In 1984 he rode in the Olympic Games at Los Angeles and then turned professional on 4 September for Reynolds. He won his first professional race a week later, a time trial in the Tour de l'Avenir. In 1985 he started the Vuelta a Espana and came second in the prologue, behind Bert Oosterbosch. Oosterbosch lost time on the second stage and Indurain became leader, the youngest rider to do it. He rode the Tour de France later that year, as he would do in each of the next 11 years, but dropped out in the fourth stage.  In 1986, Indurain again rode the Tour, dropping out on the 12th stage. He started the 1987 Vuelta a Espana with bronchitis from the Tour of Belgium. He rode the 1988 Tour de France as teammate of the winner Pedro Delgado. In 1989, he escaped during the ninth stage of the Tour de France. He won the stage and became leader of the mountains classification, wearing the polkadot jersey the next stage, the only time in his career. In 1990, Indurain rode the Tour de France again for Delgado, but Delgado could not win. Indurain finished 10th place, sacrificing several places by waiting for Delgado.  Indurain was a strong time trialist, gaining on rivals and riding defensively in the climbing stages. Indurain won only two Tour stages that were not individual time trials: mountain stages to Cauterets (1989) and Luz Ardiden (1990) in the Pyrenees. During his five consecutive Tour de France wins he never won a stage that was not a time trial. These superior abilities in the discipline fit perfectly with the TT heavy Tours of the era, with many featuring between 150 and 200 km of time trialling vs the more common 50-80 km today.

Answer this question "When did his professional career begin?" by extracting the answer from the text above.
In 1984 he rode in the Olympic Games at Los Angeles and then turned professional on 4 September for Reynolds.