Canine Levothyroxine Side Effects

If your dog is diagnosed with hypothyroidism, it's likely your veterinarian will prescribe a daily dose of levothyroxine for treatment. At normal dosages, levothyroxine doesn't cause side effects in canines. However, if your dog receives an excessive dose, he'll experience some of the symptoms of hypothyroidism's opposite condition, hyperthyroidism.

Thirsty Dog
Excess water consumption is a common side effect in a dog receiving too much levothyroxine.
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Canine Hypothyroidism

Canine hypothyroidism occurs when a dog's thyroid glands no longer produce a sufficient amount of levothyroxine, or thyroid hormone. This hormone helps regulate nearly every system in the canine body, so insufficient amounts produce a variety of symptoms. These include weight gain, excessive drinking and urination, poor coat, skin infections, hair loss and lethargy. Your vet diagnoses hypothyroidism via blood tests. Once diagnosed, she'll likely prescribe levothyroxine, which your dog must receive for the rest of his life.

Levothyroxine Sodium

Marketed under the brand names Soloxine or ThyroTabs and available only by prescription, levothyroxine sodium is available in chewable tablet form for dogs. It can take a while for you to notice whether the drug is working once you start its daily administration. Your dog's hypothyroidism symptoms might not noticeably start resolving for a few weeks after the start of drug therapy. However, they should not get worse.

High Dose Side Effects

If your vet prescribed too high a dose of levothyroxine, he could show symptoms of thyrotoxicosis, better known as hyperthyroidism, or too much thyroid hormone in his system. Signs include constant hunger and excessive water consumption. That leads to increased urine production. Some dogs exhibit heat intolerance, and also might undergo personality changes, including excitability. If your dog experiences any side effects once receiving levothyroxine, call your vet as soon as possible. Your vet's adjusting of the dosage usually clears the problem. Expect to bring your dog to the vet for regular blood tests and monitoring.

Precautions and Contraindications

Your vet will exercise caution in prescribing levothyroxine if your dog is pregnant or nursing. She might wait until the puppies are weaned before starting the dog on the drug. If your dog has cardiac disease or high blood pressure, levothyroxine treatment is contraindicated. Dogs with diabetes or Addison's disease may require special veterinary monitoring while receiving levothyroxine, as their other medication requires adjustment. Very old dogs probably shouldn't receive levothyroxine. Some dogs might experience an allergic reaction to the drug. Symptoms include breathing issues, hives and facial swelling. If your dog experiences an allergic reaction, take him to the vet immediately.