Thyroid autoregulation: impact on thyroid structure and function in rats.

Global response of the thyroid to a 10-fold increase in plasma iodide concentration results only in modifications of basolateral transfer for iodide where, by a balance phenomenon between influx and efflux, a net thyroid intake of 1.2 micrograms I/day is constantly maintained. The other main steps of thyroid iodine metabolism, thyroglobulin (Tg) iodination, endocytosis, and hydrolysis, remain constant. A stationary hormone secretion delivery results. This constant state is not found at a cellular level, where structural changes are observed in correlation with functional regulation of the cell dynamic state. Thus, an increase in plasma iodide concentration results in an increase of the apical membrane area (40%), whereas the basolateral membrane area decreases (18%). The volume of the follicle lumen increases (76%). Nevertheless, neither epithelial cell volume nor the structure of microvilli are modified. Comparison of iodine fluxes through the apical and basolateral membrane of the epithelial cell shows that an increase in plasma iodide concentration is correlated to a decrease of the Tg iodination and endocytotic fluxes (45%). A regulation also appears for basolateral transfer of iodide, whereas the lysosome system does not modulate the hormone secretion mechanism.