Levothyroxine Side Effects in Dogs

Levothyroxine is prescribed for dogs that have underactive thyroid glands, a condition known as hypothyroidism. Because hormones produced by the thyroid gland are vital to the normal function of every cell in the body, the effects of hypothyroidism can be felt everywhere, including the heart, muscles, brain, kidneys, nervous system and skin and coat. Levothyroxine can restore hormone levels to normal and make most symptoms subside. The tricky part is getting the dose right, as every dog's needs are different and the dose may require substantial adjustment. Certain symptoms will let you know that your dog's medication must be adjusted.

  1. Correct Dose

    • Levothyroxine causes no side effects when given at the correct dose. If you notice your dog acting differently while taking the medication, it may indicate either an allergic reaction or an improper dose. Both conditions can vary widely in severity, so consult your vet as soon as possible.

    Allergic Reaction

    • An allergic reaction to levothyroxine can take many forms. Swelling of the face, hives and compulsive scratching are typical signs that most people associate with an allergic reaction, but it can also affect the digestive system, causing vomiting and diarrhea. More severe cases may include pale gums, cold limbs, shock or even a coma, so immediate medical treatment is necessary. Your vet will attempt to purge the medication from your dog's system or, failing that, to neutralize what has already been digested. Once the danger is removed, the dog will be treated for his symptoms until he is back to normal. Unfortunately, an allergic reaction means you and your veterinarian will have to find another way to manage your dog's hypothyroidism.

    Overdose

    • There are two kinds of overdose: an acute overdose occurs when too much medication is taken at the same time; a chronic overdose occurs when a slightly higher than normal amount is given over a long period of time. Acute overdoses may occur when two family members unknowingly medicate their dog at the same time, or when the dog finds the pill bottle and eats the pills. In these situations, the dog may react violently with vomiting and diarrhea. Some dogs exhibit behavior changes like lethargy or excitement that can be attributed to another source, so it is important to keep track of your dog's medication at all times. An acute overdose is poisoning and requires immediate medical treatment.

      Chronic overdoses are a different story and generally occur when the dog's correct dose is not yet fully understood. Because levothyroxine is meant to treat an underactive thyroid, it makes sense that the symptoms of a chronic overdose mimic those of an overactive thyroid. This may include increased appetite and water consumption, increased urination, increased activity and behavioral changes. This situation is usually rectified by lowering the dog's dose, but many vets check the hormone levels in the blood first so they know how much they need to lower the dose while keeping the medication effective.