MONA Lisa's mysterious smile has baffled the world for centuries but a medical professor thinks he may have cracked the key behind it.\r\nUS based experts now think the smirk was caused by an under-active thyroid gland.\r\nKnown as hypothyroidism it can cause swollen hands, thinning hair and a lump in the neck - all visible in the portrait.\r\nAccording to new research this was the condition triggered the enigmatic facial expression of Leonardo da Vinci's famous model.\r\n\u00a0\r\nIt was spurred by not eating enough dairy products, seafood and meat - and her earlier pregnancy.\r\nThe smile may be due to disability caused by muscle weakness, US medics suggested.\r\nProfessor of Medicine Dr Mandeep Mehra, medical director of the Heart & Vascular Centre at Brigham and Women's Hospital in Boston, said: "The enigma of the Mona Lisa can be resolved by a simple medical diagnosis of a hypothyroidism-related illness.\r\n"In many ways, it is the allure of the imperfections of disease that give this masterpiece its mysterious reality and charm."\r\n\u00a0\r\nPrevious examinations of the 16th century masterpiece have suggested she was suffering high cholesterol, heart disease - or even syphilis.\r\nBut Prof Mehra and colleague Dr Hilary Campbell of the California University in Santa Barbara said this is not possible owing to the fact she lived to the age of 63.\r\nThe limited treatments available at the time would have meant she would have died much younger.\r\nInstead the new analysis published in Mayo Clinic Proceedings reveals noblewoman Lisa Gherardini - widely believed to be the subject - displays classic signs of hypothyroidism.\r\nHer hands look swollen and it appears she's losing her hair. She also has yellowed skin and what seems to be a goiter - or lump - in the neck.\r\nHypothyroidism in which the thyroid gland does not produce enough hormones can cause all these symptoms.\r\nThe researchers said: "The painting suggests a yellowish discolouration of the skin, which is known to occur in hypothyroidism.\r\n"The black veil that hangs below what appears to be a large forehead indicates a receding hair line, with hair that appears to be thinned.\r\nA complete lack of eyebrows or other hair throughout the pale skin further supports this diagnosis, and cascading hair down the side appears coarse in character.\r\n"Curiously, a close look at the neck does insinuate the possible presence of a diffuse enlargement such as a goiter."\r\nThey said swelling in the right hand further supports the theory as this is often noted in advanced stages of hypothyroidism.\r\nThe researchers said: "In this circumstance, if Lisa Gherardini was indeed suffering from severe hypothyroidism or its consequences, the mysterious smile may at one level be representative of some psychomotor retardation and muscle weakness leading to a less than fully blossomed smile."\r\nThe condition can be caused by a lack of iodine - found in dairy products, seafood, meat, bread and eggs.\r\nThis was a common problem among Italians during the Renaissance. Resulting goiters were commonly depicted in paintings and sculptures.\r\nThe researchers said: "Thus, the diet was one that was often iodine deficient and more importantly, the eating habits promoted the development of goiters."\r\nDr Mehra and Dr Campbell also pointed out Lisa Gherardini gave birth shortly before she began sitting for da Vinci.\r\nPregnancy can also contribute to inflaming the gland.