Hypothyroidism & Its Causes
Deficiency of thyroid gland activity, with underproduction of thyroxine, or the condition resulting from it. Common in adult dogs, particularly certain breeds, as a result of atrophy of the thyroid or lymphocytic thyroiditis.
Hypothyroidism is the most commonly diagnosed canine endocrine disease. Half of the cases are associated with autoimmune thyroiditis.
Primary hypothyroidism is characterized by progressive destruction of the thyroid gland. The gland atrophies and contains many lymphocytes. A rare form of primary congenital hypothyroidism is seen in Scottish Deerhounds.
Secondary hypothyroidism follows congenital or acquired abnormalities in the pituitary gland. Female dogs are more frequently affected than male dogs.6 Drugs used in veterinary medicine that are most likely to alter circulating thyroid hormone concentrations include the:
Myxedema coma is a rare, but life-threatening result of chronic, severe hypothyroidism in dogs. Multiple organ systems are involved, including the heart, lungs, skin, and CNS. Early recognition of the syndrome may improve survival chances, although the mortality rate is nearly 50%.4
Function Of Thyroid Glands
The thyroid glands are located in the neck on either side of the trachea (windpipe). These glands require sufficient quantities of iodine for normal production of the thyroid hormone. The thyroid hormone affects the metabolism of all tissues, particularly the metabolic rate. It is required for normal growth and development.
Hypothyroidism causes a reduced metabolic rate. Thyroid hormone (TH) is regulated through the hypothalamic-pituitary-thyroid system. During normal process, thyroid-releasing hormone (TRH) stimulates thyroid-stimulating hormone (TSH) released from the pituitary gland, which in turn stimulates production and release of T4 hormone from the thyroid gland. Active T4 subsequently inhibits further TRH and TSH release.5
Signs & Symptoms
Common signs include lethargy, increased appetite, weight gain, infertility, sensitivity to cold, a brittle, dry, dull coat, loss of hair, especially over the neck, back, and tail areas, and in some cases an oily skin, excessive wax in the ears, and chronic skin infections. Any dog with chronic skin infections should be tested for hypothyroidism.
As the disease advances, the skin darkens in color, thickens, and feels cool to the touch. The dog becomes more dull and irritable, and heart problems may develop. Only about 50% of hypothyroid dogs show the skin and hair changes. Older male dogs may develop cognitive disturbances characterized by aggression and seem to bite out of the blue.
Clinical disturbances associated with hypothyroidism vary among affected animals, and not every sign is seen in each animal patient. Many clinical signs associated with hypothyroidism are due to a reduction in metabolic rate.
A gain in body weight without an associated change in appetite occurs frequently. The weight gain may vary from slight to striking obesity. The animal usually is less active, and the owner may observe a reluctance to play or take walks. The inactivity also contributes to weight gain.
Dogs with hypothyroidism may have difficulty in maintaining normal body temperature and are often heat seekers. They will lie on or near sources of heat, such as registers, radiators, and electric blankets, and be reluctant to venture outdoors in cold weather. Excessive shivering may be observed, and the skin frequently feels cool.
In case of congenital hypothyroidism of Toy Fox Terriers, there is delayed ear and eye-opening, enlarged thyroid gland, stunted growth, and mental dullness.
Treatment involves the use of replacement thyroid hormone drugs, usually for life. Establishing the dosage requires veterinary supervision and periodic monitoring of the affected dog is absolutely necessary.
If the diagnosis is correct, the prognosis is good. Many dogs have increased activity in 10 days. Hair regrows in 4 – 6 months, and other signs improve in 2 months.
- Charles C. Capen – Thyroid Disorders in Animals
- Dr. Sandra Merchant – Hypothyroidism
- Jim E. Riviere, Mark G. Papich. Veterinary – Pharmacology and Therapeutics
- Wayne E. Wingfield, Marc R. Raffe. – The Veterinary Icu Book
- Shelly L. Vaden, Joyce S. Knoll, Francis W. K. Smith, Jr., Larry P. Tilley. – Blackwell’s Five-Minute Veterinary Consult
- Steven R. Lindsay. – Handbook of Applied Dog Behavior and Training: Procedures and protocols