Ar-kom Praditsuwan, director of the Bureau of Sanatorium and Art of Healing under the the Department of Health Service Support (HSS), said the probe is not limited to the baby Gammy case, but is being extended to other surrogacy programmes and doctors.
He said authorities are looking into surrogacy programmes. So far, they have found 12 surrogacy service clinics, seven of which are legally registered with the HSS and the others are not.
If the HSS finds physicians of clinics performing surrogacy services without licences, they will be investigated by the Medical Council of Thailand and the HSS will shut down the clinics.
In the baby Gammy case, Dr Ar-kom said a large and legally registered hospital located in a province near Bangkok provided the surrogacy service.
Officials will see if the hospital is licensed to offer the assisted reproduction technology and if the doctor who performed the procedure has a permit to carry it out, he said.
The results of the inquiry are expected this week. A practitioner must be a specialist under the Royal Thai College of Obstetricians and Gynaecologists and have a licence to offer assisted reproduction technology, the doctor said.
Doctors found to have breached Medical Council of Thailand regulations involving this matter face criminal charges and an ethics probe.
Samrit Khomrit, secretary-general to the Medical Council of Thailand, said 45 specialists have been granted a licence to perform the surrogacy procedure.
He said the council will ask for information from the HSS about the doctors who perform the procedure.
In Thailand, there are no specific laws regulating surrogacy services. Medical Council of Thailand regulations govern commercial surrogacy, which can proceed only if the egg and sperm come from a blood relative.
But Dr Ar-kom said human trafficking or consumer protection laws may be examined if state authorities are considering taking action.
A panel has been set up to look into surrogacy services agencies and consultants attached to the industry. The Consumer Protection Board will be invited to join.
He said Thailand is considered a "paradise" for couples looking for surrogacy services. There are some 20 surrogacy agencies, most of them foreign-owned, which bring in about four billion baht a year.
Meanwhile, the Australian father of Gammy, a baby with Down's syndrome born to a surrogate mother in Thailand, has denied abandoning the child and taking only his healthy twin sister, Australian media said yesterday.
The father, who has not been identified, denied he and his partner had rejected the boy, and said a Thai doctor had only told them about the girl, broadcaster ABC reported.
The report said the man complained of problems with the surrogacy agency, which he said he was told no longer existed.
After the Australian father spoke out, the Thai surrogate mother, Pattharamon Janbua, 21, expressed disappointment.
"I am upset that the father said that. I really want him to come to Thailand to see me," she said. "I would like to talk to him in front of the media. "The truth will come out in public," she said. "People who don't know me will otherwise think I'm a bad person."
Ms Pattharamon yesterday declined to say where she had been taken to receive the surrogacy procedure. She said an Australian foundation will be responsible for the baby's treatment costs.
She also raised concern about laws related to surrogacy in Thailand, saying the surrogate mother should be better protected than biological parents. "If a surrogate mother is not responsible for a baby abandoned by a couple, the baby would become a burden on society," Ms Pattharamon said.
The boy is now receiving care at Samitivej Hospital in Sriracha. Wijit Panayingpaisarn, of the hospital's public relations department, said baby Gammy's condition had improved. "A specialist is taking care of Nong Gammy. The baby is in safe hands," Mr Wijit said.
Australian Immigration Minister Scott Morrison said the child may be eligible for Australian citizenship, and receive care in that country. The government is taking "a close look at what can be done here, but I wouldn't want to raise any false hopes or expectations. We are dealing with something that has happened in another country's jurisdiction", he said.