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Hyperthyroidism - Issue Description

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Issue Description

Hyperthyroidism in dogs is a condition that occurs when the gland overproduces thyroxine. It is a rare condition for dogs. When it does occur, it normally affects middle-aged and older dogs.


  • Elevated heart rate
  • Weight loss
  • Increased appetite
  • Nervousness
  • Increased water intake and urination
  • Increased activity

  • Less commonly seen are:

  • Decreased appetite
  • Decreased activity
  • Weakness
  • Labored breathing

  • Diagnosis

    Diagnosis of hyperthyroidism is based upon clinical signs and simple blood tests called T4. This is a simple and innexpensive test, but it can be falsely low due to drugs or non-thyroid illness for both hyperthyroidism and hypothyroidism. A test called Free T4 (or FT4) is much better because it reflects true thyroid function. Free T4 testing is relatively expensive.

    Normal levels of thyroid hormones in the blood do not necessarily rule out hyperthyroidism since these levels are effected by other organ systems involved. A second blood test one to two weeks later is necessary in these cases.


    Hyperthyroidism is very rare in dogs (occurring in less than 1 or 2% of dogs), who instead tend to have the opposite problem: hypothyroidism. When hyperthyroidism does appear in dogs, it tends to be due to over-supplementation of the thyroid hormone during treatment for hypothyroidism. Symptoms usually disappear when the dose is adjusted.

    Occasionally dogs will have functional carcinoma in the thyroid; more often (about 90% of the time) this is a very aggressive tumor that is invasive and easily metastasizes or spreads to other tissues (esp. the lungs), making prognosis very poor. While surgery is possible, it is often very difficult due to the invasiveness of the mass in surrounding tissue including the arteries, the esophagus, and windpipe. It may only be possible to reduce the size of the mass, thus relieving symptoms and also allowing time for other treatments to work.

    If a dog does have a benign functional carcinoma (appears in 10% of the cases), treatment and prognosis is no different from that of the cat. The only real difference is that dogs tend to appear to be asymptomic, with the exception of having an enlarged thyroid gland appearing as a lump on the neck.


    Hyperthyroidism is treated medically with a drug called Tapazol, which inhibits the production of thyroid hormone. There are numerous side effects, which may effect the treated animal including anorexia, vomiting and lethargy. Surgical removal of one or both thyroid glands is an option, which has fewer complications, although it is often necessary to normalize thyroid hormone levels medically prior to surgery. Hypothyroidism is the most common complication of surgery. This condition is easily treated with oral medication, Thyroxine.

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