levothyroxine
noun
le·vo·thy·rox·ine
variants:
or British laevothyroxine \ ˌlē-vō-thī-ˈräk-ˌsēn, -sən 
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Medical Definition of levothyroxine
: the levorotatory isomer of thyroxine that is administered in the form of its sodium salt C15H10I4NNaO4 in the treatment of hypothyroidism — see levothroid, levoxyl, synthroid