First, I would suggest talking to your primary care doctor
. Although some people are genetically thin regardless of what they do (and therefore this can be normal), it is often worthwhile having a thorough medical check up in cases of extreme thinness that do not have an explanation.
This is because there are some relatively common medical conditions that can cause people to be underweight and can have subtle symptoms that might not be picked up. Some of these common conditions might include an over active thyroid gland and also something called celiac disease, which is a condition characterized by an allergic reaction to a wheat protein called gluten, leading to intestinal inflammation and weight loss. Inflammatory bowel disease
, which is another inflammatory condition of the intestine, can also occasionally present subtly as just extreme thinness.
Assuming your medical check up with your primary care doctor does not reveal any underlying medical reason for your thinness, then you could consider next stopping at a nutritionist
. Your doctor should be able to refer you. They will be able to give you specific recommendations about how to improve your nutrition and your caloric intake in a way that is safe and healthy for you.