The Mona Lisa is one of the most famous paintings of its time mainly due to the alluring eyes and mysterious smile of Lisa Gherardini, the subject of the painting. After a closer look at the painting, Dr. Mandeep Mehra, a medical director of the Heart and Vascular Center at Brigham and Women’s Hospital in Boston, and his co-author, Hilary Campbell, propose that the allure is simply due to a disease known as hypothyroidism.
They claim that “In many ways, it is the allure of the imperfections of disease that give this masterpiece its mysterious reality and charm.”  Their findings are published in the September journal of Mayo Clinic Proceedings. This is not the first medical examination of the famous painting.
In 2004, doctors hypothesized that Gherardini had familial hyperlipidemia, or high fat levels in her blood due to a genetic condition, which often times led to heart disease. This hypothesis was based on several details in the painting that showed symptoms of the disease. These symptoms included skin lesions and swelling in Gherardini’s right hand. But this hypothesis was recently refuted by Mehra and Campbell. 
They pointed out that Gherardini lived to age 63, which would have been unlikely if she had familial hyperlipidemia due to the limited medical advancements at the time. Instead, doctors leaned more towards hypothyroidism, a condition where the thyroid gland does not make enough thyroid hormones to meet the body’s needs. This can lead to fatigue, weight gain, pain or swelling in the joints, thinning hair or hair loss, and high fat (cholesterol) levels in the blood.
This diagnosis explains many features in the painting such as the subject’s apparent yellow skin, thinning hair, receding hairline, and an enlarged neck, which suggesting an enlarged thyroid gland. The subject’s enticing smile is just another side effect of the onset of the disease. 
Her famous expression may represent muscle weakness and reduced movement “leading to a less than fully blossomed smile” as Mehra and Campbell wrote.
The imperfections of this masterpiece are what make the painting not only compelling but relatable. Many people see peace in the painting and Gherardini’s expression because she is not perfect. It’s intriguing that the symptoms of a medical condition could have caused the timeless allure of The Mona Lisa.