It has been known for some time that the thyroid gland is a strong regulator of the body’s metabolism, making it key to many health conditions. But the molecular details of how thyroid hormone acts on cells in the body have never been fully understood. Now researchers at the Perelman School of Medicine at the University of Pennsylvania have taken a big step toward the resolution of this mystery by showing that it doesn’t operate as a straight on/off switch, but more like a dimmer.