The concern is that exposure to extremely high bilirubin levels (> 20) for prolonged periods of time increases the risk of neurological injury. A bilirubin level of 10 should not be toxic for your baby.
It is unusual, however, for the bilirubin level to remain at a level of 10 at 5 weeks of life, even in a baby born 3 weeks premature. Your pediatrician has already checked the thyroid function, which was the most important test to perform. If the bilirubin remains elevated, you may want to discuss with your pediatrician a referral to a pediatric gastroenterologist for further evaluation.