Winter Doldrums or Thyroid Disease?

March 2007 - by Doctor Denise E. Roche, D.V.M.  

Lingering cloudy winter days in anticipation of spring can make all of us feel like lying around. But is the lethargy in your pet due to the weather or due to thyroid disease?

The thyroid is a bi-lobed gland in the neck that releases thyroid hormone- levothyroxine.  The thyroid is controlled by the pituitary gland, which is located in the brain.  The pituitary gland secretes TSH (thyroid –stimulating hormone) that causes the thyroid to secrete levothyroxine.  The thyroid gland is the master gland of the body’s metabolism.  An overactive gland raises the body’s metabolism and causes hyperthyroidism. An under active thyroid gland slows the body’s metabolism which results in hypothyroidism. Hyperthyroidism occurs primarily in cats while conversely dogs are afflicted with hypothyroidism.
Ninety-five percent of canine hypothyroidism occurs by one of two processes: idiopathic thyroid atrophy or lymphocytic thyroiditis.  Idiopathic atrophy occurs when normal thyroid tissue is replaced by fat or scar tissue.  Lymphocytic thyroiditis is an immune mediated disease in which antibodies attack and destroy the thyroid gland.  This form of hypothyroidism is more common in purebred dogs such as Golden Retrievers, Doberman Pinschers, Great Danes, Boxers, Dachshunds, Poodles and Cocker Spaniels.  Hypothyroidism is more prevalent in large breed dogs and most commonly occurs in dogs from 4 to 10 years of age.  About 1 in every 500 dogs will be diagnosed with the disease.

Dogs with hypothyroidism have one or more of the following symptoms; weight gain, inactivity, lethargy, cold intolerance, dry or lusterless haircoat, increased skin pigmentation, recurrent skin or ear infections, lack of hair regrowth after clipping, excessive shedding and hair loss. Often the hair epilates easily and begins thinning on the sides, the belly and the tail.  The hair loss on the tail can become so prominent that it will be referred to as “rat tail”. Early in the disease process the hair loss may be asymmetric, but as the disease progresses it usually becomes bilaterally symmetrical -especially over the flanks. The hair loss is not itchy unless secondary bacterial infections are occurring on the skin.  The lack of itchiness helps veterinarians differentiate between thyroid disease or allergies as the cause of the skin lesions.  Occasionally hypothyroidism can lead to generalized weakness, incoordination, dragging of the hind feet and head tilt.

Symptoms of hypothyroidism develop slowly and are progressive.  Often the clinical signs can be confused with other diseases. Blood testing must be done to diagnose hypothyroidism.  A total thyroxine (Total T4) blood level is used as a screening test.  If the patient has classic symptoms of hypothyroidism and has a low screening Total T4, usually no further testing is needed.  A diagnostic thyroid panel may be needed in subclinical cases.  This panel measures Total T4, Free T4, TSH and thyroglobulin autoantibodies. A low T4, a low Free T4 along with an elevated TSH is definitive for hypothyroidism.  TSH levels rise because the pituitary gland continues to increase its signaling hormone to the ineffective thyroid gland. Dogs with lymphocytic thyroiditis will often be positive for thyroglobulin autoantibodies.  Dogs can have gradational blood values so careful interpretation of the results is required. Nonthyroidal illness can drop Total T4 levels. This is known as sick euthyroid syndrome and these patients do not have thyroid disease.

Hypothyroidism is not a curable disease, but it is easily treatable. Daily oral thyroid replacement hormone medication can be administered.  Often a patient is started on a twice daily supplement of Levothyroxine and then weaned to once a day therapy when treatment response is evident.  Medication dose adjustments are based on monitoring T4 levels. A T4 level is usually checked 4 weeks after initiating therapy and then as needed thereafter.  Different brands of Levothyroxine frequently have different gastrointestinal absorption rates, so try staying with one name brand drug for consistent results. Hormone replacement therapy must be continued for the remainder of your dog’s life, but it is well worth it.