Overview of Levothyroxine for Dogs and Cats 

 Levothyroxine, also known as Soloxine® or ThyroTabs®, is used in dogs and cats for the treatment of hypothyroidism.

Thyroid hormones are produced by two thyroid glands located in the neck. These hormones support normal cell function in almost every organ of the body.

The main thyroid hormone in dogs and cats is levothyroxine (also called L-thyroxine or T4). L-thyroxine is later converted to another more active thyroid hormone called T3.

The thyroid hormones have significant beneficial effects on brain, heart, muscle, nerve and kidney functions and also support a healthy skin and hair coat.

When an individual cannot manufacture enough thyroid hormone to support normal cell metabolism, a condition called hypothyroidism develops.

In dogs, an autoimmune disease directed against the thyroid glands usually causes hypothyroidism, and this is often suspected in a dog with lethargy, poor hair coat, obesity and weakness.

In cats, hypothyroidism is relatively rare, and usually occurs when the thyroid glands are destroyed following thyroid gland surgery or after treatment for an overactive thyroid gland (a common condition in cats called hyperthyroidism – the opposite of hypothyroidism).

A veterinarian can only make a diagnosis of hypothyroidism after analysis of special blood tests.

It should be noted that some sick pets have slightly low thyroid hormone levels in the blood; however, this does not necessarily mean the pet has hypothyroidism. Other illnesses can temporarily depress thyroid hormone concentration.

In cases of confirmed hypothyroidism, supplemental levothyroxine is usually needed to reverse the medical symptoms. In properly diagnosed cases, thyroid hormone supplementation often leads to dramatic improvement in symptoms and activity.

Levothyroxine is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.

Brand Names and Other Names of Levothyroxine

This drug is registered for use in humans and animals.

Human formulations: Synthroid® (Knoll)

Veterinary formulations: Soloxine® (Daniels) and Thyro-Tabs® (Vet-A-Mix), Thyrosyn®, Thyromed®, Nutrived®,Thyrokare®, Levosyn®, Thyroxine-L®, Leventa® Oral Solution. 

Uses of Levothyroxine

• Levothyroxine is used for the treatment of hypothyroidism in dogs and cats.  For more information on hypothyroidism, please see hypothyroidism in dogs.

Precautions and Side Effects

Levothyroxine should never be administered to animals without a definite diagnosis of hypothyroidism.

While generally safe and effective when prescribed by a veterinarian, levothyroxine can cause side effects in some animals.

Levothyroxine should not be used in animals with known hypersensitivity or allergy to the drug.

Levothyroxine may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with levothyroxine . Such drugs include amiodarone, oral antacids, corticosteroids, ketamine, phenobarbital, sertraline, sucralfate, insulin, epinephrine, digoxin and ketamine.

Excessively high doses of levothyroxine can accumulate and lead to excitement, tremors and excessive stimulation of the heart. This is a relatively common problem and may easily be overlooked.

To insure effective and safe dosing of levothyroxine, blood tests that measure thyroid hormone levels must be done once the pet is receiving a consistent dose of levothyroxine.

How Levothyroxine Is Supplied

Levothyroxine tablets are available in sizes ranging from 0.1 mg to 0.8 mg and 1 mg pills.  It is also suppled as an oral solution in 1 mg/ML in 30 mL bottles. 

Dosing Information of Levothyroxine for Dogs and Cats

Medication should never be administered without first consulting your veterinarian.

The typical dose administered to dogs is 10 micrograms per pound (22 mcg/kg) orally every 12 hours, but some dogs require less than this.

The dose for cats is 5 to 10 mcg per pound (10 to 20 mcg/kg) per day orally.

Final dose adjustments are made on the basis of blood tests to measure thyroid hormone levels.

The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.