**6. Conclusion**

Loss of follicular structure during the first 24 h of culture has been the main drawback of *in vitro* thyroid studies and, therefore, hormone synthesis. It is not enough to conserve thyrocytes' apical-basal polarity in culture in this specific tissue for maintaining colloid's extracellular functions for the enzymes implicated in iodide fixation on Tg and hormone synthesis.

We have shown that follicular architecture must be conserved in culture, especially the follicular cavity isolated from extracellular medium as this is indispensable for maintaining ultra-structure and the polarity of thyrocytes around the follicular cavity; such premise conserving the idea of Tg19S synthesis usually being glycosylated and iodised, as also T3 and T4 hormone synthesis. Follicular morphological conservation is necessary for reproducing the Wolff-Chaikoff effect *in vitro*, as has been described *in vivo.* NIS symporter localisation in thyrocytes depends on I and TSH concentration.

This culture may be used for obtaining follicles from pathologies of human tissue whose epithelium may be thin plate-like cells for *in vitro* studies in controlled and homologous conditions regarding the pathology *in vivo*. It will also enable studying normal or pathological thyroid's physiological, cellular and molecular mechanisms (for example CLC-5 channel) in a homologous model of the gland *in vivo*.
