Which protein carries T3 and T4 in the blood?

Thyroxine-binding globulin is the primary protein responsible for transporting the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) in the bloodstream. This protein binds to these hormones, allowing for their stable circulation and delivery to target tissues. The binding of T3 and T4 to thyroxine-binding globulin is crucial, as it regulates the availability of these hormones in the circulation, maintaining their levels and prolonging their half-life.

In contrast, corticosteroid-binding globulin primarily transports corticosteroids, such as cortisol, and does not play a role in the transport of thyroid hormones. Growth hormone is a peptide hormone that stimulates growth and metabolism but does not function in the transport of thyroid hormones. Insulin, while vital for glucose metabolism and energy regulation in the body, is unrelated to the transport of thyroid hormones. Hence, the exclusive function of thyroxine-binding globulin in carrying T3 and T4 makes it the correct answer.