The sentence "She is underweight" is perfectly correct and usable in written English.
You could use it, for example, in a medical report or in an article about health and nutrition. Example: The doctor noted that, while her height was within the normal range, the patient was underweight and recommended that she increase her caloric intake.
We read of crabs, period-stained jeans, what happens to her labia when she is underweight.
Richie later admitted to Vanity Fair that she is underweight and Lohan owned up to an eating disorder.
Born to an anaemic mother, she is underweight from birth but does not receive the rich colostrum or early breast milk that can help to overcome those earliest deficits.
Each child is weighed and, if he or she is underweight, the mother is given Plumpy'nut, a sweet-tasting paste made from peanuts, peanut oil, powdered milk and sugar, and fortified with vitamins and minerals.
Gertrude Atkinson's baby appeared normal, though she was underweight -- just 4 pounds 12 ounces when she was born -- and threw up nearly every day.
At the time, Ms. Walsh's daughter, Ava, was 8 months old, and, because she was underweight, Ms. Walsh found herself nursing "constantly".
She was underweight and had barnacles growing on her tail fin, or fluke, "which indicates an animal that wasn't moving very fast".
After citing the beatings that Mr. Rodriguez said he regularly inflicted on Nixzmary and the fact that she was underweight at the time of her death, the lead prosecutor, Ama Dwimoh, asked Dr. Wetli, "Doctor, isn't this classic child abuse syndrome?" Dr. Wetli replied that it was not.
And what if she were underweight -- wouldn't she therefore need health care all the more?
She ate lots of sugar and junk food, and when she started living with Hicks and O'Brien, she was underweight.
She echoed the sentiments in a recent interview with DISfunkshion magazine where she also admitted that Cutler will tell her when she's underweight.
