Kennedy said doctors who offer gender-affirming care to minors are betraying the Hippocratic Oath to do no harm and that sex-changing surgeries create “lasting physical and psychological damage.”

“This is not medicine. It is malpractice,” he said.

Other measures announced Thursday include warning letters set to go out from the FDA to 12 manufacturers of breast binders, tight garments used to flatten the chest under clothing, for “illegal marketing” of the products to children as a treatment for gender dysphoria. Kennedy also said his Department of Health and Human Services is moving to reverse the Biden administration’s attempt to include gender dysphoria within the definition of disability.

While the rules would not take effect immediately, they represent a significant escalation of its attacks on the medical needs of transgender individuals. Following Trump’s January executive order declaring that there are only two sexes, male and female, and that “these sexes are not changeable,” federal officials have intensified scrutiny of providers. Two Connecticut pediatric hospitals have already scaled back services amid threats of losing Medicaid funding and in response to subpoenas seeking patient information. Boston’s Fenway Health also recently scaled back care and will no longer offer hormones or puberty blockers to transgender patients under 19.

“This is another attempt by President Trump and Secretary Kennedy to hurt the health and well-being of young people,” Governor Maura Healey said in an email to the Globe Thursday. “I want families across Massachusetts, especially parents of LGBTQ youth, to know that the health care you need is still legal and available in our state. We have been preparing for this, along with the Legislature and providers, and will ensure continued access to health care in Massachusetts.”

Nationally, a dozen hospitals have curtailed care for transgender patients, even though the services remain legal in their states. According to HHS data, nearly 14,000 minors received gender-affirming care between 2019 and 2023.

The administration will also end NIH support for research promoting gender transition interventions and remove remaining related portfolio items by the end of the year.

The moves arrive as congressional Republicans intensify their own push. A House bill introduced by Rep. Marjorie Taylor Greene would make providing gender-affirming care to minors a federal crime, carrying penalties of up to ten years in prison. Another bill by Rep. Dan Crenshaw, expected to receive a vote this week, would bar Medicaid reimbursement for such care. Neither bill has passed the Senate.

For transgender youth and their families, the consequences of the proposed rules could be far-reaching. Gender-affirming care—particularly puberty blockers, counseling, and hormone treatment—is considered medically necessary by major professional organizations, including the American Academy of Pediatrics and the Endocrine Society. For many young people, this care aims to reduce gender dysphoria and improve mental-health outcomes, including suicide risk. Critics of gender-affirming care argue that minors are too young to make decisions with potential long-term consequences and that such treatments should be delayed or prohibited until adulthood.

Massachusetts has become a safe-haven for transgender youth to seek gender affirming care as 27 states, including New Hampshire, already ban or severely restrict pediatric gender-affirming care.

Legal challenges are expected. Multiple states, including Connecticut and Massachusetts, have sued over the federal policy changes targeting transgender care. Civil rights attorneys have filed discrimination complaints against two Connecticut hospitals for halting care in July.

About court actions, Kennedy said, “the number of lawsuits with my name on it right now is almost beyond counting. We know what we’re doing is legal.”

Lawyers, advocates and leaders from Massachusetts’ community health centers and several hospitals, along with the state’s attorney general’s office started plotting strategies Wednesday night on how to push back on the Trump administration’s tightening ban on gender-affirming care.

In a Zoom webinar, one of the first things that was made clear was that the new rules could not legally take effect before a public comment period, likely to be somewhere between 60 and 90 days, said Nina Selvaggio, executive director of Greater Boston PFLAG, an LGBTQ advocacy group, who was on the webinar.

“What the administration and HHS will be careful in doing is they will want to follow the exact procedures so nothing they do will be overturned based on operating procedures,” Selvaggio said.

“Last night we encouraged everyone to subscribe to emails from HHS to know when that period begins,” Selvaggio said. “When [it gets published in the federal register] that’s when the clock starts ticking.”

Earlier this year, when the Trump administration subpoenaed records from Boston Children’s Hospital and several others nationwide that provide gender-affirming care, Massachusetts Attorney General Andrea Joy Campbell led a coalition of 19 states in fighting the administration. That pushback, Selvaggio said, helped quash the Trump administration’s action.

Selvaggio said there was discussion during Wednesday night’s webinar of a similar tactic again.

In New Hampshire, some families and doctors said Thursday’s announcement came as a blow even after the state passed its own ban on gender affirming care for minors earlier this year.

“It’s pretty devastating,” said Sara Tirrell of Plymouth, N.H. Her transgender daughter Parker Tirrell, 16, is currently receiving puberty blockers and hormone replacement therapy at Dartmouth Health’s Transgender Health Program.

Parker Tirrell has been able to continue receiving that care in spite of a statewide ban on gender affirming care for minors because of an exception allowing minors who were already receiving care to continue to do so after it takes effect in 2026.

Sara Tirrell said she needs to consult with her daughter’s doctor to learn how the new rules will affect her care.

But, she said, preserving her daughter’s access to transgender health care is “non-negotiable,” and the family has discussed whether they need to move to another state or country in order to do so.

“For our family and for Parker in particular, this is life or death,” she said.

Frances B. Lim-Liberty, an endocrinologist who provides gender-affirming care in New Hampshire, said Thursday’s announcement did not come as a surprise.

“My main objective is to mitigate the harm these families are experiencing,” she said in an email. “These restrictions do not change their identities and who they are; but it now makes it that much harder for them to get the healthcare they need.”

This is a breaking story and will be updated.

Material from wire reports was used in this story.

Sarah Rahal can be reached at sarah.rahal@globe.com. Follow her on X @SarahRahal_ or Instagram @sarah.rahal. Amanda Gokee can be reached at amanda.gokee@globe.com. Follow her @amanda_gokee. Kay Lazar can be reached at kay.lazar@globe.com Follow her @GlobeKayLazar.