The secret behind the world's most famous smile apparently escaped as a cardiologist spent 1.5 hours in line for a full viewing at the Louvre. "The enigma of the 'Mona Lisa' can be resolved by a simple medical diagnosis of a hypothyroidism-related illness," Mandeep Mehra of Brigham and Women's Hospital in Boston writes in a letter to the editor in Mayo Clinic Proceedings, per Live Science. Together with coauthor Hilary Campbell, he claims the subject of Leonardo da Vinci's 1503 painting had an underactive thyroid gland, explaining Lisa Gherardini's yellowed skin, receding hairline, lack of eyebrows, swelled hands, even that enigmatic smile. As Mehra tells Inverse, "When you have hypothyroidism you're a little depressed, and your facial muscles are puffy and weak. You can't even bring yourself to a full smile."
Hyperlipidemia can also lead to high levels of fat in the blood, which doctors previously proposed as Gherardini's ailment. But Mehra and Campbell say Gherardini wouldn't have lived to 63 with hyperlipidemia, for which there were few treatments at the time. Instead, they suggest she was among the many Florentines with thyroid problems from a diet low in iodine, per Smithsonian. As an anatomy expert, da Vinci "would have captured every small embellishment," including a likely cholesterol deposit near Gherardini's inner eye and possible goiter on the neck, Mehra says. Further boosting his theory is the fact that Gherardini was reportedly painted after childbirth, and pregnancy sometimes precedes hypothyroidism. Still, Mehra concedes it's possible Gherardini plucked her eyebrows, and da Vinci's paintbrush did the rest. (First came a nude sketch.)