Is 100 lbs severely underweight for a 26 year old woman who is 5'5" with a very small frame? there are no protruding bones or health problems.

You need blood test: You need to have physical and basic lab work to rule out conditions that causes weight loss such as hyperthyroidism .

What's in a name?: I doesn't matter what you call it, severely or not severely; 100 pounds for 65 inches is too small. The worry here is that the patient says she has no protruding bones. Since it is impossible for a woman NOT to have protruding bones at 100 lbs and 65 inches, she may have a perception problem. That being said, there are some women who are ok at that height and weight, but they're better at 115 lbs.