A new study suggests the thyroid doesn’t just receive instructions from hormones—it contains a tiny built-in “switch” inside its own receptor protein. How can a 10–amino-acid segment inside a cell-surface receptor help explain both normal thyroid signaling and runaway hormone production? For years, the prevailing assumption was straightforward: thyroid-stimulating hormone (TSH) binds the thyroid-stimulating hormone receptor (TSHR), and that binding directly turns hormone production on. The puzzl