The clinical signs of hypothyroidism are caused by a decrease in normal thyroid hormone activity. The disorder may be acquired (a progressive deficiency of thyroid hormone) or congenital (meaning the animal is born with the disorder). The acquired form is the most common disorder of the endocrine system in dogs. It occurs as a result of gradual atrophy of the thyroid gland or of gradual infiltration and replacement of the thyroid gland with lymphocytes due to an autoimmune process (lymphocytic thyroiditis).
Acquired hypothyroidism is generally seen in middle-aged (4 to 10 years) mid - to large breed dogs. Congenital hypothyroidism is very rare.
The changes due to gradually decreasing levels of circulating thyroid hormone are slow and insidious. Early signs (which are usually not recognized as being related to hypothyroidism) include lower energy levels and increased susceptibility to infections. As the disease progresses, you will likely notice changes in your dog's hair coat - symmetrical hair loss with or without darkening of the skin, and dry or greasy hair. Other signs of hypothyroidism include a slow heart rate, lethargy, mental dullness, intolerance to cold, infertility in males and females, constipation, and weight gain. Less commonly, a dog with hypothyroidism may experience heart disease, a bleeding disorder, profound muscular weakness associated with abnormalities in the muscles or nerves, or another endocrine disorder such as diabetes mellitus.
Thyroid hormones are essential for normal growth and maturation of the nervous and skeletal systems. Puppies with congenital hypothyroidism will have stunted growth as well as many other abnormalities. Severely affected puppies most likely die before weaning.
Because there is such a broad range of possible clinical signs, hypothyroidism can be quite difficult to diagnose. Blood tests may show certain suggestive (but non-specific) abnormalities. If your veterinarian suspects this condition, s/he will perform laboratory tests to assess thyroid function.
The standard treatment is levo-thyroxine given once a day. This must be continued for life. Within a week of starting treatment, your dog's attitude and activity levels should improve. It can take up to 6 weeks before there is noticeable improvement in the skin and haircoat, but eventually all abnormalities should completely resolve.
Dogs on thyroid replacement can live a normal life. Your veterinarian will monitor thyroid hormone levels periodically and adjust the dose if required. Certain other illnesses may also necessitate dosage adjustment.
Congenital hypothyroidism is treated the same way. Clinical signs are usually completely reversed with subsequent normal physical and mental growth and development, as long as the condition is recognized and treatment started very early (3 to 4 months of age).
- CLINICAL PATHOLOGY: Suggestive findings include hypercholesteremia, mild nonregenerative anemia, elevated serum creatine kinase (CK), and hypoglycemia
- CONFIRMATION: Measurement of free T4 (FT4) and canine thyroid-stimulating hormone (cTSH) are the tests of choice to diagnose hypothyroidism, and to differentiate the primary from the secondary form. [Secondary hypothyroidism is much less common.] Some laboratories can also test for thyroid autoantibody levels (TgAA) which is helpful in the early diagnosis of autoimmune thyroiditis. This may provide useful information for breeders. [See reference 2 for a good discussion of tests.]
- RADIOGRAPHS: In congenital hypothyroidism, there are typical radiographic changes including epiphyseal closure, shortened vertebral bodies, kyphosis, and arthritis. Epiphyseal dysgenesis (ragged epiphyses with a few foci of calcification) is pathognomonic for congenital hypothyroidism.
Although inheritance of this disorder has not been determined, it is advisable not to breed affected dogs. The OFA (Orthopedic Foundation for Animals) maintains a thyroid registry based on assessment of FT4, cTSH, and TgAA, which is useful for breeders attempting to choose dogs free of hypothyroidism for a breeding programme.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
Chastain, C.B., Panciera, D.L. 1995. Hypothyroid diseases. In S.J. Ettinger and E.C. Feldman (eds.) Textbook of veterinary Internal medicine. p. 1487-1501. W.B. Saunders Co., Toronto.
Peterson ME, Melian C, Nichols R. Measurement of serum total thyroxine, triodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. JAVMA 1997; 211(11):1396-1402. This reference clearly explains the different tests available, and the significance and interpretation of the results.
Orthopedic Foundation for Animals: www.offa.org
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