Hyperthyroidism is diagnosed through a physical examination and blood tests. If a blood test shows that your child’s TSH level is below the reference range and their T4 blood level is above the reference range, this usually indicates an over-active thyroid. Another blood test can measure the antibody levels in the blood to find out whether or not it is Graves’ disease that has caused the over-active thyroid so that it can be treated correctly. The doctor may also scan your child’s thyroid.