The astronaut with the medical concern, whom she did not identify, was in a stable condition on the orbiting laboratory, a NASA spokeswoman said.
"Safely conducting our missions is our highest priority, and we are actively evaluating all options, including the possibility of an earlier end to Crew-11's mission," she said in a statement on Wednesday night.
The news came hours after NASA cancelled a planned spacewalk for Thursday.
NASA said in an earlier statement it was "monitoring a medical concern with a crew member that arose Wednesday afternoon".
Astronauts typically live in six to eight-month rotations on the ISS, with access to basic medical equipment and medications for some types of emergencies.
The four-person Crew-11 crew includes US astronauts Zena Cardman and Mike Fincke, Japanese astronaut Kimiya Yui and Russian cosmonaut Oleg Platonov.
They have been on the space station since launching from Florida in August and were scheduled to return about May.
Fincke, the station's designated commander, and Cardman, assigned as flight engineer, were scheduled to conduct a 6.5-hour spacewalk on Thursday to install hardware outside the station.
NASA's astronaut corps regards medical situations on the ISS as closely held secrets, and astronauts rarely acknowledge or describe publicly their medical conditions.
Spacewalks are arduous and risky missions that require months of training, involving bulky spacesuits and carefully coordinated instructions while tethered to the ISS.
NASA in 2024 called off a planned spacewalk last-minute because an astronaut experienced "spacesuit discomfort".
US astronaut Mark Vande Hei in 2021 called off his spacewalk over a pinched nerve.
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