By Rania Gollakner, BS, DVM, MPH

What is levothyroxine?

Levothyroxine (brand names: Thyro-Tabs Canine®, Synthroid®, Levothroid®, Levoxyl®, Unithroid®, Levo-T®, Eltroxin®, PMS-Levothyroxine®) is a thyroid hormone replacement used to treat hypothyroidism (low thyroid hormone levels).

Its use in cats to treat hypothyroidism is 'off label' or 'extra label'. Use of the human version of this drug to treat dogs or cats is also off label or extra label. Many drugs are commonly prescribed for off label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their directions may be significantly different from those on the label.

How do I give my pet levothyroxine?

Levothyroxine is given by mouth in the form of a tablet or capsule, or may be compounded into a liquid. This medication can be given with or without food, but should be given the same way every day. Be sure to follow the instructions on the label, especially for compounded medications.

This medication can take days to weeks until effects are noted, and sometimes effects are not visibly noticeable.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

Side effects are rare if this medication is given at appropriate doses. Very rarely, skin reactions have been observed. With certain forms of the medication that contain magnesium stearate and polyvinylpyrrolidone, an allergic reaction can occur.

At high doses, signs of hyperthyroidism can occur and include increased heart rate, appetite, drinking, and urination, excitability, nervousness, and panting. In cats, high doses can cause anorexia and listlessness.

This short-acting medication should not last more than 24 hours, but may last longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Levothyroxine should not be used by pets with certain heart problems, hyperthyroidism, or untreated Addison’s disease. It should be used cautiously in pets with managed Addison’s disease, heart disease, diabetes, or that are elderly. If your pet is pregnant or nursing, speak to your veterinarian about using this medication.

Are there any drug interactions I should be aware of?

Levothyroxine may interact and should be used cautiously with the following medications: amiodarone, oral antacids, tricyclic antidepressants, antidiabetic medications, corticosteroids, digoxin, ferrous sulfate, fiber (high amounts), ketamine, phenobarbital, propylthiouracil, rifampin, sertraline, sucralfate, sympathomimetics, and warfarin.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is there any monitoring that needs to be done with this medication?

While taking levothyroxine, your veterinarian will monitor your pet to make sure it is working and will check blood thyroid hormone levels via thyroid blood work panels. These will be monitored more frequently at first and then at regular intervals once your pet’s levels are regulated.

How do I store levothyroxine?

The tablets should be stored at room temperature in a dark and dry place away from moisture and light. For compounded liquids, follow the instructions on the label for storage recommendations.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

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