What do Catholic traditions say was the real name of St. Thomas? The Gospels do not mention the real name of St Thomas, unlike in the case of other apostles. The name Thomas (Koine Greek: Θωμᾶς) given for the apostle in the Gospels is derived from the Aramaic or Syriac: ܬܐܘܡܐ‎ Toma, equivalently from Hebrew Teom, meaning &quot;twin&quot;. The equivalent term for twin in Greek, which is also used in the New Testament, is Δίδυμος Didymus. {Courtesy: Wikipedia}
So, if Didymus and Thomas were in fact nick-names, what was the real name of St Thomas the Apostle?
What do Catholic traditions have to say on this?
The 1913 Catholic Encyclopedia article on St. Thomas the Apostle says:


  The principal document concerning him is the "Acta Thomae", preserved to us with some variations both in Greek and in Syriac, and bearing unmistakeable signs of its Gnostic origin (...) it represents Thomas (Judas Thomas, as he is called here and elsewhere in Syriac tradition) as the twin brother of Jesus. The Thomas in Syriac is equivalant to didymos in Greek, and means twin.


So we have an ancient Syriac-language tradition which gives St. Thomas a birth name of Judas — which is the name of at least two other apostles, Judas Thaddaeus and Judas Iscariot. Since, however, we have a lot of repeated names among the Gospel characters, this is not an issue.

The Church does not regard the names that St. Thomas might have had or not particularly important to the Gospel message, so Catholics are free to believe or disbelieve in this Syriac tradition; but at any rate it exists.