What is thyroid disease?
All mammals have a thyroid gland, including humans. It has many different functions but mostly controls the body’s rate of metabolism by producing different thyroid hormones. There are two important diseases to dogs and cats of the thyroid gland: hypothyroidism and hyperthyroidism. Hypothyroidism is most common in dogs. It occurs when the thyroid gland's hormone production is impaired and results in a decreased metabolism. Hyperthyroidism is most common in cats. This occurs when excessive thyroid hormone is produced and results in signs that reflect an increased metabolism.

What are the signs of thyroid disease?
In dogs, hypothyroidism is most common in 4 to 10 year olds. It will affect either gender and usually mid-to large size breeds and very rarely the toy and miniature breeds. A deficiency of thyroid hormone affects the function of all organ systems and clinical signs are often non-specific. Many of the clinical signs associated with canine hypothyroidism are directly related to slowing of metabolism, which results in development of mental dullness, lethargy, intolerance of exercise, and weight gain without a corresponding increase in appetite. Mild to marked obesity usually develops. It may be difficult to maintain body temperature so the classic hypothyroid dog is a heat seeker. Skin and hair coat changes are common. The skin may be dry with excessive shedding or have slow hair growth. Very often the hair will be thin or bald on both sides of the body, the backs of the thighs and top of the tail.

In cats, hyperthyroidism is most common in middle age to older cats. The most common signs include weight loss, increased appetite, hyper-excitability, excessive drinking, excessive urinating and enlargement of the thyroid gland. Gastrointestinal signs are also common and may include vomiting, diarrhea, and increased stool volume. Cardiovascular signs include rapid heart rate, heart murmurs, difficulty breathing, enlarged heart and congestive heart failure.

How can you diagnose thyroid disease?
The veterinarian will get a good history from the owner about the animal, then perform a thorough physical examination. In dogs, hypothyroidism is probably one of the most over-diagnosed diseases because many diseases and conditions mimic hypothyroidism. A definitive diagnosis requires careful attention to clinical signs and a routine blood test for thyroid hormone levels. The most important thyroid hormone for testing is T4. Finding a low free T4 concentration is consistent with hypothyroidism and it is just the opposite in cats with hyperthyroidism. Finding high thyroid hormone concentrations is the hallmark of hyperthyroidism and confirms the diagnosis.

How do you treat thyroid disease?
In dogs with hypothyroidism, we have to replace the thyroid hormone which the body is not producing enough of and we do have to do this for the rest of the dog’s life. The medication is simple, cheap and effective. Thyroxine, also known as the thyroid hormone T4, is the drug of choice for thyroid hormone replacement. The most important indicator of the success of therapy is improvement of the clinical signs and reversal in changes in the hair coat, skin and body weight which may take 1-2 months.

Cats with hyperthyroidism can be treated by radioiodine therapy, surgical removal of the thyroid gland or long-term administration of an anti-thyroid drug. Radioactive iodine is a simple, effective and safe treatment where the radioiodine is concentrated within the thyroid, where it selectively destroys the hyper-functioning thyroid tissue. Treatment with the anti-thyroid drug methimazole is very common and it controls hyperthyroidism by blocking thyroid hormone production. Then we will routinely test the blood thyroid hormone levels to ensure we are giving the correct dose for both dogs with thyroid hormone replacement and with cats getting the anti-thyroid drug.