Paracrine Regulation of Thyroid-Hormone Synthesis by C Cells

Thyroid hormone secretion by follicular cells is mainly regulated by thyroid-stimulating hormone (TSH) from the pituitary gland, which in turn is controlled by thyrotropin releasing hormone (TRH) of hypothalamic origin. However, there are a number of additional endocrine mediators, such as estrogens and corticosteroids, which are also involved in the regulation of the biosynthesis of T3 and T4. Besides this extrathyroidal main regulation, thyroid function and growth are under the influence of intrathyroidal peptides. Thus, there is evidence of the role played by some local factors released by nerve endings, such as vasointestinal peptide (VIP) or neuropeptide Y [1], by follicular cells themselves, such as insulin-like growth factors (IGFs) [2], and, specifically, by C cells through a paracrine mechanism, acting locally upon neighboring follicular cells [1]. This last intrathyroidal mechanism of regulation of follicular-cell activity by C cells is the main subject of the present review.