Diseases of the thyroid gland are very common, affecting millions of Americans. The most common diseases are an over- or under-active gland. These conditions are called hyperthyroidism (e.g., Grave's disease) and hypothyroidism. Sometimes the thyroid gland can become enlarged from over-activity (as in Grave's disease) or from under-activity (as in hypothyroidism). An enlarged thyroid gland is often called a "goiter." Sometimes an inflammation of the thyroid gland (Hashimoto's disease) will cause enlargement of the gland. Thyroid drugs supplies your body with a synthetic hormone to replace the thyroid hormone your body is not producing effectively.