I can’t say it’s definitely not a hoax, but in view of the testimony from an expert endocrinologist at the University of Sheffield, it’s unlikely. It’s also been picked up by the Telegraph, not just the Sun tabloid.
It’s also happened before. It’s rare, less than twenty other cases worldwide, and only two of them well documented. All previous cases had a female gender identity - they were transsexual - so this came as an enormous relief. Especially to those who had no history of cross-dressing, nor exhibited anything other than male behaviour.
There appears to be 4 types : Type I involves excess prolactin due to a damaged pituitary, and takes 7 years to have full effect. Type II is the most common, and least understood. It’s associated with rapid estrogen absorbtion by cells, and excess estrogen for reasons unknown. It takes 3 or so years for full feminisation at normal pubertal rates. Type III is the spectacular and dangerous one, the change happens over months, not years. It appears to be the result of simultaneous CAH and AIS Intersex conditions. Types I-III happen at age 45-50, usually.
Type IV happens at puberty, and appears to be an Intersex condition.
Female to Male natural changes, due to 5ARD or 17BHDD are far more common - maybe 1 in 70,000 rather than 1 in several million.
One of the cases blogged it as it happened. Also the only known survivor of Type III.

She’s a member of the Australian Sceptics, BTW. There’s a number of witnesses to testify that it was her body that got photoshopped, not the images.
Google “Zoe Brain”. Me. And before May 2005, I had no idea of any of this stuff. There’s still nothing in PubMed about it. Lots on 5ARD a 17BHDD, but these MtoF cases are so rare they haven’t been studied.