Synthroid for dogs

In this article:

• What is Synthroid used for in dogs?
• Is Synthroid safe for dogs?
• Synthroid dosage for dogs
• Synthroid alternatives for dogs
• Summary

In this article

• What is Synthroid used for in dogs?
• Is Synthroid safe for dogs?
• Synthroid dosage for dogs
• Synthroid alternatives for dogs
• Summary

Synthroid is a brand-name prescription drug that treats underactive thyroid (hypothyroidism) in people, but it’s not commonly used in dogs. The active ingredient in Synthroid is levothyroxine sodium, a synthetic version of the thyroid hormone thyroxine (T4). Thyroxine regulates many critical functions in the body, including metabolism, energy, oxygen consumption, protein synthesis, blood volume, body temperature, chemical reactions, heart rate, and growth. An underactive thyroid causes several health problems. 

Like people, dogs commonly experience underactive thyroid and are treated with levothyroxine. However, levothyroxine doses are substantially higher in dogs than in people. Synthroid and generic levothyroxine tablets prescribed to humans come in tablets that are too low for most dogs. Veterinarians may prescribe Synthroid to a dog but will more often use higher-dose levothyroxine tablets, which the U.S. Food and Drug Administration approves for use in dogs.

RELATED: Save up to 80% on Synthroid

What is Synthroid used for in dogs?

Synthroid can treat underactive thyroid in dogs, but veterinarians are more likely to use levothyroxine formulated specifically for dogs.

Hypothyroidism

The thyroid gland produces two critical hormones: thyroxine (T4) and triiodothyronine (T3). Hypothyroidism is when the thyroid gland does not produce enough thyroid hormones. It is a common medical condition in people and dogs, particularly middle-aged and older dogs between four and 10 years of age. Some breeds are more predisposed to hypothyroidism, such as Labrador Retrievers, Golden Retrievers, Doberman Pinschers, Beagles, Boxers, and Border Collies. 

When the thyroid produces too little thyroid hormone in a dog, owners will notice changes in the dog, including lack of energy, decreased activity level, weight gain, hair loss, “rat tail,” poor hair coat quality, dullness, and cold sensitivity (the dog will exhibit heat-seeking behaviors). Low thyroid hormone also causes multiple abnormalities, such as high cholesterol, that can eventually cause serious health issues in the dog.

Hypothyroidism has many causes. The problem can be that the thyroid gland does not secrete enough thyroid hormone (primary hypothyroidism) or that the pituitary gland does not secrete enough hormone (TSH) to tell the thyroid to produce thyroid hormone (secondary hypothyroidism). No matter what the cause, hypothyroidism is treated in one way: thyroid hormone replacement. Among the possible thyroid hormone supplementation therapies, the standard choice is daily doses of oral levothyroxine. 

In most dogs, hypothyroidism is a lifelong condition, so pet owners will give the dog levothyroxine tablets every day for the rest of the dog’s life. 

RELATED: How to treat hypothyroidism in dogs

Is Synthroid safe for dogs?

When used at the appropriate dose, Synthroid will only rarely cause problems in a dog. 

However, Synthroid should never be given to a dog that has recently had a heart attack. Other contraindications are overactive thyroid (hyperthyroidism) and untreated Addison’s disease, a condition in which the adrenal gland does not produce enough cortisol, the body’s primary “stress” hormone. Synthroid should never be given to a euthyroid dog (a dog with healthy thyroid function).

Because of potential problems, veterinarians use levothyroxine cautiously in dogs with an overactive adrenal gland, heart disease, or diabetes. They will also be careful when using it in senior dogs. 

Side effects of Synthroid in dogs

When the Synthroid dose is appropriate, dogs rarely experience adverse reactions. Levothyroxine is chemically very similar to thyroxine, a natural hormone always present in a dog’s system.

Dogs may experience side effects if the dose is too high, but dogs are more resistant to levothyroxine overdoses than other animals. Symptoms of a levothyroxine overdose closely resemble the symptoms of hyperthyroidism, a condition in which the thyroid gland produces too much thyroid hormone.

Interactions of Synthroid with other pet meds

Before giving a dog levothyroxine tablets, tell the prescribing veterinarian about all the drugs, supplements, and foods that the dog takes. Since thyroxine hormones affect nearly every system in a dog’s body, levothyroxine has many significant drug and food interactions. The most significant include:

• High-fiber diets
• Iron supplements
• Heartburn and ulcer medications such as antacids, sucralfate, or stomach acid reducers
• Insulin
• High-dose corticosteroids
• Antidepressants
• The heart rhythm drug amiodarone

Synthroid dosage for dogs

The recommended levothyroxine starting dosage in dogs is 0.02 milligrams (mg) of levothyroxine per kilogram (kg) of body weight (0.01 mg/lb) taken once per day or divided into two daily doses. Current guidelines advise once-daily dosing, though some veterinarians may still use twice-daily dosing. 

