Use cautiously when rapid metabolic stimulation or arrhythmias would be hazardous.
Untreated adrenal insufficiency or thyrotoxicosis; cardiac disease, diabetes and older age require a conservative starting amount.

Interactions

Food may reduce absorption markedly.
Acid suppressants, calcium, iron, aluminium and sucralfate may reduce absorption.
Phenobarbital, sulfonamides, NSAIDs and corticosteroids may alter thyroid measurements or metabolism.

Adverse effects

Excessive supplementation causes thyrotoxicosis.
IV treatment can cause arrhythmias.

Toxicity

Reduce or stop excess supplementation and assess cardiovascular signs and thyroid concentrations.

Preparations

Preparation	Form	Strength	Route
Levothyroxine 25 mcg tablet	Tablet	25 mcg / tablet	PO
Levothyroxine 50 mcg tablet	Tablet	50 mcg / tablet	PO
Levothyroxine 75 mcg tablet	Tablet	75 mcg / tablet	PO
Levothyroxine 88 mcg tablet	Tablet	88 mcg / tablet	PO
Levothyroxine 112 mcg tablet	Tablet	112 mcg / tablet	PO
Levothyroxine 125 mcg tablet	Tablet	125 mcg / tablet	PO
Levothyroxine 137 mcg tablet	Tablet	137 mcg / tablet	PO
Levothyroxine 150 mcg tablet	Tablet	150 mcg / tablet	PO
Levothyroxine 175 mcg tablet	Tablet	175 mcg / tablet	PO
Levothyroxine 200 mcg tablet	Tablet	200 mcg / tablet	PO
Levothyroxine 300 mcg tablet	Tablet	300 mcg / tablet	PO
Equine levothyroxine powder 1 g/453.6 g Specified product: 1 level teaspoon contains 12 mg T4 and 1 tablespoon 36 mg; these measures apply only to this exact powder.	Powder	1 g / 453.6 g	PO

Monitoring and Administration

Give consistently in relation to meals; do not assume brands or human and veterinary products are bioequivalent.

Recheck T4 after dose or product changes. Recommended post-tablet sampling is 4-6 hours, oral liquid 2-2.5 hours and equine post-dose sampling 1-2 hours.

Protect from heat, moisture and light. Compounded formulations can lose potency; published stability is vehicle-specific.

Body-surface-area alternative 0.5 mg/m² is described for large dogs, but BSA dosing is not supported here. Do not mix the injectable product with other drugs or fluids; reconstitute immediately before use and discard unused solution. Long-term equine 12-24 mg/day “maintenance” for weight control lacks supporting evidence.