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).  The thyroid gland regulates the rate of metabolism. If it is less functional than normal, metabolism slows down, and is the basis for the clinical signs of hypothyroidism.

Hypothyroidism is almost always caused by one of two diseases: lymphocytic thyroiditis or idiopathic thyroid gland atrophy.

Lymphocytic thyroiditis 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.

  • 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 (TSH levels) can be performed. These are more expensive, so they are not used as first line tests.


Your veterinarian will examine your dog to exclude other causes of coughing, such as heart disease, fungal and parasitic infections like heartworm disease, a collapsing trachea, and cancer. Dogs with kennel cough usually have a history of exposure, i.e. newly acquired pets from a shelter, pet store, or breeder, or pets that have recently been boarded, to a groomer, dog training classes, dog shows, or outings to the beach or dog parks. Based on the examination and history, your veterinarian will determine whether they suspect kennel cough.

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.