Underactive describes something that isn't working as hard or as much as it should. When a body part or system is underactive, it's functioning below its normal level, like an engine running too slowly to do its job properly.

The most common use is medical. An underactive thyroid (called hypothyroidism) means the thyroid gland isn't producing enough hormones, which can make someone feel tired, cold, or sluggish. Doctors can usually treat an underactive thyroid with medication that replaces what the body isn't making on its own.

The word can describe other situations too. An underactive immune system doesn't fight off infections as well as it should. An underactive imagination might struggle to come up with creative ideas. Even a classroom discussion could be called underactive if students aren't participating much.

The opposite is overactive, when something works too hard or too much. Think of underactive as the “too little” problem and overactive as the “too much” problem. Both suggest something out of balance, not operating at the level it needs to function properly.