The Corrections Department issued a statement on Monday saying it cannot provide details about inmates' health or other medical details without their consent, to protect their right to privacy.
The statement comes amid continuing calls for the department and the police to release updated details on the health of convicted former prime minister Thaksin Shinawatra, who has been in the Police General Hospital since shortly after his sentencing more than a month ago.
There are suspcions Thaksin, who has a year-long sentence to serve, is being given privileged treatment in being allowed to stay out of prison.
His daughter Paetongtarn said last week that her father had undergone surgery since arriving back in Thailand and was recovering, but she gave no further details.
In its statement on Monday, the Corrections Department said it must comply with human rights protections and related laws, which treat health information as confidential. It could not disclose inmates' health details without their consent.
This was also in line with the UN's Nelson Mandela Rules, which are also guidelines for the protection of prisoners.
The department said that in the 2023 fiscal year it had in its care 276,686 inmates and more than 30,000 of them required medical care outside the prison. About 140 of them spent more than 30 days in medical care.
Political activist and persistent petitioner Srisuwan Janya has demanded the department disclose an official report by corrections officials guarding Thaksin at the hospital.
According to Mr Srisuwan, the officials' daily report to their supervisor and the department chief is a detailed record of what happened that day, and includes information about meals and visits and is considered official information.
He said the report is not about Thaksin's medical condition, and he had the right to access it under the Official Information Act of 1997. He said he would petition the Official Information Board initially, and take the matter further if necessary.
"And if there is an attempt to keep this document away from the public, I'll file a petition asking the Administrative Court to order its disclosure," he said.