Levothyroxine

Major Uses

Levothyroxine is used successfully to treat hypothyroidism in many animals, including dogs, cats, and horses. It is used for replacement therapy in animals that cannot maintain a high enough T3 level on their own.

Intravenously injected levothyroxine can be used to treat acute hypothyroidism in animals with myxedema coma.

Common Precautions

There are few side effects associated with levothyroxine. Very high doses of the medication can produce thyrotoxicosis, but this is uncommon. Possible adverse side effects from intravenously injecting levothyroxine include pneumonia and arrhythmias.

There are no known contraindications assocaited with levothyroxine. If the brand of the product is switched, further monitoring of the animal's T3 levels is needed to ensure the two products are therapeutically equivalent.

Levothyroxine should not be used in animals with a known allergy or hypersensitivity to the drug. If overdose occurs, a poison control center or veterinarian should be contacted immediately.

Storage

Levothyroxine is inherently unstable and can be greatly affected by heat, light, and humidity. This medication must be stored in a tightly sealed container away from any source of light and at room temperature. The oral solution, Levanta, has an 18 month shelf life when stored at refrigerated temperatures.

Administration

Levothyroxine is available in tablets ranging from 0.025 to 0.8 mg in 0.1 mg increments. It is also available in 0.1 to 0.8 mg chewable tablets, 1 mg/mL oral solution for dogs, and 1 g per 453.6 of oral powder (1 gram per pound) used for horses.

A typical starting dose in canines is 22mcg/kg, by mouth, every 12 hours. This dose may not be high enough, so extensive monitoring when beginning this medication is necessary in order to adjust the dosage appropriately.

Doses may vary in different species, when the drug is given by a different route or concurrently with other medications, and with regards to a patient's age, breed, and health status. A veterinarian's dosing instructions and/or those printed on the medication label should be followed closely.