Although thyroxine is an exception, non-steroid hormones do not cross the plasma membrane of the target cell, acting instead through second messengers (cyclic AMP, cyclic GMP, IP3). Remember this. Non-steroid hormones do not cross the plasma membrane of the target cell, acting instead through second messengers. Second messengers are an omnipresent mechanism in signal transduction between regulatory systems and target cells. A small amount of bound hormone can result in large second messenger production, amplifying the message. Within the cell, the second messenger can itself initiate further amplification (binding to a kinase, for example).