active principle of the thyroid gland, 1915, from thyro-, combining form of thyroid, + oxy- (apparently a reference to the oxygen atom present in it, but OED has it as a shortening of oxy-indol) + chemical suffix -ine (2), denoting an amino acid.
1690s (in reference to both the cartilage and the gland), from Greek thyreoeides "shield-shaped" (in khondrosthyreoeides "shield-shaped cartilage," used by Galen to describe the "Adam's apple" in the throat), from thyreos "oblong, door-shaped shield" (from thyra "door," from PIE root *dhwer- "door, doorway") + -eides "form, shape" (see -oid). The noun, short for thyroid gland, is recorded from 1849.
word-forming element in chemistry, often interchangeable with -in (2), though modern use distinguishes them; early 19c., from French -ine, the suffix commonly used to form words for derived substances, hence its extended use in chemistry. It was applied unsystematically at first (as in aniline), but now has more restricted use.
The French suffix is from Latin -ina, fem. form of -inus, suffix used to form adjectives from nouns, and thus is identical with -ine (1).
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Harper, D. (n.d.). Etymology of thyroxine. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/thyroxine