The UCI said an anti-doping test that the 32-year-old took in Spain on 7 September showed more than the permitted level of 1,000 ng/ml of the drug Salbutamol in his body.
Team Sky, for whom Froome rides, said the test showed the presence of 2,000 ng/ml - twice the allowed limit - but said Froome had taken the medicine because he had suffered from asthma towards the end of the race.
The UCI has asked the Briton to provide more information but, in line with World Anti-Doping Agency guidelines, has not suspended him.
Team Sky said in a statement: "During the final week of the Vuelta, Chris experienced acute asthma symptoms. On the advice of the Team Sky doctor, he used an increased dosage of Salbutamol (still within the permissible doses) in the run-up to the 7 September urine test."
Sky said the rider had declared his use of the medication. It added: "The notification of the test finding does not mean that any rule has been broken."
Froome added the 2017 Tour de France victory to triumphs in 2016, 2015 and 2013. He won the 2017 Vuelta becoming only the third man after Jacques Anquetil in 1963 and Bernard Hinault in 1978 to win in France and Spain in the same year.
"The UCI is absolutely right to scrutinise the test results," Froome said. "As always, I took the greatest care to ensure that I did not use more than the permissible dose. Together with the team, I will provide whatever information the UCI requires."