Thyroid hormones have a major influence on all major organs/systems and adequate levels are important for optimal function. Thyroid dysfunction is a common condition that affects between 3% and 21% of the population with prevalence being more common in women and in older individuals. Hypothyroidism is a common condition of thyroid hormone deficiency, which is readily diagnosed and managed but potentially fatal in severe cases if untreated. The definition of hypothyroidism is based on statistical reference ranges of the relevant biochemical parameters and is increasingly a matter of debate. Clinical manifestations of hypothyroidism range from life threatening to no signs or symptoms. The most common symptoms in adults are fatigue, lethargy, cold intolerance, weight gain, constipation, change in voice, and dry skin, but clinical presentation can differ with age and sex, among other factors.This is a comprehensive book on Hypothyroidism, including an Introduction of the disease, epidemiology, its pathophysiology, followed by clinical features, investigations and management. A summary and bibliography has been provided at the end.