Hyperthyroidism is a condition in which the thyroid is working at below-normal levels. Symptoms of hyperthyroidism in dogs include hair loss, weight gain, intolerance to cold, a slow heart rate, lethargy and chronic constipation.
The thyroid produces two types of hormones that control metabolism. Hypothyroidism most commonly occurs in middle-aged, medium- and large-breed dogs, including golden retrievers, Irish setters and cocker spaniels. Dogs that are found to have elevated serum cholesterol levels during routine check-ups should be tested for hypothyroidism.
There is no cure for hypothyroidism, but dogs can be treated with hormone replacement therapy. Treatment consists of providing a synthetic hormone called L-thyroxine to affected dogs one to two times daily, depending on the dog's weight and hormone levels.