After starting the dog on levothyroxine, the veterinarian will examine the dog again four weeks later and either increase or decrease the dose based on the dog’s clinical signs and thyroid hormone levels measured by a blood test. The veterinarian will continue to take blood tests to measure hormone levels every two to four weeks. As necessary, the veterinarian will keep adjusting the dose until the appropriate dose is reached.

Veterinarians do not commonly prescribe human versions of levothyroxine, such as Synthroid, because the dosage strengths are too small. The levothyroxine doses required in dogs are substantially higher than in people. In dogs, the half-life of levothyroxine is much shorter than in humans. In people, the half-life of levothyroxine is seven days. In dogs, it’s 10 to 16 hours. 

Synthroid tablets just aren’t big enough for dogs. The largest dose of Synthroid available is a 0.3 mg tablet. The smallest dosage strength of Thyro-Tabs or Thyro-Kare, FDA-approved levothyroxine tablets for dogs, is 0.1 mg, and the largest is 1 mg. For most dogs, owners will have to administer more than one Synthroid tablet of varying dosage strength. For instance, if a dog is prescribed 0.7 mg of levothyroxine per day, that might take two 0.3 mg Synthroid tablets and an additional 0.1 mg tablet. That’s too hard to do correctly and consistently. The alternative is a single 0.7 mg Thyro-Kare or ThyroTabs tablet.

Can dogs overdose on Synthroid?

Dogs can overdose on levothyroxine but seem to be less affected than other species. In dogs, the most common side effects of levothyroxine overdose include:

• Weight loss
• Increased appetite
• Excessive panting
• Excessive thirst
• Increased urination
• Excitability
• Hyperactivity

Veterinary literature does not specify a maximum dosage of levothyroxine. Veterinarians determine the appropriate dose by assessing the clinical signs and using blood tests to measure thyroid hormone levels.

Can I give my dog Synthroid every day?

Veterinarians are unlikely to prescribe Synthroid to dogs. Instead, they will prescribe products specifically formulated for canine use. It is safe and necessary to give a hypothyroid dog daily doses of levothyroxine. Levothyroxine treatment usually lasts for the life of the dog.

How to give your dog Synthroid

Most pet owners will give their dogs levothyroxine tablets formulated for dogs. Some levothyroxine medications for dogs are available in powder form but lack FDA approval. The veterinarian may suggest giving the dog an oral solution prepared by a compounding pharmacy for easier administration of levothyroxine.

• Administer levothyroxine to the dog as instructed by a veterinary professional.
• Do not give more than prescribed.
• Levothyroxine tablets can be given to a dog with or without food. However, always give the tablet the same way each time.
• If you don’t know how to give a dog a tablet, ask a veterinary technician at the vet’s office to demonstrate.
• If you give the dog an oral solution, always measure the dose with a calibrated oral syringe. A veterinarian can provide one for free.
• Do not miss a dose. If a levothyroxine dose is missed, give it when remembered. However, if it’s almost time for the next dose, skip the missed dose and give the dog the next dose on schedule. Never give the dog a double dose to compensate for a missed dose. 
• Store levothyroxine tablets at room temperature, protected from light and moisture.
• Keep this medicine out of the reach of children.

Do not discontinue levothyroxine until you talk to a veterinarian. Levothyroxine is a lifelong treatment in most cases.

Synthroid alternatives for dogs

Levothyroxine is the treatment of choice for hypothyroid dogs. It allows for precise control because each tablet contains a consistent and predictable dose. The only alternative to levothyroxine is dried and powdered thyroid products called desiccated thyroid. There are no FDA-approved desiccated thyroid medications for dogs. Most veterinarians will avoid them, and most doctors do, too. Desiccated thyroid does not allow for precise control of hypothyroidism because the doses vary from one pill to the next. Quality is a significant issue in many of these products.

Summary

Hypothyroidism is a common condition in middle-aged and older adult dogs. It is treated with oral levothyroxine. Human versions of levothyroxine, like Synthroid, are not commonly used. Instead, veterinarians rely on levothyroxine formulated in higher dosage strengths for use in dogs.