A dog's endocrine system consists of hormone-producing glands and tissues. The thyroid gland is part of the endocrine system and secretes thyroid hormones, which regulate metabolic function. By far the most-common thyroid disorder is hypothyroidism, when the gland produces insufficient hormones, although hyperthyroidism occasionally occurs in older dogs. Thyroid-disorder symptoms are many and varied, although classic hypothyroidism is marked by several hallmark symptoms.
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Causes
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There are various causes of thyroid disease, and they are not well understood. The most common theory is that a malfunctioning thyoid is a result of an autoimmune disease, and it might be "turned on" by exposure to some medications or chemicals. Two other suspected causes are a disruption of natural hormone production by spaying and neutering, and invasion of the gland by fat cells in overweight dogs.
It's also strongly suggested that genetics play a significant role in thyroid disease, and affected dogs should not be bred.
Symptoms
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Symptoms of hypothyroidism (insufficient thyroid hormone production, particularly the hormone thyroxine) are extremely varied. The classic symptoms are unexplained weight gain, lethargy and mental dullness, skin problems and dull or brittle fur. Other symptoms can include darkened skin, joint swelling and lamneness, skin and ear infections and behavioral changes. Hyperthyroidism (overproduction of thyroid hormones) is uncommon in dogs, and the symptoms are weight loss, nervousness or aggression and hyperactivity.
Time Frame
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Thyroid disease typically manifests in middle-aged and older dogs, from age 4 onward. It is rare in younger dogs. Since testing for thyroid disease is fairly simple, most vets will routinely run a thyroid test on a middle-aged and older dog that present the common symptoms of the disease.
Diagnosis
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Since thyroid-disease symptoms are so varied, they can be easily missed. Typically a veterinarian will consider no one single symptom, but a cluster of symptoms, along with the age and size of the dog. Hypothyroidism is rare in very small dogs and it tends to affect medium-to-large breeds like retrievers, setters, Rottweilers, standard dachshunds and cocker spaniels. It affects both sexes equally but some clinicians have noted greater prevalence in spayed females.
Diagnosis is made by one of several blood tests, ranging from the baseline T-4 test to several other blood tests that can narrow down the specific diagnosis and quantify insufficient thyroid hormones.
Treatment
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Hypothyroid is easily treated by giving the dog synthetic thyroxine (levothyroxine). The initial dose is based on the dog's weight, but since there will be individual differences in how the dog reacts and utilizes the hormone, he will be retested four-to-six months after starting the medication. Depending on the results of the blood tests and management of symptoms, the dose may be adjusted up or down.
Hyperthyroid is treated with Tapazol, which inhibits hormone production. One of the two thyroid glands can be surgically removed, but this sometimes results in hypothyroidism.
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