The pituitary is a gland in the brain, and it acts like a hormone thermostat. As blood flows through the pituitary, it measures your hormone levels. As your pituitary senses that your body needs more of any given hormone, it releases a stimulating hormone into the blood stream.  That stimulating hormone travels to its respective gland, signaling that gland to produce more hormone.  For example, to stimulate the thyroid gland to produce more thyroid hormone, the pituitary will release TSH (or thyroid stimulating hormone).  As levels of that hormone increase to appropriate levels, the pituitary shuts the stimulating hormone down. As the body ages, the pituitary will produce less and less stimulant.  At the same time, as the body ages, glands become less and less sensitive to stimulating hormones. The result:  hormone levels decline.

An interesting footnote to the above: research suggests that men are experiencing the symptoms of andropause at much younger ages in recent years. The going theory is that this trend is the result of the environmental toxins we’re all exposed to today, and that this exposure is causing premature slowing of the pituitary in much of the population.