Hormonal Regulation

Because the hormones that act through nuclear hormone receptors are nearly all fat-soluble, they are readily absorbed into the body, freely transported, and stored and accumulated in fatty tissues. Steroids, retinoic acid, thyroid hormone, and vitamin D3 are active at extremely low concentrations, ranging from about 0.3 to 30 parts per billion, with 3 parts per billion considered a physiological concentration for retinoic acid and many steroids. Since the hormones are present and act at such low concentrations, it is critical that their levels be precisely regulated. Consequently, hormone synthesis and degradation is regulated by the activity of specific biosynthetic and catabolic enzymes.

It should also be noted that some chemicals in the environment and natural compounds found in the diet can affect the activity of hormone receptors, particularly the estrogen receptor. Such interaction can potentially lead to disturbances in hormone homeostasis and inappropriate regulation of target genes. These xenobiotic "endocrine disrupting chemicals" have the potential to impact many body systems by inappropriately activating or interfering with the activity of hormone receptors. As a result, endocrine disruption is a growing concern that is being studied intensively in many laboratories around the world.