Hormones carry messages from glands to cells to maintain chemical levels in the bloodstream that achieve homeostasis. "Hormone" comes from a word that means, "to spur on." This reflects how the presence of hormones acts as a catalyst for other chemical changes at the cellular level necessary for growth, development, and energy.

As members of the endocrine system, glands manufacture hormones. Hormones circulate freely in the bloodstream, waiting to be recognized by a target cell, their intended destination. The target cell has a receptor that can only be activated by a specific type of hormone. Once activated, the cell knows to start a certain function within its walls. Genes might get activated, or energy production resumed. As special categories, autocrine hormones act on the cells of the secreting gland, while paracrine hormones act on nearby, but unrelated, cells.

There are two types of hormones known as steroids and peptides. In general, steroids are sex hormones related to sexual maturation and fertility. Steroids are made from cholesterol either by the placenta when we're in the womb, or by our adrenal gland or gonads (testes or ovaries) after birth. Cortisol, an example of a steroid hormone, breaks down damaged tissue so it can be replaced. Steroids determine physical development from puberty on to old age, as well as fertility cycles. If we are not synthesizing the correct steroidal hormones, we can sometimes supplement them pharmaceutically as with estrogen and progesterone.

Peptides regulate other functions such as sleep and sugar concentration. They are made from long strings of amino acids, so sometimes they are referred to as "protein" hormones. Growth hormone, for example, helps us burn fat and build up muscles. Another peptide hormone, insulin, starts the process to convert sugar into cellular energy.

Hormones so perfectly and efficiently manage homeostasis due to negative feedback cycles. Our goal is to keep the concentration of a certain chemical, such as testosterone, at a constant level for a certain period of time, the way that a thermostat works. Using negative feedback, a change in conditions causes a response that returns the conditions to their original state. When a room's temperature drops, the thermostat responds by turning the heat on. The room returns to the ideal temperature, and the heater turns off, keeping the conditions relatively constant.