My 8-year-old son was diagnosed with type 1 diabetes at the age of three years. He also has Hashimoto's thyroiditis. His blood results show TSH is 49 mU/L and T4 is 1.1 micrograms per deciliter. He is currently using Humalog insulin and tablet Levothyroxine. What is the reason for his high TSH levels? Also, what are the normal levels of thyroxine?