Yes. Under the Americans with Disabilities Act (ADA), if an employer has a reasonable belief that an employee may be unable to perform her job or may pose a direct threat to herself or others, the employer may ask for medical information. However, the employer may only obtain the information needed to make an assessment of the employee's present ability to perform her job and to do so safely. In this instance, because the reporter's job frequently requires her to meet short deadlines, the employer may ask her to provide a doctor's note or other documentation indicating whether there are any limits on how many hours a day she can work
Source: EEOC Guidance: Questions and Answers about Diabetes in the Workplace and the Americans with Disabilities Act (ADA), http://www.eeoc.gov/laws/types/diabetes.cfm; reported in Employment Practices Guide, New Developments 5372.