Our hormones have a molecular structure that is identical to hormones that are naturally produced by the human body, and are intended to replace those hormones when levels decline due to aging, disease, or surgery. In order for replacement hormones to fully replicate the function of hormones which were originally naturally produced and present in the human body, the chemical structures must match exactly. Synthetic non-bio-identical hormones have similar but NOT identical structures, so they are not the perfect fit for hormone replacement.
Researchers have long held that there are significant differences between hormones that are natural to humans and synthetic preparations. The structure of the hormone, not the sources is the key.
Natural hormones are chemically processed from precursors found in yams or soy plants, yet they are identical to the hormones produced by the human body. These hormones are able to follow normal metabolic pathways so that essential active metabolites are formed in response to hormone replacement therapy.
Synthetic, or conventional, and commercially available, or artificial, or horse hormones have been chemically altered, and are not identical in structure or activity to the naturally occurring hormones that are intended to replace.
One last important fact to mention is that natural substances can not be patented, so scientific studies are less numerous on natural hormones. Side chains can be added to a natural substance to create a product that can be patented by a manufacturer and profitable to mass produce as a pharmaceutical drug. Therefore a drug company has an incentive to fund research on the use and effectiveness of synthetic hormones. Structural differences inherent to synthetic hormones may be responsible for side effects that are experienced when synthetic hormones are used for replacement therapy.