What is the thyroid gland and what does it do?
The thyroid gland is one of the most important glands in the body. It is located in the neck and is composed of two lobes, one on each side of the trachea (windpipe). This gland is controlled by the body’s master gland, the pituitary gland, located at the base of the brain.
The thyroid gland regulates the rate of metabolism. If it is hyperfunctional, metabolism speeds up. If it is less functional than normal, metabolism slows down. The latter is the basis for the clinical signs of hypothyroidism.
What causes hypothyroidism?
Hypothyroidism is almost always caused by one of two diseases: lymphocytic thyroiditis or idiopathic thyroid gland atrophy.
The former disease is the most common cause of hypothyroidism and is thought to be an immune-mediated disease. This means that the immune system determines that the thyroid is abnormal or foreign and attacks it. It is not known why the immune system does this.
Idiopathic thyroid gland atrophy is also poorly understood. In this case, normal thyroid tissue is replaced by fat tissue in what is considered a degenerative disease.
These two causes of hypothyroidism account for more than 95 percent of cases. The other five percent are due to uncommon diseases, including cancer of the thyroid gland.
What are the clinical signs?
When the rate of metabolism slows down, virtually every organ in the body is affected in some manner. Most affected dogs have one or more of several “typical” physical and/or chemical abnormalities.
These include:
  • Weight gain without an increase in appetite
  • Lethargy and lack of desire to exercise
  • Cold intolerance (dog gets cold easily)
  • Dry hair coat with excessive shedding
  • Very thin hair coat to near baldness
  • Increased pigmentation in the skin
  • Increased susceptibility to skin and ear infections
How is it diagnosed?
The most common method is to test for the T4 level. This is a measurement of the main thyroid hormone in a blood sample. If it is below normal and the correct clinical signs are present, the test is diagnostic.
However, testing for the T4 level can be misleading because some dogs that are not hypothyroid may have subnormal levels. This happens when another disease is present or when certain drugs are given.
If hypothyroidism is suspected but the T4 level is normal, confirmatory tests can be performed. These are more expensive, so they are not used as first line tests.
Can hypothyroidism be treated?
Hypothyroidism is treatable, but not curable. It is treated with oral administration of a thyroid replacement hormone. This drug must be given for the rest of the dog’s life.
How is the proper dose determined?
A standard dose is used initially based on the dog’s weight. However, after about one month of treatment, further testing is done to verify that the thyroid hormone levels are normal. In some dogs, the dose will need to be further adjusted every six to 12 months.
What happens if the medication is overdosed?
If the medication is overdosed, signs of hyperthyroidism can be seen. These include hyperactivity, lack of sleep, rapid weight loss and an increase in water consumption. If any of these occur, notify us and a proper adjustment can be made.