In addition to an increased appetite leading to severe weight problems from childhood, she had Type-2 diabetes, learning difficulties and reproductive problems.
They found that she had inherited two copies of a harmful genetic change which meant she could not make a protein called carboxypeptidase-E (CPE).
CPE is important in processing of a number of hormones, brain transmitters controlling appetite, insulin and other hormones important in the reproductive system.
Studies have previously shown that CPE deficiency causes obesity, diabetes, and impaired memory in mice, but no humans with the condition have been found before.
"These are serious disorders that affect the body's ability to regulate hunger and fullness signals. They are inherited in the same way as other genetic diseases are inherited and sufferers should not be stigmatised for their condition," said professor Alex Blakemore, who led the study.
The examined patient's parents and cousins, gave her relatively high likelihood of inheriting the same genetic change from both parents. She had an older brother with similar symptoms who died aged 21, the study said.
"Finding a genetic cause for the patient's problems has helped her and her family to understand and manage her condition better," first author of the study Sanne Alsters, who carried out the genetic tests, said.
"There are now increasing number of single-gene causes of obesity and diabetes known. We do not know how many more have yet to be discovered, or what proportion of the severely obese people in our population have these diseases - it is not possible to tell just by looking," Blakemore said.
The study was published in the journal PLOS ONE.
