'athyroid' originates from Greek-derived elements: the prefix 'a-' meaning 'without' and 'thyroid' (from New Latin 'thyreoideus', ultimately from Greek 'thyreoeidēs') where 'thyreoeidēs' meant 'shield-like' (used for the thyroid cartilage/gland).

'thyroid' entered English via New Latin 'thyreoideus' from Greek 'thyreoeidēs' ('shield-like'); in modern medical English the negative prefix 'a-' (Greek) was attached to 'thyroid' to form 'athyroid' meaning 'without a thyroid'.

Initially a straightforward compound meaning 'not having a thyroid gland'; the term's medical sense has remained consistent, referring specifically to absence of thyroid tissue.