Levothyroxine for dogs: Dosage and safety

In this article:

• Uses
• Safety
• Dosage
• Alternatives
• Summary

In this article

• Uses
• Safety
• Dosage
• Alternatives
• Summary

Veterinarians prescribe levothyroxine to dogs that don’t produce enough thyroid hormone, a condition called hypothyroidism. Levothyroxine is a synthetic formulation of the thyroid hormone thyroxine (T4), and levothyroxine doses are given to replace the missing thyroxine. Because levothyroxine is similar to a natural hormone in the dog, it is safe to give dogs with an underactive thyroid under the direction of a veterinarian. 

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What is levothyroxine used for in dogs?

Veterinarians use levothyroxine in dogs whose thyroid glands aren’t producing enough thyroid hormone, a medical condition called hypothyroidism. Levothyroxine is a synthetic thyroid hormone, so levothyroxine treatment is a type of hormone replacement therapy. 

Hypothyroidism

Thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are critical for animal health. Thyroxine regulates metabolism, energy use, oxygen consumption, protein synthesis, blood volume, body temperature, heart rate, growth, and many other processes in the body. Dogs with an underactive thyroid will experience a number of health problems.

The thyroid gland produces thyroid hormones. Two structures in the brain, the hypothalamus and the pituitary gland, regulate thyroid hormone production by producing thyroid-stimulating hormone (TSH). When the whole system works as it should, TSH is produced when thyroid hormones are low, and the thyroid gland releases the right amount of thyroid hormone to compensate. When thyroid hormones are too high, less TSH is produced, and the thyroid gland produces less thyroid hormone. The result is healthy thyroid hormone levels.

Hypothyroidism is a condition in which the body doesn’t produce enough thyroid hormone. Either the thyroid gland isn’t working right (primary hypothyroidism), or the body isn’t producing enough TSH (secondary hypothyroidism). It’s a common problem in both people and dogs, particularly middle-aged and older dogs between 4 and 10 years of age. Some breeds, such as golden retrievers, Labrador retrievers, Doberman pinschers, beagles, border collies, huskies, Maltese, Shetland sheepdogs, and boxers, are more likely to be affected by the condition. 

Hypothyroidism affects every system in the body, causing noticeable symptoms and potentially serious health problems, such as weight gain or high cholesterol. Signs of hypothyroidism in a dog include lack of energy, sluggishness, weight gain, poor hair coat quality, “rat tail,” and hair loss. The dog becomes more sensitive to cold, as evidenced by shivering or efforts to keep warm.

Although there are several signs, hypothyroidism is diagnosed conclusively through blood tests that measure TSH and thyroid hormone levels. Treatment will also be guided by blood testing.

Whether in people or pets, primary hypothyroidism and secondary hypothyroidism are always treated with thyroid hormone replacement therapy. The first and standard choice is oral levothyroxine. It allows for precise and predictable doses that allow veterinarians to stabilize thyroid hormone concentrations. 

Hypothyroidism is usually a lifelong condition in pets. Levothyroxine doses must be faithfully given every day for the rest of the dog’s life.

RELATED: How to treat hypothyroidism in dogs

Is levothyroxine safe for dogs?

Most dogs will have no problems with levothyroxine once the correct dose is established.

There are exceptions. Levothyroxine is never used in dogs that have:

• Normal thyroid hormone levels and a healthy thyroid gland (euthyroid)
• High thyroid hormone levels (hyperthyroidism)
• Addison’s disease (underactive adrenal gland and low levels of cortisol)
• Recently had a heart attack

Veterinarians will need to regularly monitor dogs taking levothyroxine who also have:

• Heart disease
• Diabetes
• Advanced age

Side effects of levothyroxine in dogs

When the correct dose is established, levothyroxine should not cause adverse reactions. 

If the dose is too high, dogs may show the same symptoms they would have with an overactive thyroid producing too much thyroid hormone, or hyperthyroidism. Symptoms include weight loss, hunger, excessive peeing, excessive drinking, and aggressiveness. These are signs the dose is wrong. When a dog shows any symptoms of hyperthyroidism, call the veterinarian. High thyroid hormone levels can’t be definitively established without a blood test. They can be easily corrected by reducing the levothyroxine dose.

RELATED: Hypothyroidism vs. hyperthyroidism: causes, symptoms, treatments & more

Interactions of levothyroxine with other pet meds

Make sure the prescribing veterinarian knows about all prescription medications, over-the-counter remedies, supplements, and foods the dog receives, particularly:

• High-fiber foods
• Iron supplements
• Heartburn and ulcer medications, such as antacids, sucralfate, or stomach acid reducers
• Insulin and other antidiabetic drugs
• Antidepressants
• High-dose corticosteroid treatment 
• The heart rhythm drug amiodarone
• The anticonvulsant phenobarbital

Levothyroxine dosage for dogs

There is no fixed levothyroxine dosage for dogs. Instead, veterinarians will prescribe the dose that results in normal thyroid hormone levels and clears up the symptoms of hypothyroidism.

