Levothyroxine for Dogs and Cats: Uses, Dosage and Side Effects

Levothyroxine for dogs and cats is a synthetic form of thyroxine (T4), the hormone produced naturally by the thyroid gland. It is the standard treatment for hypothyroidism (a condition in which the thyroid gland does not produce enough thyroid hormone) in dogs. It is also used in cats under similar circumstances, typically when thyroid levels drop too low following treatment for an overactive thyroid. In most cases, treatment is lifelong, and correct dosing is key to keeping thyroid levels stable.

Levothyroxine replaces the natural thyroid hormone your dog's body is missing. Once absorbed, it converts into the active hormone needed to regulate energy, metabolism, heart rate, and coat health. It essentially acts as a daily supplement to restore your pet's vitality and balance.

Without sufficient thyroid hormone, dogs develop a range of symptoms including weight gain despite normal eating, lethargy, hair thinning or loss, skin thickening, and a slowed heart rate.

Levothyroxine for cats works the same way, replacing the missing T4 and allowing the body to convert it into active T3 (triiodothyronine). However, cats metabolise and absorb Levothyroxine differently from dogs.

Cats process the hormone more quickly and absorb it less efficiently from the gut. Because of this, cats typically need a relatively higher dose per kilogram of body weight than dogs.

It is rare to see naturally occurring hypothyroidism in cats. Most cats who need Levothyroxine have developed low thyroid levels as a result of treatment for hyperthyroidism (an overactive thyroid, which is common in older cats). Instead of just looking at symptoms, a vet will usually run bloodwork to confirm low thyroid levels before starting Levothyroxine in a cat.

The standard Levothyroxine dosage for dogs is 0.02 mg/kg given orally once or twice daily. Many dogs are started on twice-daily dosing initially, with once-daily dosing considered once thyroid levels are well controlled and stable.

For the best absorption, Levothyroxine should be given on an empty stomach, at least 30 minutes before a meal. Because dose requirements vary between individual dogs, follow-up blood tests are done 4 weeks after starting treatment, and then every 6 to 12 months once levels are stable. Never adjust the dose without veterinary guidance.

Large breeds may need a larger absolute dose of Levothyroxine, still calculated at 0.02 mg/kg. Breeds including Dobermans, Golden Retrievers, Labrador Retrievers, and Boxers are more commonly diagnosed with hypothyroidism and may require long-term management.

In large dogs with concurrent health conditions such as diabetes or heart disease, your vet may start at a lower dose. They may increase the doses gradually to allow the body to adjust without placing additional stress on the dog’s cardiovascular system.

The Levothyroxine dosage for cats is typically 0.05 to 0.1 mg (50 to 100 micrograms) given orally once or twice daily. Because cats metabolise thyroid hormone more rapidly than dogs, some cats require twice-daily dosing to maintain consistent blood levels.

Treatment should be given on an empty stomach for the best absorption. Your cat will need thyroid (blood) tests a few weeks after starting treatment and then at regular intervals. This allows the vet to adjust the dose based on how well the medication is working. Its use in cats is generally off-label (used outside the originally approved indication), which is common in veterinary practice.

Most side effects from Levothyroxine occur when the dose is too high, causing signs of an overactive thyroid rather than problems from the drug itself. These signs typically ease when the dose is reduced.

Signs that the dose may be too high in dogs include increased thirst and urination, increased appetite, weight loss, restlessness or hyperactivity, panting, vomiting, diarrhoea, and a fast or irregular heart rate. These are signs of excess thyroid hormone, sometimes called iatrogenic hyperthyroidism (hyperthyroidism caused by medication).

Dogs receiving the correct dose generally experience very few side effects.

Cats given too much Levothyroxine may develop increased appetite, weight loss, restlessness, vomiting, diarrhoea, fast heart rate, and increased thirst or urination. These are the same signs seen in cats with naturally overactive thyroids.

Store Levothyroxine:

Levothyroxine for dogs and cats is a well-established thyroid hormone replacement used to manage hypothyroidism. When prescribed at the right dose and monitored regularly, most pets do very well on this medication and see a significant improvement in their quality of life.

The dose often needs precise adjustment over time. Because of this, consistent follow-up blood tests are absolutely essential. Keeping your vet informed of any changes in your pet's behaviour or weight is just as important as giving the pill itself.