Thyroid morphological and functional heterogeneity: impact on iodine secretion.

Thyroid iodine turnover heterogeneity includes morphological (cellular and colloidal distribution space for iodide) and functional heterogeneity (hormone synthesis in the colloid). In 'normal' rats, both iodide actively trapped by the epithelial cell and that coming from deiodination of iodotyrosines present the same probability for thyroglobulin (Tg) iodination (Tg iodination flux: 4.0 +/- 0.3 micrograms I/day). A portion of the thyroid iodide is sequestered in the colloid lumen and is inoperative in the Tg iodination mechanisms. The masses of cell and colloid compartments are equivalent (0.018 +/- 0.002 micrograms I) while colloid iodide concentration is twice that of the cell (0.11 and 0.06, respectively). The turnover of about 3 micrograms I of colloid iodine (Tg) is follicle diameter-dependent (inter-follicular heterogeneity) and it is mainly characterized by 2 different half lives of 8 and 16 hours, respectively. Ninety percent of the thyroid iodine (hormone) secretion (1.10 +/- 0.11 micrograms I/day) is provided by this compartment rich in iodotyrosine residues (70%). The remaining 10% of iodine secretion is provided by a Tg pool (7 micrograms I) characterized by 2 compartments (intra-follicular heterogeneity) with slow and very slow turnovers. The longer the transit time of Tg molecules in the colloid, the higher their iodothyronine content.