What Is Levothyroxine and What Does It Treat?
Levothyroxine is a synthetic thyroid hormone used to treat hypothyroidism in dogs — a common condition in which the thyroid gland does not produce enough thyroid hormone. It is sold under brand names such as Soloxine, Thyro-Tabs Canine, Leventa and others, and as generic levothyroxine sodium.
The thyroid hormone thyroxine (T4) acts like the body’s “thermostat and accelerator,” regulating metabolism in nearly every tissue. When a dog’s thyroid is underactive, that metabolism slows down, producing the classic signs of hypothyroidism:
- Weight gain without an increase in appetite
- Lethargy and reduced activity or exercise tolerance
- Hair loss, a dull or thinning coat, and skin problems (often symmetrical, and frequently a “rat tail”)
- Cold intolerance — seeking warmth, reluctance in cold weather
- Recurrent skin and ear infections
- Sometimes a sad or “tragic” facial expression, slow heart rate, or neurological signs
Levothyroxine treats this by replacing the missing hormone. It is essentially identical to the body’s own T4, restoring normal metabolism. The effect is often dramatic — many dogs become noticeably brighter, more active and re-grow their coat over the following weeks and months.
Two points are essential. First, hypothyroidism must be properly diagnosed with blood testing before treatment — the signs overlap with many other conditions, and starting thyroid hormone in a dog that does not need it is harmful. Second, there is no single fixed dose: the starting dose is just a starting point, and the correct long-term dose is found through follow-up blood tests. The calculator above gives an educational starting estimate; a veterinarian must diagnose, prescribe and monitor. See PuppaDogs’ guide to levothyroxine for dogs.
How the Levothyroxine Dosage Calculator Works
The calculator estimates a starting dose from your dog’s body weight and the chosen dosing frequency. It then:
- Shows the dose per administration and the estimated daily total.
- Optionally converts it into a number of tablets if you enter the strength.
- Flags conditions — heart disease, diabetes, adrenal insufficiency — that change how treatment is started.
Enter your dog’s weight, choose the dosing frequency, optionally enter the tablet strength, tick any health considerations, then press Estimate Starting Dose.
How Levothyroxine Doses Are Calculated for Dogs
The Starting Dose
The usual starting dose of levothyroxine for dogs is approximately:
0.02 mg/kg of body weight (about 0.01 mg/lb) per administration.
Traditionally this is given every 12 hours (twice daily), which remains the standard starting approach. Dogs absorb and use thyroid hormone less efficiently than humans, which is why canine doses can look surprisingly high compared with human doses. Many dogs are later found to do well on once-daily dosing, and your veterinarian decides this based on monitoring.
The Dose Is Set by Bloodwork, Not Weight
This is the crucial point. The 0.02 mg/kg figure is only a starting estimate. After your dog has been on levothyroxine for about 4–6 weeks, your veterinarian will run a post-pill (or “therapeutic monitoring”) T4 blood test — typically a blood sample taken 4–6 hours after a dose — to see how the dog is actually responding. The dose is then fine-tuned, and the process repeated until the thyroid level and the clinical signs are both right. Body weight gets you to the starting line; bloodwork sets the real dose.
Tablet Strengths
Levothyroxine for dogs comes in a wide range of tablet strengths, measured in milligrams — commonly 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8 mg — which lets the dose be matched closely to each dog. (Note that 0.1 mg is the same as 100 micrograms.)
Levothyroxine Dosage Chart for Dogs
This chart shows the starting-dose estimate at 0.02 mg/kg per dose. It is a reference only — the final dose is set by blood testing.
| Dog weight | Per dose (0.02 mg/kg) | Twice-daily total |
|---|---|---|
| 5 kg (11 lb) | ~0.1 mg | ~0.2 mg/day |
| 10 kg (22 lb) | ~0.2 mg | ~0.4 mg/day |
| 20 kg (44 lb) | ~0.4 mg | ~0.8 mg/day |
| 30 kg (66 lb) | ~0.6 mg | ~1.2 mg/day |
| 40 kg (88 lb) | ~0.8 mg | ~1.6 mg/day |
How to Give Levothyroxine to Your Dog
- Be consistent. Give it at the same times each day, in the same way relative to meals. Some foods and the timing of feeding can affect how much levothyroxine is absorbed, so keeping the routine identical makes the blood levels — and the monitoring — reliable.
- Hide the tablet in a small treat or pill pocket.
- Do not skip doses. Steady, consistent dosing is what keeps a dog’s thyroid level stable.
- Keep the monitoring appointments. The post-pill T4 tests are how the dose is kept correct.
- Missed dose: give it when you remember unless the next dose is near, then skip it — never double up.
Side Effects and Safety
Because levothyroxine simply replaces a hormone the body should be making, a correctly dosed dog usually has no side effects at all — it just returns to normal. Problems arise mainly from getting the dose wrong or from underlying conditions.
