Levothyroxine
Levothyroxine supplies T4, which is converted to active T3; bioavailability and requirements vary by patient and formulation.
Group
Thyroid hormone replacement
Mechanism of Action
Restores thyroid-hormone receptor signalling and normal metabolic regulation.
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
- Confirmed hypothyroidism
- Acute myxoedema coma
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.