The recommended starting dosage in dogs is 0.02 milligrams (mg) of levothyroxine per kilogram (kg) of body weight (0.01 mg/lb) daily. The full dose can be given once daily, or half doses twice daily. Veterinarians will most likely use once-daily dosing, as it aligns with current guidelines. 

The starting dose is not the final dose. Veterinarians usually increase the dose in steady increments every two to four weeks. They use clinical signs and blood tests to determine how well each dosage is working. They continue to adjust the dosage until thyroid hormone levels are normal and the dog no longer has clinical signs of hypothyroidism. 

Human versions of levothyroxine, such as Synthroid or generic levothyroxine, are inappropriate for dogs’ use. The dosage strengths are too low. Dogs require substantially more thyroid hormone than people do because they eliminate levothyroxine more quickly. The half-life of levothyroxine in people is seven days, but it takes a dog 10 to 16 hours to eliminate half a dose.

Thyroid pills for dogs range in dosage strength from 0.1 mg per pill to 1 mg. For people, levothyroxine tablets range from 0.013 mg to 0.3 mg. There’s a slight overlap, but not much. Because of this difference, veterinarians usually prescribe levothyroxine tablets formulated for dogs that contain high-delivery doses. Thyro-Tabs Canine is a brand-name levothyroxine product approved by the FDA for use in dogs and available in appropriate dosage strengths.

RELATED: Synthroid for dogs

Can dogs overdose on levothyroxine?

A levothyroxine overdose is the same as having too much thyroid hormone, so it produces the same problems as an overactive thyroid. Symptoms appear about one to nine hours after the overdose. The most common side effects of a levothyroxine overdose in a dog include:

• Vomiting
• Diarrhea
• Increased appetite
• Excessive thirst
• Increased urination
• Excessive panting
• Excitability
• Hyperactivity
• Aggressive behavior

Fortunately, dogs are more resistant to levothyroxine overdoses than people and other animals. They eliminate the drug much more quickly from the body than other animals.

No maximum dosage has been established. Blood tests and clinical signs will determine the appropriate dose. Any dose that produces normal thyroid hormone levels and resolves hypothyroid symptoms is the maximum dose for that particular dog.

Can I give my dog levothyroxine every day?

Most dogs will need daily levothyroxine for the rest of their lives. Daily levothyroxine doses are not only safe but necessary in hypothyroid dogs. 

How to give your dog levothyroxine

Pet parents can give their dog levothyroxine in three ways: as a tablet, liquid, or powder, though levothyroxine powder is intended for horses. Pet parents should provide their dog with levothyroxine formulated for dogs, not human formulations.

Veterinarians are most likely to prescribe levothyroxine tablets. It’s important to give every dose as instructed by the veterinarian:

• Do not give more levothyroxine than prescribed.
• You can give the dog the dose with or without food, but it must be given the same way every time. Food affects how much levothyroxine is absorbed, so changing how you deliver it will change the dose.
• If you don’t know how to give a dog a tablet, a veterinary technician or the veterinarian can show you the appropriate technique.
• If you give the dog an oral solution, always measure the dose with a calibrated oral syringe. If one is missing from the package, ask the veterinarian for one.
• Do not miss a dose. If a levothyroxine dose is missed, give it when remembered.
  • If the dog is getting once-daily doses, skip the missed dose if it’s the next day. Give that day’s dose as scheduled.
  • If the dog gets twice-daily doses, skip the missed dose if it’s almost time for the second dose. 
  • Never give two doses to make up for a missed dose. 
• Do not stop giving the dog levothyroxine unless advised to do so by a veterinarian.
• Store levothyroxine tablets at room temperature, protected from light and moisture.
• Keep levothyroxine out of the reach of children.

Be careful when switching formulations, such as moving from levothyroxine tablets to the oral solution, or vice versa. They may not be fully equivalent to what you are giving the dog, so switching may require dose adjustments.

Levothyroxine alternatives for dogs

Veterinarians choose levothyroxine as the hypothyroid drug of choice. Unlike the alternatives, levothyroxine can be precisely and predictably dosed. 

The only alternative thyroid medication for dogs is desiccated thyroid hormone made from dried and powdered pig thyroid glands, like Armour Thyroid or NP Thyroid. However, the amount of thyroid hormone can vary from product to product and pill to pill. The quality may also be questionable, since the U.S. Food and Drug Administration (FDA) does not regulate them. Veterinary professionals do not recommend their use.

RELATED: NP Thyroid vs. Synthroid: differences, similarities & side effects

Summary

Companion animals, particularly dogs, can experience low thyroid hormone levels as they age, especially as they enter middle age (6 to 7 years old). Daily doses of levothyroxine help return them to their normal selves, but they must be given levothyroxine formulated for dogs. Once the veterinarian has determined the correct dose, the dog should feel better and not experience serious problems if the medicine is given faithfully.