The growth and function of the thyroid are controlled by thyrotropin¹ through the activation of its receptor, which belongs to the large family of G protein–coupled receptors. Despite the extreme diversity of their ligands, all receptors from this family have a common molecular architecture: seven transmembrane segments, three extracellular loops, three intracellular loops, an extracellular amino terminal, and an intracytoplasmic carboxy terminal (Figure 1). The glycoprotein hormone receptors constitute a subfamily that is characterized mainly by a particularly long amino-terminal extracellular domain that confers binding specificity.¹,²¹

The thyrotropin receptor is encoded by 10 exons spread over 58 kilobases on . . .