The young woman went off to college—which began, as it often does these days, with an invitation to state her name, sexual orientation and “pronouns.” When her anxiety flared during her first semester, she and several of her friends decided their angst had a fashionable cause: “gender dysphoria.” Within a year, the lawyer’s daughter had begun a course of testosterone. Her real drug—the one that hooked her—was the promise of a new identity. A shaved head, boys’ clothes and a new name formed the baptismal waters of a female-to-male rebirth.

This is the phenomenon Brown University public-health researcher Lisa Littman has identified as “rapid onset gender dysphoria.” ROGD differs from traditional gender dysphoria, a psychological affliction that begins in early childhood and is characterized by a severe and persistent feeling that one was born the wrong sex. ROGD is a social contagion that comes on suddenly in adolescence, afflicting teens who’d never exhibited any confusion about their sex.

Like other social contagions, such as cutting and bulimia, ROGD overwhelmingly afflicts girls. But unlike other conditions, this one—though not necessarily its sufferers—gets full support from the medical community. The standard for dealing with teens who assert they are transgender is “affirmative care”—immediately granting the patient’s stated identity. There are, to be sure, a few dissenters. “This idea that what we’re supposed to do as therapists is to ‘affirm’? That’s not my job,” said psychotherapist Lisa Marchiano. “If I work with someone who’s really suicidal because his wife left him, I don’t call his wife up and say, ‘Hey, you’ve got to come back.’ . . . We don’t treat suicide by giving people exactly what they want.”

But giving in to patients’ demands is exactly what most medical professionals do when faced with ROGD. Like fashionable and tragic misdiagnoses of the past, this one comes with irreversible physical trauma. “Top surgery,” a euphemism for double mastectomies. Infertility. Permanent rounding of facial features or squaring of the jawline. Bodily and facial hair that never goes away.

Planned Parenthood furnishes testosterone to young women on an “informed consent” basis, without requiring any psychological evaluation. Student health plans at 86 colleges—including those of nearly every Ivy League school—cover not only cross-sex hormones but surgery as well.

ROGD-afflicted adolescents typically suffer anxiety and depression at a difficult stage of life, when confusion is at least as pervasive as fun, and there is everywhere the sense that they ought to be having the times of their lives. I spoke with 18 parents, 14 of them mothers—all articulate, intellectual, educated and feminist. They burst with pride in daughters who, until the ROGD spell hit, were highly accomplished, usually bound for top universities. Except for two mothers whose daughters have desisted, all insisted on anonymity. They are terrified their daughters will discover the depth of their dissent and cut them off. They are determined to use whatever influence they have left to halt their daughters’ next voluntary disfigurement.

Nearly every force in society is aligned against these parents: Churches scramble to rewrite their liturgies for greater “inclusiveness.” Therapists and psychiatrists undermine parental authority with immediate affirmation of teens’ self-diagnoses. Campus counselors happily refer students to clinics that dispense hormones on the first visit. Laws against “conversion therapy,” which purports to cure homosexuality, are on the books in 14 states and the District of Columbia. These statutes also prohibit “efforts to change a patient’s . . . gender identity,” in the words of the New Jersey law—effectively threatening counselors who might otherwise dissuade teens from proceeding with hormone treatment or surgery.

Reddit, Tumblr, Instagram and YouTube host an endless supply of mentors, who cheerfully document their own physical transitions, omitting mention of dangerous side effects and offering tips on how to pass as a man and how to break away from unsupportive parents. For anxious teens who tend toward obsession, these videos can be mesmerizing. Though the stars are typically pictured alone in a bedroom, they project exuberance and social élan. As one female-to-male YouTube guru who goes by “Alex Bertie” puts it: “Taking testosterone is the best decision I’ve ever made. I’m so happy within myself. It did not solve all of my problems, but it’s given me the strength to make the most out of life and to battle my other demons like my social issues.”

Brie Jontry, a spokeswoman for Fourth Wave Now, an international support network for these families, is one of the two mothers who spoke on the record. She tells me ROGD teens often come from politically progressive families. Many of the mothers I spoke with say they enthusiastically supported same-sex marriage long before it was legal anywhere. Some of them describe welcoming the news when their daughters came out as lesbians. But when their daughters suddenly decided that they were actually men and started clamoring for hormones and surgery, the mothers begged them to reconsider, or at least slow down.

“If your kid went off and joined the Moonies, people would feel sorry for you, and they would understand that this is a bad thing and that your kid shouldn’t be in the Moonies,” one mother, a former leader of the pro-gay organization Pflag, said. “With this, I can’t even tell anybody. I talk to my husband, that’s it.” The couple have faithfully covered their daughter’s tuition, health-care and cellphone bills—even though she refuses to speak to them.

Under the influence of testosterone and the spell of transgression, ROGD daughters grow churlish and aggressive. Under the banner of civil rights, they assume the moral high ground. Their mothers take cover behind pseudonyms. As ROGD daughters rage against the biology they hope to defy, their mothers bear its burden, evincing its maternal instinct—the stubborn refusal to abandon their young.

Ms. Shrier is a writer living in Los Angeles.