Correlation between thyroid-follicle fusion and structural modifications of the epithelial cells. A quantitative study in the adult rat.

Changes in thyroid structure induced by a decrease in TSH or iodide-dependent stimulation were quantified by stereological analysis of light micrographs. Studies were carried out on intact (R5) and hypophysectomized (R5H) rats receiving 5 micrograms iodide, and on intact rats (R5O) receiving 50 micrograms iodide daily. For R5H- and R5O-thyroids, the mean parameters of the epithelial cells, height, volume and lateral membrane area, were smaller than those of R5-thyroids. An inverse shift was observed for the apical membrane area, whereas the peripheral membrane area was unchanged. The number of epithelial cells was similar in each of the three groups; however, the number of follicles was greater in R5-thyroids, suggesting that follicular fusion occurs in R5O- and R5H-thyroids. This was confirmed by direct observation. The present results demonstrate that in adult rats a lack of TSH or an increased iodide diet (insufficient to produce a physiopathological state) induce follicle fusion probably by means of cellular reorganization. This increase in follicle size could be involved in the regulation of thyroid iodine turnover.