Side Effects of Thyroxine in Dogs

Written by shelley moore
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Side Effects of Thyroxine in Dogs
Thyroxine is generally very safe for dogs. (dog image by Krzysztof Gebarowski from

Levothyroxine or L-thyroxine is a synthetic thyroid hormone available in brands such as Soloxine, Synthroid and ThyroTabs. This medication is useful for treating canine hypothyroidism, a condition involving inadequate thyroid hormone production. Hypothyroidism in dogs, as in people, often requires lifetime treatment. Thyroxine is a prescription-only drug that is generally very safe when administered at proper dosages.

Elevated Thyroid Hormone Effects

In recommended doses, canine side effects should not occur with thyroxine. Possible side effects are mainly associated with an elevated thyroid hormone. They can include coughing, dry mouth, loss of appetite, nausea, vomiting, diarrhoea, dizziness, sleep problems and itching, as listed in pharmaceutical information provided by 1800PetMeds.


You should not give your dog thyroxine without a veterinary diagnosis of hypothyroidism, because excessive thyroid hormone can cause serious side effects. Additionally, if you miss providing a dose, do not double up if it gets close to the time for the next dosage. A thyroxine overdose can have symptoms of increased thirst and urination, increased hunger, aggressiveness and other behaviour changes, hyperactivity, excitement, tremors, panting and abnormal discomfort in warm environments.


Thyroxine can cause complications in dogs with cardiovascular disorders, because this medication can elevate the metabolic rate. Your dog might not be able to safely take thyroxine if he has high blood pressure or any type of heart disease. Because thyroxine has hormonal effects, it can create problems for dogs with other hormone-related conditions, such as diabetes, Addison's disease or a pituitary gland disorder.

Allergic Reaction

Although unlikely, some dogs may experience an allergic reaction to thyroxine. Signs include trouble breathing, hives and itching and swelling of the face, mouth or tongue. An allergic reaction to thyroxine calls for immediate veterinary attention.

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