How are protein hormones released from endocrine cells?

Protein hormones are synthesized within endocrine cells through a complex process involving the endoplasmic reticulum and the Golgi apparatus.1

The synthesis begins when mRNA directs the production of a biologically inactive precursor known as a prohormone. This prohormone is transported to the Golgi apparatus, where it is packaged into vesicles called secretory granules. As these granules migrate toward the cell surface, a proteolytic enzyme cleaves the prohormone, separating its inactive region from the active hormone.12

The active hormone is then discharged from the cell into the extracellular fluid through a process known as exocytosis. Typically, the same signal that stimulates the synthesis of a protein hormone also triggers the immediate release of already synthesized hormones from stored secretory granules.1