Thai health officials on Wednesday banned doctors from performing castrations, pending an ethics review of complaints about minors undergoing the operation as the first step toward a sex change.
The ban on all castrations came after parents and gay rights activists last week filed a complaint that doctors were castrating underage boys. They argue that boys should wait until they are 18 to receive the surgery.
The Medical Council of Thailand, which sets ethical standards for doctors, is reviewing the practise and will develop guidelines for castrations, a health ministry official told AFP.
"The ban will last until the Medical Council reaches a decision over the castrations," the official told AFP.
"These castrations are unethical," he added.
The ban affects more than 10,000 hospitals and clinics, which could face closure if they violate it.
The boys hope that by removing their testicles, their bodies will not develop masculine features and they will appear more feminine when they eventually complete their gender reassignment surgery.
Most clinics require parental approval before performing castrations on minors, but the order bars the operation even with approval of parents.
Castration costs as little as 4,000 baht (130 dollars), a tiny fraction of the total cost of gender reassignment surgery.
Thailand is famously tolerant of transsexuals, known locally as "kathoey", or the third gender. While they have traditionally been allowed roles in festivals and cabarets, they have in recent years sought to make inroads into mainstream society.