Introduction:
The thyroid is a very small gland found in the neck region of the animal. The thyroid gland is responsible for producing many hormones important for the regulation of metabolism. The most common thyroid problem in dogs results in low levels of thyroid hormones being produced (hypothyroidism). Hyperthyroidism, or high thyroid hormone levels, can occur in dogs, but it is rare.Hypothyroidism most often occurs in dogs that are 4-10 years of age. The disease is more common in the Golden Retriever, Doberman Pinscher, Dachshund, Miniature Schnauzer, Airedale Terrier, Poodle, and Cocker Spaniel breeds.
Causative Agents: Hypothyroidism is most often caused by a loss of active thyroid tissue. This loss can be due to shrinkage or destruction of the thyroid gland itself. Some dogs with hypothyroidism, however, may actually have problems with the pituitary gland. The pituitary gland is a master gland controlling other glands in the body and is found at the base of the brain. A healthy pituitary is necessary to stimulate the thyroid to produce its hormones.
Clinical Signs: The typical hypothyroid dog is lethargic, may gain weight, and often seeks out warm areas to rest. These dogs often have some hair loss or thinning on the back, tail, and under the neck. A dry hair coat with darkened, thick, itchy skin is not uncommon. Some dogs may experience reproductive problems and changes in blood chemistry (see page D80). Less common complications that may arise include breathing problems and paralysis of certain muscles controlling the "voice box."
Diagnosis: The thyroid hormone levels can be measured in a blood sample taken by a veterinarian. The results often require some interpretation as not all hypothyroid cases are straight-forward. Sometimes additional testing is required to pin-point the exact source of the problem.
Treatment: Once a diagnosis has been made, the dog is usually placed on one of the many synthetic or natural thyroxine containing products. Thyroxine is the major replacement hormone required in most dogs. Thyroxine absorption from the intestinal tract is extremely variable in dogs. For this reason, regular blood testing for continued thyroid monitoring is recommended to ensure proper and adequate hormone dosage. In general, a positive response should be noticed within 1-2 months after treatment begins. Behavioral changes such as increased activity level are usually noted much sooner than the correction of skin problems which usually require the full 2 months or more.