How Pathologic Conditions causing the underweight?

Illness can affect weight status in a number of ways. For example, fevers and infections increase the demand for energy, which if not met because of poor appetite, lead to loss of weight. Food intake may be severely limited by nausea, vomiting or diarrhoea in gastrointestinal disturbances. Metabolic rate may be greatly increased in hyperthyroidism resulting in underweight. Drug therapy may also alter taste or reduce appetite, leading to weight loss.