The main reason for this research study is to learn more about hydroxyurea and the treatment of sickle cell anemia (SCA). Hydroxyurea is a medication that has been studied for many years and has been shown to provide benefits for people with SCA. In this research study, the investigators hope to learn more about how to improve the dosing and monitoring of hydroxyurea and learn more about the long-term effects of hydroxyurea over time. Hydroxyurea is usually dosed based only on your weight. Our study will use a new way to select a starting dose that is based on how each patient absorbs hydroxyurea.