set point

A set point is the target value a regulatory system tries to hold, just like the temperature you dial into a thermostat. The body compares the actual level of something — blood calcium, glucose, thyroid hormone — against this internal target, and feedback nudges the level back whenever it drifts away. The set point is what gives homeostasis a destination to aim for.

Set points are not single rigid numbers but narrow ranges, and they can be deliberately shifted. The body resets its temperature set point upward during fever, and the cortisol and growth hormone systems carry set points that change predictably over the day. In disease, an abnormally reset set point can be the core problem rather than a broken gland.

Crucially, a set point is enforced by feedback, so defending it can resist outside change. This is why losing weight is hard if the body's energy set point pushes appetite and metabolism back toward the old weight, and why suppressing one part of an axis can fail if the system keeps trying to restore its target.