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Dog: Hypothyroidism

General information

Hypothyroidism occurs when the thyroid glands do not produce enough thyroid hormone. The thyroid glands in the dog are located on either side of the trachea (windpipe) and exist as two separate halves. The thyroid gland produces two thyroid hormones, thyroxine (T4) and triiodithyronine (T3).

Thyroid hormones affect multiple body systems. They play an essential role in regulating growth, metabolism, immune function and heart function.

Production of thyroid hormone is controlled by the pituitary gland located at the base of the brain. The pituitary gland produces thyroid stimulating hormone (TSH) which stimulates the thyroid gland to produce thyroid hormones.

Causes

Immune-mediated disease or primary hypothyroidism is the most common cause for hypothyroidism and is recognized as a heritable condition. The body’s own immune system becomes over-reactive and attacks the normal thyroid gland, destroying the thyroid hormone producing cells. The initiating factor of this destruction is unknown but thyroid autoantibodies are known to be present. Puppies can be born with (congenital) hypothyroidism. While this is uncommon, these puppies are also born with physical defects and malformations. Other causes of hypothyroidism include a dietary deficiency in iodine and rare tumors of the thyroid which infiltrate and destroy thyroid tissue.

Secondary hypothyroidism is caused by a diseased pituitary gland and decreased secretion of TSH. Tumors of the pituitary gland, malnutrition and chronic corticosteroid use can result in decrease TSH secretion.

Cardinal symptom

Lethargy

Symptoms

Middle aged dogs (4-10 years) are most commonly affected. Breeds prone to hypothyroidism include the Doberman Pinscher, Golden Retriever, Irish Setter, Great Dane and Dachshund.

While clinical signs can vary between individual dogs, they do involve multiple organ systems.

  • General changes: Lethargy, mental dullness, weight gain with decreased appetite.
  • Skin changes: Symmetrical hair loss, dry, dull haircoat, change in haircoat color, poor wound healing. thickened, pigmented skin.
  • Reproductive changes: Infertility, shortened, irregular heat cycles, low sperm count, lack of libido.
  • Cardiovascular changes: Slow heart rate, abnormal heart rhythms.
  • Nervous system changes: Disorientation, seizures, megaesophagus and laryngeal paralysis.
  • Musculoskeletal changes: Weakness, muscle wasting and exercise intolerance.

Diagnosis

Diagnosis of canine hypothyroidism is not simple and straightforward. It should include a complete physical examination and a combination of laboratory tests including a complete blood count (CBC), serum biochemistry and a urinalysis to rule out other disease. Testing of thyroid hormones, TSH and autoantibody levels must be interpreted carefully and used in conjunction with the physical examination and history of the dog. A biopsy of the thyroid gland is the most effective method to diagnose hypothyroidism. However, this procedure requires anesthesia and is considered invasive.

Treatment

Hypothyroidism is treated by administering an oral synthetic replacement thyroid hormone called levothyroxine. The dosage will vary with the individual dog. Initially, laboratory results should be rechecked monthly after starting hormone therapy. Once the thyroid hormone levels are stable and within normal range, annual rechecks are recommended.

Prognosis

Following diagnosis, hypothyroidism is relatively easy and inexpensive to treat. While life-long medication is necessary, the prognosis is good and most dogs respond well to treatment.

Update version: 4/24/2014, © Copyright by www.enpevet.de
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