Hyperthyroidism in Dogs

Hyperthyroidism in Dogs

Hypothyroidism is the most common endocrine skin disease diagnosed in dogs. It most often affects dogs between the ages of four to ten years, mostly medium to large breeds, and is rarely seen in the small or miniature breeds. Hypothyroidism occurs equally in both males and females. The thyroid gland has several key functions, but the most notable is the role it plays in regulating the metabolism.  

Signs & Symptoms of Hypothyroidism

  • Low energy/lethargy
  • Hair loss
  • Dry hair coat or excessive shedding
  • Weight gain with no increase in appetite
  • Heat seeking behavior
  • High blood cholesterol
  • Anemia
  • Reproductive disturbances in intact dogs

Causes of Hypothyroidism

The thyroid gland is located in the neck, below the voicebox or larynx. This gland produces two hormones Thyroxine (T4) and Triiodothyronine (T3), both playing vital roles in maintaining the metabolic rate of a dog’s body. When insufficient amounts of these hormones are released, the dog’s metabolism slows. More than 95% of hypothyroid cases are caused by the destruction of the thyroid gland, either because of an autoimmune disorder or due to an unknown cause. In fewer cases, a tumor of the pituitary gland may be the cause.

Diagnosis of Hypothyroidism

Diagnosis always begins with a complete history and a physical exam. There are several different tests used to diagnose hypothyroidism. The diagnostic path chosen will depend largely on the symptoms your dog has and the availability of diagnostic tools your veterinarian has. Your veterinarian will be most likely to recommend some variation of the following:

  • CBC/Chemistry Panel - These blood tests will evaluate various internal organ functions, including the heart, liver, kidneys, pancreas, metabolism, and electrolyte balance. The CBC is a measure of the amount and different kinds of red and white blood cells that are present in the body.
  • Baseline T4 or Total T4 (TT4) - This blood test will determine the level of T4 thyroid hormone in the bloodstream. Dogs that are having a failure of the thyroid gland will reveal a low level of T4 in the bloodstream. However, there are other conditions that can cause a lowering of T4, so if this test is positive for hypothyroidism an additional, more definitive test is run to confirm the diagnosis of hypothyroidism.
  • Baseline T3 Test - This blood test will determine the level of T3 thyroid hormone present in the bloodstream. T3 is another form of thyroid hormone produced by the thyroid gland, known as triiodothyronine. This test is not nearly as accurate in the early stages of hypothyroidism and occasionally will be normal even when the T3 level is low. The T3 test is NOT a reliable test to diagnose hypothyroidism when used alone. Because of this, this test is usually used in combination with the T4 test or with the TSH stimulation test. When the T3 is used with the other two tests, it can give a much clearer picture of the hormone levels found the bloodstream.
  • TSH Stimulation Test - This is generally thought to be the most definitive blood test used for confirming hypothyroidism in dogs. This is a timed test, so you should expect to have your dog stay in the hospital for a day. A small amount of thyroid stimulating hormone (TSH) is injected into the bloodstream. Then, in about six hours, a blood sample is drawn for evaluation of the T4 level. If your dog has hypothyroidism, he/she will not have a higher T4 level than that of the baseline value. This is because they are not exhibiting the ability to stimulate the thyroid hormone following an injection of the TSH. NOTE: Dogs that have high T4 following the TSH test have some other disease process going on that is giving them a low T4 on the baseline T4 test. Your veterinarian will now know that hypothyroidism is not the cause of the low level T4, and will continue to diagnose.     

Treatment for Hypothyroidism

Even though the diagnosis of hypothyroidism can seem long and complicated, the good news is that hypothyroidism is easily treated. Hypothyroidism can be treated by replacing the T4 hormone in the gland, through use of the synthetic hormone thyroxine for the remainder of your dog’s life. Your dog will need to be routinely checked once or twice a year to make sure their hormone levels are in check and the dosage of medication is still pertinent. Once hormone replacement begins, the symptoms will start to diminish and they will eventually disappear.

Prevention of Hypothyroidism

There is no way to prevent hypothyroidism, but early detection can prevent your dog from displaying severe symptoms. Having a T4 test done annually in your yearly wellness exam can be helpful for early detection. The following are the breeds that appear to have the most predisposed markers for developing hypothyroidism:

  • Golden Retrievers
  • Doberman Pinschers
  • Greyhounds
  • Irish Setters
  • Dachshunds
  • Cocker Spaniels
  • Airedale Terriers
  • Miniature Schnauzers

On a side note, German Shepherds and mixed breeds appear to be at a reduced risk for contracting the disease.