Levothyroxine sodium is a synthetic form of the thyroid hormone thyroxine (T4) and is the standard treatment for hypothyroidism in dogs. It is available under veterinary brand names including Soloxine (Virbac) and ThyroTabs (Lloyd Laboratories). Hypothyroidism is one of the most common endocrine disorders in dogs, typically caused by immune-mediated thyroiditis or idiopathic thyroid gland atrophy, resulting in insufficient production of thyroid hormones. Levothyroxine supplementation restores normal circulating T4 levels, reversing the metabolic slowdown that characterizes hypothyroidism. Most dogs require lifelong therapy with periodic blood level monitoring.
What Is Levothyroxine Used For?
Levothyroxine is prescribed for the treatment of:
- Hypothyroidism in dogs, which may present with:
- Weight gain despite normal or reduced appetite
- Lethargy and exercise intolerance
- Hair loss (bilaterally symmetric alopecia), dry or dull coat
- Skin thickening, hyperpigmentation, recurrent skin infections
- Cold intolerance
- Bradycardia (slow heart rate)
- “Tragic facial expression” (myxedema) in severe cases
- Neurological signs (rare)
Diagnosis of hypothyroidism requires blood testing (total T4, free T4, TSH) performed by your veterinarian. Do not start levothyroxine without a confirmed diagnosis, as supplementing a euthyroid (normal thyroid) dog can suppress natural thyroid function.
Dosage
The typical starting dose of levothyroxine in dogs is 0.02 mg/kg (0.01 mg/lb) orally twice daily, though some dogs are managed with once-daily dosing after stabilization. Your veterinarian will individualize the dose based on blood level monitoring.
| Dog’s Weight | Starting Dose (0.02 mg/kg BID) | Common Tablet Sizes |
|---|---|---|
| 10 lbs (4.5 kg) | 0.1 mg twice daily | 0.1 mg |
| 25 lbs (11 kg) | 0.2 mg twice daily | 0.2 mg |
| 50 lbs (23 kg) | 0.4-0.5 mg twice daily | 0.5 mg |
| 75 lbs (34 kg) | 0.6-0.7 mg twice daily | 0.7 mg or combination |
| 100 lbs (45 kg) | 0.8-0.9 mg twice daily | 0.8 mg |
Levothyroxine tablets are available in many strengths (0.1 mg through 0.8 mg). Large-breed dogs may need disproportionately lower per-kilogram doses. Ideally, administer on an empty stomach or at a consistent time relative to meals, as food can affect absorption. Your veterinarian will check a post-pill T4 level (typically 4-6 hours after dosing) at 4-8 weeks after starting therapy and adjust accordingly.
Important: Do not switch between brands without veterinary approval, as bioavailability may differ between products.
Side Effects
At the correct dose, levothyroxine side effects are rare because it simply replaces a deficient natural hormone. Side effects typically indicate overdose (iatrogenic hyperthyroidism):
| Signs of Overdose (Hyperthyroidism) | Other Concerns |
|---|---|
| Increased thirst and urination | Worsening of pre-existing cardiac disease |
| Restlessness, hyperactivity | Excessive panting |
| Weight loss | Rapid heart rate (tachycardia) |
| Diarrhea | Behavior changes (anxiety) |
| Increased appetite |
If you notice signs of overdose, contact your veterinarian. Dose reduction typically resolves these signs. At appropriate therapeutic doses, most dogs tolerate levothyroxine very well.
Drug Interactions
- Calcium, iron, and aluminum-containing antacids: Can reduce levothyroxine absorption. Separate administration by at least 4 hours.
- Corticosteroids: May alter thyroid hormone metabolism and test results.
- Phenobarbital and other enzyme inducers: May increase levothyroxine metabolism, requiring dose adjustment.
- Insulin: Thyroid hormone replacement may alter insulin requirements in diabetic dogs; monitor closely.
- Warfarin and anticoagulants: Levothyroxine may increase anticoagulant effects.
- Ketamine: Concurrent use may cause tachycardia and hypertension.
Always inform your veterinarian of all medications and supplements your dog is receiving.
Contraindications
Levothyroxine should not be used in:
- Dogs with uncorrected adrenal insufficiency (hypoadrenocorticism/Addison’s disease): Thyroid supplementation before cortisol replacement can precipitate an adrenal crisis. Treat Addison’s disease first.
- Dogs with thyrotoxicosis (excess thyroid hormone)
- Dogs with known hypersensitivity to levothyroxine or product components
Use with caution in dogs with heart disease, diabetes, or adrenal disorders. Your veterinarian will evaluate your dog’s overall health before starting therapy.
Storage
- Store at controlled room temperature, 59-86 degrees F (15-30 degrees C).
- Protect from light and moisture.
- Keep in original packaging.
- Keep out of reach of children and pets.
Frequently Asked Questions
How long does levothyroxine take to work? Clinical improvement (increased energy, reduced hair loss, weight loss) typically begins within 4 to 6 weeks of starting therapy. Complete coat regrowth may take 3 to 6 months. Your veterinarian will recheck thyroid levels at 4-8 weeks to ensure the dose is appropriate.
Is levothyroxine a lifelong medication? In most cases, yes. Hypothyroidism in dogs is a permanent condition requiring lifelong supplementation. Stopping the medication will cause signs to return. Consult your veterinarian before making any changes.
Can I use human levothyroxine for my dog? Veterinary-specific formulations (Soloxine, ThyroTabs) are preferred because bioavailability is better characterized for dogs. Some veterinarians may prescribe human-label levothyroxine (Synthroid) but dosing and monitoring must still be veterinary-directed. Never give your dog thyroid medication without veterinary guidance.
Why does my vet check blood levels 4-6 hours after giving the pill? This timing captures the peak blood level (post-pill T4), which helps your veterinarian assess whether the dose provides adequate but not excessive thyroid hormone levels throughout the day.
My dog was diagnosed with hypothyroidism. Will the hair grow back? Yes. With appropriate levothyroxine therapy, most dogs experience significant to complete coat regrowth, though it may take several months. Skin infections and other secondary complications also typically resolve with treatment. Consult your veterinarian if improvement is not seen.
This content is for informational purposes only and is not a substitute for professional veterinary advice. Always consult your veterinarian before starting, changing, or stopping any medication for your dog.