By Lauren R. Forsythe, PharmD, MBA, DICVP; Rania Gollakner, BS, DVM, MPH

What is levothyroxine?

Levothyroxine (brand names: Thyro-Tabs Canine®, Synthroid®, Levothroid®, Levoxyl®, Unithroid®, Levo-T®, Eltroxin®, PMSLevothyroxine®) 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 carefully, as they 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.
  • It 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.

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 as soon as you remember, and then continue with the regular dosing schedule. However, if it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.

Are there any potential side effects?

Side effects are rare if levothyroxine is given at appropriate doses. Rarely, it may cause the following side effects:

  • skin reactions
  • allergic reaction with forms of the medication that contain magnesium stearate and polyvinylpyrrolidone

In cats, high doses may cause the following side effects:

  • hyperthyroidism (signs include increased heart rate, urination, eating and drinking; excitability, nervousness, and panting)
  • anorexia
  • 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 that have:

  • certain heart problems
  • hyperthyroidism
  • untreated Addison’s disease

It should be USED WITH CAUTION in pets that:

  • have managed Addison’s disease
  • have heart disease
  • have diabetes
  • 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 with other medications and should be used cautiously with the following:

  • oral antacids
  • tricyclic antidepressants
  • anti-diabetic medications
  • corticosteroids
  • ferrous sulfate
  • fiber (high amounts)
  • ketamine
  • phenobarbital
  • propylthiouracil
  • sertraline
  • sucralfate

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

Is any monitoring needed with this medication?

  • Your veterinarian will monitor your pet to make sure the medication is working.
  • Your veterinarian 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?

  • 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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