Hypothyroidism in Dogs


The thyroid glands located in the neck produce hormones that affect the function of many parts of the body. Although thyroid disease in cats often causes over productive glands and too much hormone, dogs with thyroid disease usually have a low production of thyroid hormones.

Overactive thyroid glands in the dog are rare and are usually associated with cancer. Hypothyroidism in dogs is usually caused by inflammation or shrinkage of the thyroid gland. Thyroid cancer can cause hypothyroidism although it does not occur commonly in dogs. Hypothyroidism occurs more commonly in medium to large breed dogs and usually in middle aged dogs. Breeds commonly affected include:

  • Golden retriever
  • Doberman pincher
  • Irish setter

The most common signs of low thyroid function in dogs include loss or thinning of the fur, dull hair coat, excess shedding or scaling, weight gain, reduced activity and reduced ability to tolerate the cold. The hair loss occurs primarily over the body, sparing the head and legs, and is usually not accompanied by itching or redness of the skin. Some dogs will have thickening of the skin and increased skin pigment, especially in areas of friction, such as the armpit (axilla). Hypothyroid dogs often have ear infections and show ear pain, redness, and odor. Hypothyroid dogs may also develop skin infections which may be itchy and result in sores on the body. The accumulation of substances called mucopolysaccharides can cause the muscles of the face to droop giving the dog a facial expression that is sometimes called “tragic”.

Less commonly recognized signs that may be seen in a small number of dogs with hypothyroidism include dilation of the esophagus (megaesophagus) causing regurgitation, and abnormal function of nerves or muscles leading to weakness or abnormal ability to walk.

Routine blood tests can be affected by hypothyroidism, although the changes are not consistent and can be subtle. Dogs may have a mild anemia and increased levels of cholesterol.

There are several blood tests that can be used to confirm a suspected diagnosis of hypothyroidism. Blood testing for hypothyroidism is often performed as a panel of several tests in order to increase the yield of these tests. The results of some of these tests can be influenced by the presence of other non-thyroid diseases, so test results must be considered in light of the whole picture.

Treatment of hypothyroidism is by giving oral replacement hormone for the rest of the dog’s life. Initially thyroid hormone is usually given twice daily. Once the hair coat begins to improve, some dogs can be maintained on once daily medication. There are two general forms of thyroid medication, T3 and T4. T4 is converted to T3 by the body. Most hypothyroid dogs given T4 will convert it to T3 so almost all hypothyroid dogs receive T4 (levothyroxine or L-thyroxine). A few dogs are unable to make this conversion and require T3 medication.

It usually takes 4 to 6 weeks before regrowth of the fur is apparent. In dogs with associated ear infection, the ears should be cleaned and treated with antibiotics either in the ear or by mouth. Blood levels of the T4 are often measured in order to fine tune the dose.


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