Levothyroxine Impurity-B (Crude)

Levothyroxine Impurity-B is a chlorinated analog of the parent thyronamine derivative, featuring a 3,5-diiodo-4-hydroxyphenyl moiety linked to a 3-iodo-4-amino tyrosine backbone. The substitution of an aromatic iodine with chlorine at the para-position relative to the hydroxyl group introduces distinct electronic and steric effects, altering its metabolic stability. This impurity arises via oxidative chlorination during synthetic pathways involving iodinated tyrosine precursors. It serves as an HPLC reference standard for quantifying this specific degradation byproduct in levothyroxine active pharmaceutical ingredients.