There is no single cause of dry skin. Dry skin causes can be classified as external and internal. External factors are the most common underlying cause and are the easiest to address. External factors include cold temperatures and low humidity, especially during the winter when central heaters are used. Internal factors include overall health, age, genetics, family history, and a personal history of other medical conditions like atopic dermatitis. In particular those with thyroid disease are more prone to developing dry skin.