Northern Ireland's only gender identity clinic (GIC) has been unable to take on any new patients since 2018.
Brackenburn Clinic has 403 people on its waiting list but has temporarily stopped its drop-in service due to limited resources and soaring demand.
A spokesperson for the Belfast Trust said: "The GIC, in line with services across the UK, has seen a significant rise in demand from individuals seeking assessment and treatment. This is upwards of 75% from 2014.
"This service, similar to others across the health service in Northern Ireland, currently does not have adequate capacity to meet demand and this is resulting in new patients referred having to wait longer than desired for assessment."
The target for an initial appointment is 13 weeks. One patient has been waiting 166.
Aaron O'Donnell (18), who was referred to the clinic earlier this year, said: "The wait makes you feel kind of hopeless. I can't stand living in this weird purgatory for the next two years.
"I have been lucky enough to have a private consultation with Ireland's specialist endocrinologist. It's not a luxury everyone can afford. The 15-minute appointment cost over £170.
"It adds extra stress to the situation due to financial pressure as surgery costs around £7,000 if private."
A spokesperson for the Department of Health said the Health and Social Care Board was carrying out a review with the aim of "identifying a service model that will meet the increasing demand for gender identity services and help reduce the unacceptably long waits currently being experienced".
Transgender NI director Alexa Moore said: "The utter failure of the gender identity services stems from the treatment of trans identity as a mental disorder.
"This outdated approach has led to increased waiting times and negative and stigmatising experiences within the services, as well as worsening mental health within the community.
"While we are optimistic about the review currently under way, no alternative arrangements have been provided to trans people on the waiting list, many of whom have been there for three or four years. This demonstrates a distinct and worrying lack of urgency regarding dealing with this public health crisis. Too many trans people are suffering and not enough is being done."
Ms Moore said she had requested a meeting with Health Minister Robin Swann, but it had been rejected.
"This very clearly shows the lack of urgency with which this is being tackled and a relative lack of willingness to engage with trans communities on the minister's behalf," she added.
The Department of Health said: "The minister is due to receive the findings and recommendations of the review group by the end of June and would welcome a meeting thereafter with representatives from the transgender community."