Hypothyroidism

GENERAL DESCRIPTION 

Hypothyroidism is the most common endocrine skin disease diagnosed in dogs.  It most often affects dogs between the ages of 4 to 10 years, and it is seen in medium to large breeds, and rarely seen in the small or miniature breeds.  Hypothyroidism occurs equally in both males and females, and very rarely occurs naturally in cats.   The thyroid gland has several key functions but the most notable is the role it plays in regulating the metabolism.  

SOURCE & CAUSE(S) 

The thyroid gland is located in the neck below the voicebox or larynx.  This gland produces two hormones Thyroxine (T4) and Triiodothyronine (T3), both play vital roles in maintaining the metabolic rate of the body, when insufficient amounts of hormone are released, the 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.

SIGNS & SYMPTOM

• 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

DIAGNOSIS & TESTS

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 pet 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 the amount and different kinds of red and white blood cells 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 have 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, it is known as Triiodothyronine.  This test is not nearly as accurate in early stages of hypothyroidism and occasionally will be normal even when the T4 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 the 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 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 and injection the TSH, thus leading the diagnosis of Hypothyroidism.  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 & MANAGEMENT

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 dogs 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 diminish.

PREVENTION & HELPFUL TIPS

There is no way to prevent hypothyroidism, but early detection can prevent  your dog from displaying severe symptoms.  Having a T4 test done annually included in your yearly wellness exam can be helpful to 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 Schnauzer
• German Shepherds and mixed breeds appear to be at a reduced risk of contracting the disease.