(CNN) For some, being transgender is an identity. For others, it's a label. At its core, it's the state of knowing that your gender is different from the one you were assigned at birth.

There's no one way to be transgender, no set of boxes to check that make it so. And while the medical community is working to figure out what causes people to be transgender, the consensus is clear on this: Being transgender is not a mental illness, though as late as 2012 the American mental health community classified it that way.

But for a transgender person to access medical care to align their body with their gender, they often must be diagnosed with a mental disorder called gender dysphoria -- especially if they plan to seek reimbursement from an insurance company.

And they must convince their insurer that the treatment is medically necessary and not cosmetic. That's where things can get even more complicated.

First, you must have a mental disorder

Gender dysphoria is defined by the distress caused by the discrepancy between a person's body and their gender identity. The American Psychiatric Association added the term to its diagnostic manual in 2013 to distinguish between the condition of being transgender and symptoms that arise from distress over being transgender.

Technically speaking, being transgender has never been considered a mental illness because the word was never adopted as diagnostic nomenclature, said Jamison Green, past president of the World Professional Association for Transgender Health (WPATH). But the medical community used other terms in its Diagnostic and Statistical Manual of Mental Disorders, known as the DSM, to pathologize gender nonconformity, including transsexualism and gender identity disorder.

For years, advocates and clinicians fought to remove those terms from the DSM . They argued that they contributed to the stigmatization of the trans community, just as the inclusion of "homosexuality" in the DSM until 1973 fostered misperceptions that being gay was a mental disorder -- one that could be treated and cured.

The DSM-5, published in 2013, replaced gender identity disorder with gender dysphoria to make clear that the diagnosis pertains to distress and not identity, per se. The World Health Organization has signaled its intention to move transgender identity from its chapter on mental illness to a new one on sexual health.

The new diagnosis was intended to clear hurdles to gender-affirming treatment, including surgical procedures. But some contend that it inappropriately pathologizes transgender identity by requiring a mental health diagnosis to access care.

Others argue that the diagnosis is essential to ensure access to care. For many seeking treatment in the American health care system -- especially those without the means to pay out of pocket -- it can be the only option available.

Everyone is different

But not all transgender individuals want or need surgical interventions to transition. The process of transitioning is different for each person. Some resist the term altogether because they don't buy into the idea that gender is binary and that there are only two options: man or woman. And not all people who identify as transgender have gender dysphoria at the level of distress that requires diagnostic treatment.

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For some, transitioning may not involve any kind of surgical intervention; it might be enough to change their name or gender designation on identity documents and come out to friends and family, known as social transitions. Some people take hormones to change features they developed in puberty. Hormone replacement therapy may aid in redistributing body fat to different parts of the body and accelerate or slow body hair growth. The hormone estrogen can produce breast tissue and halt sperm production, and testosterone tends to stop menstruation and increase sex drive.

For some who are transitioning, hormone therapy is sufficient. But it doesn't change primary sex characteristics, such as genitals -- changes that some consider essential. That's when they might seek gender-affirming surgery -- and to get insurance coverage for it requires a diagnosis of gender dysphoria.

WPATH lists protocols to treat gender dysphoria in its standards of care, a set of flexible clinical guidelines that the world's leading medical associations and courts of law follow. The association stresses that each person requires a different course of treatment, and what's medically necessary for one person might not be for another. As far as the association is concerned, patients and their physicians -- those trained to work with the community -- are the best arbiters of what's medically necessary for that person.

Surgical interventions for gender dysphoria, according to WPATH, can include feminizing or masculinizing hormone therapy, voice therapy, chest augmentation and reduction, or genital surgery.

According to the association's standards of care , "persistent, well documented gender dysphoria" is a prerequisite for hormone treatment and most surgical interventions. For genital surgery, the association says a patient should take hormones and live for at least one year "in the gender role that is congruent with their gender identity" before seeking surgical interventions.

"Within those standards of care are the best-known treatments for gender dysphoria, so by the time [patients] see me, they have seen one to two physicians who know them well and have given them this diagnosis," said Dr. Sidhbh Gallagher, a WPATH member and plastic surgeon at Indiana University Health's Gender Affirmation Surgery program.

Competing definitions of medical necessity: Doctors vs. insurance

Leading medical organizations in the United States agree that gender-affirming care are the most effective treatment for gender dysphoria. The American Medical Association , the American Psychological Association and the American Psychiatric Association have issued position statements supporting coverage for medically necessary treatment as determined by a patient and their health care provider.

Insurers tend to require at least one physician's letter documenting the patient's gender dysphoria and attesting to the procedure's medical necessity. Some require additional documentation, such as multiple letters from Ph.D.-level physicians, making the barrier to entry even higher for an already vulnerable patient population, Gallagher said.

Others use their own criteria for medical necessity to the frustration of patients and surgeons.

"Why do the insurance companies set their own standards when they know nothing about transgender surgery?" said Dr. Beverly Fischer, a plastic surgeon in Baltimore who specializes in chest surgery. "Insurance companies take it upon themselves -- at extra expense to patients who have already been through enough -- to create additional requirements. In my opinion, they're trying to make it more difficult for them to get coverage."

The trade association America's Health Insurance Plans disputed the suggestion that health plans apply arbitrary criteria to transition-related benefits.

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"Health plans support evidence-based care that ensures safety for the patient, and in many cases may require a patient undergoing gender reassignment treatment to receive other services before they go into surgery," Kristine Grow, senior vice president of communications at the insurance association, wrote in an email.

To evaluate a procedure's medical necessity, Grow said that many insurers use criteria outlined in what's known as the Stanford definition. Unlike the American Medical Association's, the Stanford definition factors in "the cost-effective(ness) for this condition compared to alternative interventions, including no intervention."

The American Medical Association, on the other hand, has said it opposes definitions of medical necessity that "emphasize cost and resource utilization above quality and clinical effectiveness."