This study is testing a new drug called mitapivat for kids with a condition called thalassemia. Thalassemia is a blood disorder where the body makes fewer healthy red blood cells, leading to frequent blood transfusions. The study aims to see if mitapivat can help reduce these transfusions. A placebo is a "fake" treatment used to compare results.

Eligibility: Kids aged 1 to 17 years who are dependent on transfusions and weigh at least 7 kilograms can join. They must have a confirmed diagnosis of thalassemia. Kids taking hydroxyurea, a medication to help blood conditions, can participate if their dose hasn't changed for 16 weeks. Girls who have started menstruating need to avoid pregnancy during the study.

• The study requires you to follow all procedures for the duration of the study.
• Pregnant or breastfeeding participants cannot join the study.
• Participants with certain health conditions or past treatments might not be eligible.