**4. Conclusion**

In conclusion, according to the data published so far in the literature and revised through this chapter, a number of regulatory factors are produced at thyroid level by C cells and have direct effects on follicular-cell activity at both proliferative and thyroid hormone synthesis levels (Figure 9). These new paracrine regulatory mechanisms would justify, from a phylogenetic point of view, the parafollicular localization of C cells in mammals and might open new molecular approaches for the treatment of human thyroid disease.

Paracrine Regulation of Thyroid-Hormone Synthesis by C Cells 71

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