Right Diagnosis lists 40 medical causes of sunken eyes. The most common of these are dehydration, malnutrition, fatigue, extreme weight loss, underactive thyroid and Horner's syndrome, a condition of the nervous system. Several genetic disorders, such as whistling face syndrome, RHYNS syndrome and Krause syndrome also cause sunken eyes, but these conditions are rare.
Dehydration occurs when the body's fluid losses exceed intake. According to the Mayo Clinic, excessive perspiration, fever, vomiting and diarrhea may cause dehydration if fluid intake is inadequate. The symptoms of mild-to-moderate dehydration are a dry mouth, severe thirst, decreased urine output, headache, tiredness and constipation. Sunken eyes, little or no urine output, dark urine, little or no tear production and decreased elasticity of the skin are signs that dehydration is severe and requires immediate medical care.
Malnutrition occurs when the body lacks adequate nutrition. According to MedlinePlus, malnutrition results from the inadequate intake of sufficient nutrients or malabsorption of nutrients due to illness, such as cancer, Crohn's disease, celiac disease and AIDS. Anorexia nervosa, a psychological condition in which a distorted body image leads to self-starvation, also causes malnutrition, according to the Mayo Clinic. Extreme weight loss, fatigue, dehydration, an obsession with food and refusing to eat are signs of anorexia nervosa. If left untreated, anorexia nervosa may lead to severe biochemical disturbances and even death.