Hypothyroidism is a condition with insidious onset and multiple etiologies. Because of this, it often goes unrecognized. Improved understanding of the pathophysiology of the various forms of hypothyroidism has led to improved screening recommendations to detect the disorder in at-risk individuals, often before the onset of symptoms. While thyroid hormone replacement remains the mainstay of therapy, there are efforts to develop treatments for the immune reaction which is responsible for hypothyroidism in most individuals, holding promise for a cure. This book provides a comprehensive review of our current knowledge of the etiology of hypothyroidism and provides recommendations for screening practices and therapeutic approaches.