A research question can serve two important functions. A thesis sentence changes your topic or research question into a statement that you will support with your research. It presents a problem that you will examine with your evidence. It should go beyond the obvious. For example, almost everyone would agree with the following thesis statement, so no research is needed.
Malnourished children suffer from many pediatric illnesses.
It expresses your position in a full, declarative sentence, is not a question, not a statement of purpose, and not merely a topic. It reflects the fact that your paper is sharply focused on a single issue or topic. It establishes an investigative, inventive edge to your research and thereby gives a reason for all your work. It points forward to the conclusion. If you have trouble discovering your thesis at first, ask yourself a few questions.