Why is free hormone more biologically active than hormone bound to a transport protein?

In the context of biochemistry, it is the free, unbound hormone that is considered biologically active because it is the only form capable of accessing target tissues to exert its effects.12

While many hormones circulate in the blood bound to specific transport proteins, these bound hormones are not directly active. Instead, they exist in a state of equilibrium with a smaller concentration of free hormone. When free hormone leaves the circulation to bind with its specific receptor on a target cell, the bound hormone is released from its carrier protein to maintain the necessary concentration of free, active hormone.12

Consequently, transport proteins function primarily as a reservoir, protecting the body against sudden surges in hormone levels and facilitating the even distribution of hormones to large organs. Because only the free molecule can interact with the receptors located on or within target cells, it is the essential component for initiating hormonal action.12