Eman Abd El Aty, said to have weighed around 500kg (1,102lb), left an Indian hospital last Thursday after doctors said she lost more than 250kg.
But doctors in UAE say she is suffering from a "cardiac issue" and infected bed sores, apart from her obesity.
Recent weeks saw a row erupt between her doctors in India and her family.
It began when her sister, Shaimaa Selim, released a short video on social media alleging that her sister was still unable to speak or move, and had not lost as much weight as the hospital was claiming. The hospital strongly denied the allegations.
A statement issued by Abu Dhabi's Burjeel Hospital said a multi-disciplinary team of 20 doctors is treating Ms Abd El Aty.
"She is currently facing several health conditions, including a cardiac issue, which is still under intensive investigation; severe urosepsis [a bacterial infection complicating a urinary tract infection]; and third degree infected bed sores."
The statement said the doctors are "currently focused on improving her health condition and ensuring that as part of the hospital's short term goal, Eman is able to sit with minimal to no assistance".
She had arrived in India on a specially-modified Airbus plane in February and was put on a special liquid diet to reduce her weight so that doctors could perform bariatric surgery.
Bariatric surgery, also known as weight loss surgery, is used as a last resort to treat people who are dangerously obese and carrying an excessive amount of body fat.
The two most common types of weight loss surgery are:
Gastric band, where a band is used to reduce the size of the stomach so a smaller amount of food is required to make someone feel full
Gastric bypass, where the digestive system is re-routed past most of the stomach so less food is digested to make someone feel full
An Egyptian woman who was believed to have been the world's heaviest is now being treated for several health conditions at a hospital in Abu Dhabi.