'antithyroid' originates from Greek and New Latin elements: the prefix 'anti-' (Greek) meaning 'against' and 'thyroid' from New Latin 'thyreoideus' (ultimately from Greek 'thyreoeides').

'thyreoeides' (Greek, meaning 'shield-like') became New Latin 'thyreoideus' and then English 'thyroid'; the compound 'antithyroid' was formed by adding the prefix 'anti-' to 'thyroid' in modern medical English.

Initially the components meant 'against' and 'shield-like (thyroid)'; over time the compound came to specifically mean 'against thyroid function' or 'suppressing the thyroid', a medical sense that has remained stable.