Hypothyroidism is a state in which the thyroid gland does not make enough thyroid hormone. Iodine deficiency is often cited as the most common cause of hypothyroidism worldwide but it can be caused by many other factors. It can result from the lack of a thyroid gland or from iodine-131 treatment, and can also be associated with increased stress. In this book, the authors present current research in the study of the diagnosis and screening, genetic influences and treatment options for hypothyroidism. Topics include low T3 syndrome and subclinical hypothyroidism; age-based TSH levels in hypothyroidism treatment; overt hypothyroidism, subclinical hypothyroidism and isolated hypothyroxinemia during pregnancy; and thyroid hypofunction in pregnancy.