The thyroid is one of the largest endocrine glands in the body. The thyroid controls how quickly the body burns energy, makes proteins, and how sensitive the body should be to other hormones. It participates in these processes by producing thyroid hormones, principally thyroxine and triiodothyronine. These hormones regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body. The thyroid also produces the hormone calcitonin, which plays a role in calcium homeostasis. As discussed in this book, both excess and deficiency of thyroxine can cause disorders. Thyrotoxicosis or hyperthyroidism, a cause of Graves Disease, is caused by an excess of circulating free thyroxine, free triiodothyronine, or both. Hypothyroidism is the case where there is a deficiency of thyroxine, triiodiothyronine, or both. Clinical depression can sometimes be caused by hypothyroidism. This book presents a wide variety of research on the role of thyroid hormones and their effects.