According to doctors, it might not be a smile at all – and is instead a symptom of an underactive thyroid Picture: File

Mona Lisa’s quizzical smile has puzzled generations of art scholars.

But, according to doctors, it might not be a smile at all – and is instead a symptom of an underactive thyroid.

The condition causes muscle weakness that could explain the curious slant of her mouth and lips. The American researchers say her yellowish skin, puffy neck and apparently receding hairline may also be signs of an underactive thyroid, which causes tiredness and weight gain.

The presumed model for Leonardo Da Vinci’s masterpiece, the noblewoman Lisa Gherardini, may have had thyroid problems because of a lack of iodine in her diet in Renaisance Tuscany.

It had previously been suggested that she had Bell’s palsy, a type of facial paralysis. Others have claimed Mona Lisa was pregnant, or a man, or in imminent danger of having a heart attack.

Dr Mandeep Mehra of 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. In many ways, it is the allure of the imperfections of disease that give this masterpiece its mysterious reality and charm.’

The researchers also ruled out the idea that the uneven smile could be explained by hardened arteries. But they admitted it might simply be due to Leonardo’s experiments with sfumato, a technique which saw him blur tones and colours.

Their theory is published in the journal Mayo Clinic Proceedings.

Painted between 1503 and Leonardo’s death in France in 1519, the Mona Lisa has been on permanent display at the Louvre in Paris since 1797.