Gender Identity Disorder, known more commonly as Gender Dysphoria, is a formal medical diagnosis used by psychologists and physicians who experience extreme discontent with the sex they were assigned at birth. There are many possible reasons for it, not necessarily limited to pscyhological/behavioral issues, but biological ones related to genetics, hormones, and brain chemistry, as well as prenatal development. Like many things on modern social networking, it's become a centerpiece of enormous controversy, despite being considered a medical condition by the medical community for roughly thirty years.
Individuals who don't believe that you do not need to possess Gender Dysphoria at all as a medical condition in order to be transgendered call themselves Tucutes, though the more common (and accurate) term is Transtrenders. They emerged from social networks like Tumblr around 2012, where claiming to be Transgendered became a trendy thing to do because it was a simple way to gain social acceptance on Social Networks.
In many cases, so-called tucutes "identify" as trans not as any particular form of gender issue, but as a political statement. Though there is some evidence to suggest that Gender Dysphoria may not be exclusively medical, there has not been any studies on the subject with extensive peer review, in no small part because the rise in social networks has caused an enormous number of people to falsely identify as transgendered.
Transtrenders and "Truscum"
Truscum is a derogatory slur for a Transgendered person (especially a female-to-male) who treats their Gender Dysphoria as a medical condition to be treated with hormonal therapy, surgery, counseling, and so on. Because claiming to be transgendered has become something of a trend on Tumblr and Livejournal, due to the perceived lack of privilege it offers, the term grew out of a group of alleged Transgendered people whom, rather than a medical condition, view Transgenderism as an identity. The tendency of people who refuse to acknowledge that Gender Dysphoria is the literal medical definition of being transgendered resulted, predictably, in people with actual Gender Dysphoria calling individuals stating that Gender Dysphoria was not required to be trans "not really transgendered," citing the enormous number of self-diagnosers and fakers claiming to be trans solely for attention, ergo truscum.
At a glance, the argument that Transgendered people seeing their Dysphoria as a medical condition exclusively might seem stigmatizing - transpeople and homosexuals alike, for example, have historically been treated abysmally in the past by the medical establishment until comparatively recently. With Gender Dysphoria now classified as a medical disorder by the ICD-10 CM and DSM-5, however, it's vastly easier for transpeople who actually do meet the diagnosis (and aren't doing it for attention) to get the therapy, hormones, and surgery they need to relieve their dysphoria, which can quite literally save lives. In this regard, the number of people now falsely claiming to be transgendered for attention is doing those with Gender Dysphoria no favors.
As is the case for certain parties claiming to have multiple personalities for attention, claiming to be transgendered for attention has become a major issue, and not just on social networks. The fact that so many people are self-diagnosing and claiming to have Gender Dysphoria and/or not having any real reason for specifying their transgendered status other than because they feel like it, they are keeping the gatekeeping for transgendered people - the all-too-critical paths required for transitioning - artificially high.
As such, because so many people that clearly do not have Gender Dysphoria are now claiming they do, mostly based on self-diagnosis, the requirements on acquiring the necessary permissions for hormone replacement therapy, surgery, and so on are vastly higher than they would otherwise be. This is a major problem for transgendered people who do actually recognize Dysphoria as the medical condition the scientific community treats it as, since delays in treatment or medication can worsen symptoms of depression and similar conditions - the suicide rate for transpeople who do not get proper treatment of Gender Dysphoria is rather high.
Secondly, for those who do not believe in Gender Dysphoria being exclusively medical without dismissing those who do (and there is some evidence to support that theory) and the possibility of this from a scientific standpoint, the multitude of individuals claiming to be transgendered specifically for attention/via self-diagnosis has essentially crippled attempts to do organized studies on it. Again, the sheer volume of individuals claiming it solely because they can is causing direct damage to the larger whole.
Tertiarily, it's also an issue for the exact opposite reason. There are transgendered people who claim their status solely to use them for identity politics. The absolutely enormous number of people who have claimed that Gender Dysphoria as a medical condition isn't necessary for being transgender means that a number of people all-too-willing to berate others for not transitioning beyond a certain point or not being sufficiently on their side (for example) now have literally endless ammunition to go act like like jerks with, enabling them to label their opponents as dangerous fakers.