Table 1.

Principles of endocrinology

Principle	Explanation
Hormones are responsible for coordinating the systems of the body, and allowing communication and integration of the various tissues of the body from conception until death.	Although hormones are often considered only for their role in the maintenance of ‘homeostasis’, they have much wider roles that include the development and differentiation of embryonic and fetal tissues, sexual reproductive functions, maintenance of bone density, body weight and cardiovascular health, and many other endpoints.
Hormones mediate effects via specific, context- dependent interactions with receptors.	The actions of hormones are dictated based on the specificity of their interactions with receptors, the concentration of the hormone in the blood (including the bioavailable fractions), the abundance and location of the receptor, and other factors.
Hormones act at low circulating concentrations.	Hormones induce significant biological changes at circulating concentrations in the part-per-billion or part-per-trillion range. This is because of the non-linear relationship between hormone concentration and the percentage of receptors that is bound, as well as the non-linear relationship between the percent of occupied receptors and induced biological effect.
Hormones often exhibit non-linear and non- monotonic dose responses.	Like vitamins, nutrients, and other pharmacological agents that interact with receptors, hormones can induce non-monotonic responses via mechanisms including receptor downregulation, receptor desensitization, receptor competition, receptor interactions, endocrine negative feedback loops, and even traditional toxicology mechanisms (i.e. the induction of cytochromes, or cytotoxicity).
The effects of hormones differ based on an individual’s stage of development.	differentiation of cells, tissues, or organs; these so-called organizational effects can be observed long after exposures have ceased because the developmental trajectory of the cell, tissue, organ, or entire organism has been changed. In contrast, activational effects are those that are observed only during the period of exposure, thus when hormone treatment ceases these effects subside; these are typically associated with adult exposures.