Hypothyroidism is more common in medium to large size dogs and is caused by a lowered production and release of thyroid hormones. A laboratory blood test called a “T4” may be ordered by a veterinarian to confirm or monitor Hypothyroidism. The condition is not common in cats. Hyperthyroidism is common in cats and is caused by an excessive concentration of and circulation of a thyroid hormone. This is a rare condition for dogs.