Diffusion of thyroglobulin in the follicular colloid. (Minireview)

Methods used for estimating in vivo diffusion velocity of thyroglobulin (Tgb), the factors affecting hydrodynamic properties of thyroidal colloid and the effects of changing diffusion properties on follicular function are briefly reviewed. The principal methods, besides pure in vitro techniques, are autoradiography of thyroid sections after in vivo labelling of Tgb and freezing autoradiography for examining colloidal diffusion of ions or other small molecules. The main factors known to affect diffusion of Tgb in the colloid space are concentration and actual physico-chemical properties of the Tgb molecule itself, the latter parameter depending on several factors such as sugar content, iodination degree, etc. Additional factors are thyrotropin which speeds up and drugs such as pentobarbital or verapamil which slow down the velocity of Tgb diffusion. High iodine supply has a retarding effect on Tgb diffusion in the colloid of mice thyroids. Any change of Tgb diffusion in the colloid may have a striking effect on follicular function. The hydrodynamic properties of the colloid components are increasingly recognized as a potentially important factor in regulating kinetics of hormone synthesis.