How does NASA handle astronaut medical issues in space?

An astronaut in a white space suit faces toward the bottom of the image amidst a series of space equipment floating above Earth. .

NASA astronauts Mike Fincke and Zena Cardman were expected to perform a spacewalk outside the International Space Station on Thursday (Jan. 8), but it was called off Wednesday afternoon (Jan. 7) due to "medical concerns" with an unidentified crew member.

Those concerns apparently grew in the hours that followed, prompting NASA to issue a follow-up statement to its EVA (extravehicular activity) cancellation, indicating that bringing astronauts home early is on the table.