A small organ in the front of your neck, just below the Adam's apple, shaped a bit like a butterfly. It makes hormones — chemical messengers that go into your blood and set the speed your body runs at: how fast you burn food for energy, your heart rate, your body temperature, your weight, even your mood and how tired you feel. Doctors talk about it constantly because it goes wrong often, and usually in one of two directions. If it makes too little (an underactive thyroid, hypothyroidism), you feel cold, slow, heavy, and tired, and you may gain weight. If it makes too much (an overactive thyroid, hyperthyroidism), you feel hot, jumpy, and anxious, your heart races, and you may lose weight while eating normally. Both are common, both are much more common in women, and both are usually manageable with daily medicine. A swelling of this organ is called a goitre, and it can also grow lumps (nodules), most of which turn out to be harmless. The word works as a noun ("the thyroid," "her thyroid") and as an adjective ("thyroid gland," "thyroid problems," "thyroid medication"). In everyday speech people shorten the whole story to the organ itself: "She has a thyroid problem" or even "It's her thyroid" means something is wrong with it.