ICD-10-CM > R00-R99 > R50-R69 > R63 > R63.6 R63.6 Underweight Billable/Specific Copy Code Copy Description Use Additional code to identify body mass index (BMI), if known (Z68.-) Type 1 Excludes abnormal weight loss (R63.4)anorexia nervosa (F50.0-)malnutrition (E40-E46)