Hypothyroidism is one of the most common endocrine (hormonal) diseases in dogs, and without treatment, this progressive, incurable condition can be life-threatening.

Fortunately, following an accurate diagnosis of an underactive thyroid, your veterinarian can prescribe levothyroxine for dogs. These easy-to-administer tablets will put your pet on a rapid road to recovery and, with lifelong treatment, prevent the symptoms of this thyroid disease from returning.

What is Levothyroxine, and How Does it Work?

Levothyroxine sodium tablets, sold under the brand names Thyro-Tabs® Canine or ThyroKare, are an inexpensive, FDA-approved treatment for hypothyroidism in dogs. Dr. Rebecka Hess, Professor of Internal Medicine at the University of Pennsylvania School of Veterinary Medicine, explains that hypothyroidism is a disease characterized by decreased thyroid synthesis and secretion. This inability to produce and release sufficient thyroid hormones causes signs including lethargy, inactivity, weight gain, and hair loss.

“Levothyroxine is a synthetic version of the body’s natural thyroid hormone [T4 or thyroxine], which is given to dogs with hypothyroidism to supplement for their thyroid hormone deficiency,” Dr. Hess says. “Proper levothyroxine administration to dogs with confirmed hypothyroidism reverses the clinical signs and clinicopathologic abnormalities (such as high cholesterol) associated with untreated hypothyroidism.”

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When to Use Levothyroxine

Currently, levothyroxine is the primary-approved drug treatment for hypothyroidism in dogs. Dr. Orla Mahony-Wages, a small animal internist at the Cummings School of Veterinary Medicine at Tufts University, says that, for dogs with a definitive diagnosis, treatment with levothyroxine leads to significant improvement in quality of life. “Multiple studies, including field effectiveness and safety studies, demonstrate the efficacy of levothyroxine,” she says.

It’s suitable for use in most dogs with confirmed hypothyroidism, but there are some caveats.

The drug has not been evaluated on breeding, pregnant, or nursing bitches. Treatment may temporarily stop or be delayed for these patients, especially since excess thyroid hormones in pregnant women are associated with thyroid gland problems in their babies.

“Dogs with a recent diagnosis of hypothyroidism and significant co-morbidities, such as diabetes mellitus, Addison’s disease, moderate-to-severe heart disease, debilitation or advanced age, should be started on a low dose of levothyroxine that can be slowly increased to effect,” Dr. Mahony-Wages says. This allows your veterinarian to monitor for any concerning side effects in these more at-risk patients if there’s an adverse reaction.

How to Administer Levothyroxine

Your veterinarian will prescribe an initial dose of levothyroxine depending on your dog’s weight and other factors. You can easily administer the pills orally to your dog at home. Dr. Hess says levothyroxine is typically given at an initial dose of 0.02mg/kg twice daily (not to exceed 0.8mg/kg). Some dogs do well on a once-daily regimen. If the thyroid hormones remain low despite supplementation, the levothyroxine dosage for dogs may have to be increased.

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Levothyroxine is typically absorbed best when your dog has an empty stomach. For optimal results, stick with a consistent schedule of administration.

The drug can affect the human thyroid gland, so it’s best to wear gloves during administration. Also, if you have a dog with a poop-eating habit, make sure you’re quick to scoop, as levothyroxine is excreted in feces.

Because hypothyroidism isn’t a curable condition, your dog will need to remain on levothyroxine for the rest of their life.

The Importance of Monitoring After Treatment Starts

The prognosis for a dog with hypothyroidism is excellent when treating with levothyroxine. However, veterinary monitoring tests are essential to ensure appropriate dosage.

Expect your veterinarian to check your dog’s thyroid hormone levels every 4 to 8 weeks initially. Hypothyroidism symptoms will continue if the dose they’re receiving is too low. These include weight gain, poor appetite, lethargy, hair loss, and skin and ear infections. If the dose is too high or if your dog has an underlying illness making it difficult to break the drug down or was misdiagnosed with hypothyroidism, taking levothyroxine can lead to thyrotoxicosis (an overdose of thyroid hormones).

Required levothyroxine levels can change over time, so even after the initial dosage levels are confirmed, expect to attend monitoring appointments once or twice a year for the rest of your dog’s life.

Breed Differences and Hypothyroidism Misdiagnosis

Some experts speculate hypothyroidism is being overdiagnosed, and research shows that breed differences in normal thyroid hormone levels exist. “Sighthounds are well known to have thyroid concentrations below the standard reference range,” Dr. Mahony-Wages says. “There are studies in some other breeds that have been helpful in establishing breed-specific ranges.”

“For dogs with classic clinical signs, supportive laboratory changes and abnormal thyroid hormone test results, the diagnosis can be straightforward,” she says. “For dogs with absent or nonspecific clinical signs and low thyroid hormone concentrations, it is important to consider breed, activity, interference from medications and other illness.”

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Monitoring thyroid levels after levothyroxine treatment starts is essential. However, to reduce the risk of hypothyroidism misdiagnosis and thyrotoxicosis from unnecessary administration of this medication, your veterinarian should check breed-specific reference intervals (RIs). This aids in the accurate interpretation of the data used to assess thyroid health.

Side Effects of Levothyroxine For Dogs

Dr. Hess says that dogs with an accurate hypothyroidism diagnosis generally tolerate levothyroxine well. Once your veterinarian establishes the correct dosage, side effects are rare.

Most commonly, issues occur when your dog is receiving an overdose of thyroid hormones. This results in them displaying signs of hyperthyroidism. If you suspect thyrotoxicosis, don’t delay contacting your veterinarian for advice.

Thyrotoxicosis symptoms include:

• Hyperactivity

• Anxiety

• Excessive drinking and urination

• Increased appetite

• Weight loss

• Rapid heart rate

• Vomiting

• Diarrhea

• Seizures

Dr. Mahony-Wages says that pruritus (itching) and allergic-type hypersensitivity skin reactions to levothyroxine have also occasionally been reported in dogs.

How Long Before Levothyroxine Has an Effect

“Clinical response could start within a week of beginning levothyroxine treatment,” Dr. Hess says. After just a few weeks, there should be noticeable improvements in your dog’s appetite and energy levels. If your dog experiences significant hair loss as a symptom of hypothyroidism, be patient—it could take a few months to grow back properly. They may also need additional medication to help control itchiness or clear up secondary skin or ear infections.

Other Treatment Options for Hypothyroidism

Dr. Mahony-Wages says that while levothyroxine is the primary treatment for dogs with a definitive diagnosis of hypothyroidism, rarely, dogs may fail to respond due to poor absorption of the drug.

“If different levothyroxine products, multiple-dose adjustments and administration on an empty stomach are ineffective, supplementation with T3 [Triiodothyronine thyroid hormone] may be considered,” she says. “However, this drug requires three times daily administration due to a short half-life [it stays in the body for a shorter period].” T3 levels also fluctuate in the body naturally, so there’s more chance of your dog experiencing hyperthyroidism-like symptoms on these supplements.

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