Thyroid hormone receptor (TR) regulates growth, development, and differentiation of mammals. TR contains both a domain that binds to T3, the thyroid hormone, and a separate domain that binds to the DNA. Which of the following BEST explains how TR signals? T3 binds to TR at the cell membrane, which then releases second messengers to regulate gene expression, T3 binds to DNA at the cell membrane, which then releases second messengers to regulate gene expression, T3 diffuses through the cell membrane to bind to TR, which then binds to DNA to regulate gene expression, or T3 diffuses through the cell membrane to bind to DNA, which then binds to TR to regulate gene expression.