Absorption of orally administered levothyroxine from the gastrointestinal tract ranges from 40–80%, with the majority of the drug absorbed from the jejunum and upper ileum. Levothyroxine absorption is increased by fasting and decreased in certain malabsorption syndromes, by certain foods, and with age.xamples include calcium and iron supplements taken within four hours of levothyroxine, as well as soy products within three hours of the medication. Other substances that reduce absorption are aluminium and magnesium containing antacids, simethicone, sucralfate, cholestyramine, colestipol, and polystyrene sulfonate.