Hypothyroidism refers to an underactive thyroid gland, and is a condition almost exclusively seen in dogs. Hyperthyroidism refers to an overactive thyroid gland, and is controversially seen almost exclusively in cats.
The thyroid gland controls the production of thyroid hormone, which in turn has a profound effect on controlling and managing the metabolic rate in the body. Dogs suffering from an underactive thyroid tend to be slow, lethargic, sluggish, seek warmth, and gain weight easily. Cats that suffer from hyperthyroidism have a very fast metabolism, with an extremely rapid heart rate, a ravenous appetite, and extreme weight loss. In dogs hypothyroidism tends to cause bilaterally symmetrical hair loss on the body, and is associated with weight gain and obesity.
Many dogs with hypothyroidism also are prone to skin disease and allergies, and it is one of the tests I would look for early when treating a dog with allergies. The condition is most common in large breed dogs from the age of 6 to 8 onwards. In dogs the condition is most commonly caused by an autoimmune disease which attacks the thyroid gland and decreases the amount of thyroid hormone produced. In America, Dr Jean Dodds has linked autoimmune thyroid failure with excessive vaccination.
In cats hyperthyroidism seems to occur much later in life, in cats from 12 years plus, and is commonly found in conjunction with kidney failure. The condition is caused by a benign tumour growing on the thyroid gland, producing excessive thyroid hormone. Treatment of hypothyroidism in dogs is based on supplementing the animals with a daily or twice daily dose of synthetic thyroid hormone, which replaces the function of the normal missing hormone, and is quite safe and effective long-term. Biologically appropriate diets with added kelp which is a natural source of iodine, can assist stimulating thyroid function in dogs.
Treatment of hyperthyroidism and cats can be managed in several ways. The nodules on the thyroid glands can be surgically removed, or treated with radioactive iodine. There is also a medical approach using a drug called neomercazole, or a topical drug called methimazole, which both act to reduce the level of thyroid hormone produced. There is also an iodine free diets becoming available for cats that should also reduce thyroid function and reduce the excessive thyroid hormone production