The thyroid is an endocrine gland, located at the front of the neck, which produces the thyroid hormones. Due to certain factors, there may arise a disruption in the normal functioning of the thyroid gland. This condition is termed as thyroid disease. When the thyroid gland does not produce sufficient thyroid hormone or produces excess hormone, it leads to the conditions called hypothyroidism and hyperthyroidism respectively. Various structural abnormalities of the thyroid gland occur such as in the case of goiter, lingual thyroid and thyroglossal duct cyst. Tumors of the thyroid can also occur and can be cancerous or non-cancerous. Side effects of medicines such as amiodarone, some types of interferon and IL-2, lithium salts, dopamine agonists, etc. can affect the thyroid gland. The diagnosis of thyroid disease is primarily determined through blood tests that measure different thyroid hormones, antithyroid antibodies, thyroidglobulin levels, calcitonin levels, etc. In some cases, ultrasound, radioiodine scanning or biopsy may be required. There are several medication pathways for the treatment of thyroid diseases. This book is compiled in such a manner, that it will provide in-depth knowledge about the study of the thyroid gland and associated disorders. The aim of this book is to present researches that have transformed the understanding of thyroid diseases. It is appropriate for students seeking detailed information in this area as well as for experts.