 The paracellular pathway allows the permeation of hydrophilic molecules that are not able to permeate through the lipid membrane by the transcellular pathway of absorption.This is particularly important for hydrophilic pharmaceuticals, which may not have affinity for membrane-bound transporters, and therefore may be excluded from the transcellular pathway.The vast majority of drug molecules are transported through the transcellular pathway, and the few which rely on the paracellular pathway of transportation typically have a much lower bioavailability; for instance, levothyroxine has an oral bioavailability of 40 to 80%, and desmopressin of 0.16%.