Is a bmi of 17.8 considered healthy?

Stability is key: Certainly this is below 18.5, which is the low end of normal. However, if you have always been thin, then this is really nothing to worry about. If there are signs of eating disorders, these in themselves have significant health consequences and need to be addressed.

Underweight.: That BMI is underweight, and therefore unhealthy. This should be investigated, as there may be a medical reason for it to be so low.