Signs of Too Much Thyroid Hormone (Thyrotoxicosis)
If the dose is too high, a dog can develop thyrotoxicosis. Watch for and report:
- Hyperactivity, restlessness or anxiety
- Excessive panting
- A fast heart rate
- Increased thirst and urination
- Weight loss despite a good or increased appetite
These signs mean the dose needs to be reduced — contact your veterinarian.
Conditions That Change How Treatment Starts
- Heart disease — thyroid hormone increases the heart’s workload, so vets usually start lower and increase slowly.
- Diabetes — starting levothyroxine can change insulin needs; glucose must be monitored.
- Addison’s disease / adrenal insufficiency — thyroid hormone should not be started until any adrenal insufficiency is treated and stable, as doing so can trigger a crisis.
- Senior dogs — often started cautiously, particularly before heart disease has been ruled out.
Treatment Duration and Monitoring
Canine hypothyroidism is a lifelong condition, so levothyroxine is almost always a permanent, daily treatment. After the dose is established, your veterinarian will recheck thyroid levels periodically — often every 6–12 months — and any time the dog’s signs change, because the right dose can drift over a dog’s life. With consistent dosing and routine monitoring, a hypothyroid dog can live a completely normal, healthy life.
Conclusion
Levothyroxine is a safe, effective, often transformative treatment for canine hypothyroidism, started at roughly 0.02 mg/kg per dose, traditionally twice daily. The calculator above gives that starting estimate and converts it into tablets. But the most important message is this: the starting dose is only a starting point. Hypothyroidism must be properly diagnosed first, and the correct long-term dose is found and maintained through post-pill T4 blood testing under your veterinarian’s care. Give it consistently, watch for signs of over-treatment, and keep the monitoring appointments — and your dog can expect a full and normal life.
Frequently Asked Questions
How much levothyroxine should I give my dog?
The usual starting dose of levothyroxine for dogs is about 0.02 mg/kg of body weight per administration, traditionally given every 12 hours. As a guide that is roughly 0.2 mg twice daily for a 22 lb dog. This is only a starting estimate – the correct long-term dose is set by your veterinarian using post-pill T4 blood tests.
Why are dog levothyroxine doses higher than human doses?
Dogs absorb and metabolise thyroid hormone less efficiently than humans, so the milligram dose needed to achieve normal thyroid levels is considerably higher in dogs. This is normal and expected – canine and human levothyroxine doses should never be compared directly, and the dog dose must be set for the dog by a veterinarian.
How is the correct levothyroxine dose for a dog determined?
Body weight only provides a starting dose. After about 4-6 weeks of treatment, your vet performs a post-pill T4 blood test – a sample taken 4-6 hours after a dose – to see how the dog is responding. The dose is then adjusted and rechecked until both the thyroid level and the clinical signs are correct.
What are the signs my dog is getting too much levothyroxine?
Too much thyroid hormone causes thyrotoxicosis. Signs include hyperactivity or restlessness, excessive panting, a fast heart rate, increased thirst and urination, and weight loss despite a good appetite. If you notice these, contact your veterinarian, as the dose likely needs to be reduced.
Does my dog need levothyroxine for life?
Almost always, yes. Canine hypothyroidism is a lifelong condition, so levothyroxine is a permanent daily treatment. The dose is monitored with periodic blood tests, often every 6-12 months, because the right dose can change over a dog’s life. With consistent treatment, hypothyroid dogs live completely normal lives.
Can levothyroxine be given with food?
Levothyroxine can be given with food, but the key is consistency. Some foods and the timing of meals affect how much is absorbed, so it should be given the same way relative to feeding every day. A consistent routine keeps blood levels stable and makes the monitoring blood tests reliable.
Related PuppaDogs Calculators
Continue building your dog’s personalised care plan with these related PuppaDogs calculators:
- Dog Pregnancy / Whelping Due-Date Calculator
- Puppy Weight Predictor (Adult Weight Calculator)
- Heatstroke Risk Calculator for Dogs
- Bloat (GDV) Risk Calculator for Dogs
- Dog Life Expectancy Calculator (Breed, Body Condition, Lifestyle)
- Spay/Neuter Timing Calculator for Dogs (Breed-Specific)
References & Further Reading
The dosing ranges and safety information on this page are drawn from the following veterinary references. Always defer to your own veterinarian and the manufacturer’s label for your specific product.
- Plumb DC. Plumb’s Veterinary Drug Handbook. 10th ed. Hoboken, NJ: Wiley-Blackwell, 2023 – levothyroxine sodium monograph.
- Merck Veterinary Manual. Hypothyroidism in dogs – diagnosis and treatment. merckvetmanual.com.
- American Animal Hospital Association (AAHA). 2023 Selected Endocrinopathies of Dogs and Cats Guidelines – canine hypothyroidism therapy. aaha.org.
- Thyro-Tabs Canine (levothyroxine sodium tablets) – FDA-approved veterinary label.
- PuppaDogs. Levothyroxine for Dogs: Benefits, Dosage, Side Effects, and More. puppadogs.com.
