The Seimas passed the amendments before closing its spring session, but the president sent them back to the parliament for reconsideration.
Grybauskaitė cited incompatibility with the Constitution and certain procedural violations made by the Seimas as the reasons for returning the package.
The amendments would have authorized the health minister to decide on a network of health facilities that would have been given priority in signing agreements with Territorial Health Insurance Funds for the costs of health services to be covered by compulsory health insurance.
Rasa Svetikaitė, advisor to the president, says that it is necessary to establish by law qualitative criteria that healthcare providers would have to meet to be eligible for funding from the Compulsory Health Insurance Fund.
"I want to underline that the president supports the reform of healthcare facilities which is necessary, because the current network of hospitals is inefficient, expensive and fails to ensure top-quality services. However, constitutional principles can't be compromised, not even for the purpose of achieving the best goals," the advisor told reporters on Tuesday.
"The Constitution guarantees a patient the right and freedom to choose his or her healthcare provider -- no matter whether it is a public or private one," she said.
The president's office says that the laws could be amended without postponing the launch of the reform, now planned for January 2019.