'thyroid-activating' is a compound of 'thyroid' and 'activate'. 'thyroid' originates from New Latin 'thyreoides', ultimately from Greek 'thyreoeidēs', where 'thyreos' meant 'shield'. 'activate' originates from Latin 'activare', from 'actus' (meaning 'do, drive') and the suffix '-are'.

'thyroid' passed into scientific New Latin from Greek 'thyreoeidēs' and entered English in the 17th–19th centuries as the anatomical name for the gland; 'activate' came into English from Latin via Late Latin/Medieval Latin 'activare' and French/English usage meaning 'make active'. The compound 'thyroid-activating' is formed in modern medical/technical English by joining the noun and present participle/verb-derived adjective.

Individually, 'thyroid' originally referred to a 'shield-like' structure (the name of the gland) and 'activate' meant 'make active'; together the compound evolved to mean 'producing activation of the thyroid (gland)', used especially in medical contexts to describe agents or effects that stimulate thyroid function.