A prohormone refers to a committed intra-glandular precursor of a hormone, usually having minimal hormonal effect by itself. The term has been used in medical science since the middle of the 20th century. Though not hormones themselves, prohormones amplify the effects of existing hormones. Examples of natural, human prohormones include proinsulin and pro-opiomelanocortin.
For peptide hormones, the conversion process from prohormone to hormone typically occurs after export to the endoplasmic reticulum and often requires multiple processing enzymes. For example, proinsulin is processed by PC 1/2, PC 3, and carboxypeptidase E to afford insulin. Proamylin, which is cosecreted with proinsulin, requires the above three factors and an amidating monoxygenase. Though commonly misdescribed as such, Vitamin D3 is not an example of a prohormone or a hormone. The word prohormones has taken on a new sense due to the presence of specific oral medications designed for athletes to affect hormone levels. For small molecule hormones, the conversion is often one step, and is often used to regulate hormone levels.
Read more about Prohormone: Prohormones of Testosterone and Other Androgenic Steroids, Prohormones Added To The List of Schedule III Anabolic Steroids, Common Types of Prohormones Prior To 2004 Anabolic Control Act, Regulation in The United States and Europe