AHD's etymology puzzles me, in view of Webster 1913 giving
no mention of relation to endocrine fluctuation.

menstruum

SYLLABICATION: men新tru暉m
PRONUNCIATION: mnstr-m
NOUN: Inflected forms: pl. men新tru暉ms or men新tru戢 (-str-)
A solvent, especially one used in extracting compounds from plant and animal tissues and preparing drugs.
ETYMOLOGY: Middle English, menstruation, from Medieval Latin mnstruum, sing. of Latin mnstrua, menses. See menstruate.