extrathyroid

The story

1. Proto-Indo-European*h₁éǵʰ
2. Proto-Indo-European*h₁éǵʰs
3. Proto-Indo-European*h₁eǵʰsteros
4. Proto-Italic*eksteros
5. Latinexter
6. Latinextrā
7. Englishextra-
8. Proto-Indo-European*dʰwer-
9. Proto-Hellenic*tʰurā
10. Ancient Greekθῠ́ρᾱ (thŭ́rā)
11. Ancient Greekθῠρεός (thŭreós)
12. Proto-Indo-European*weyd-
13. Proto-Indo-European*wéydos
14. Proto-Hellenic*wéidos
15. Ancient Greekεἶδος (eîdos)
16. Ancient Greekθῠρεοειδής (thŭreoeidḗs)
17. New Latinthyreoīdēs
18. Englishthyroid
19. English todayextrathyroid

From extra- + thyroid.