athyroidal

The story

1. Proto-Indo-European*né
2. Proto-Indo-European*n̥-
3. Proto-Hellenic*ə-
4. Ancient Greekἀ- (a-)
5. Englisha-
6. Proto-Indo-European*dʰwer-
7. Proto-Hellenic*tʰurā
8. Ancient Greekθῠ́ρᾱ (thŭ́rā)
9. Ancient Greekθῠρεός (thŭreós)
10. Proto-Indo-European*weyd-
11. Proto-Indo-European*wéydos
12. Proto-Hellenic*wéidos
13. Ancient Greekεἶδος (eîdos)
14. Ancient Greekθῠρεοειδής (thŭreoeidḗs)
15. New Latinthyreoīdēs
16. Englishthyroid
17. Proto-Indo-European*h₂el-
18. Proto-Indo-European*h₂élisder.?
19. Englishthyroidal
20. English todayathyroidal

From a- + thyroidal.