ABSTRACTExtracts obtained from defatted soybean flour by water extraction and acetone fractionation caused a decrease in radioiodine uptake by rat thyroid “in viva” up to 24 hr after administration of the extract by stomach tubing. The inhibitory effect was evident at 24 hr only when either the extract or the flour had been previously autoclaved. Similar results were recorded when extracts from commercial products such as soybean milk, protein concentrate, and toasted flour were used. However, using extracts from unheated raw soy flour, the inhibitory effect was only present 6 hr after ingestion and it subsequently disappeared. The active factor is not precipitated by 90 ethanol. Isolated soybean isoflavones either autoclaved or nonautoclaved did not show any inhibitory activ
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