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**Chapter 3** 

© 2012 Martín-Lacave et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

**Paracrine Regulation of Thyroid-Hormone** 

Rocío García-Marín, José Carmelo Utrilla and Inés Martín-Lacave

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

The thyroid gland consists of two lobes connected by a narrow band of thyroid tissue called the isthmus, located in the midportion of the neck, immediately in front of the larynx and trachea. Each lobe of the thyroid gland consists of numerous follicles, the **thyroid follicles,**  which constitute the structural and functional unit of the gland. Each follicle consists of a simple layer of cuboidal epithelial cells, the **follicular epithelium,** enclosing a central lumen containing a **colloid** substance rich in **thyroglobulin,** an iodinated glycoprotein, yielding a periodic-Schiff (PAS)-positive reaction. The shape of the normal follicles varies from round to oval and they show a considerable size variation depending on the degree of gland

and reproduction in any medium, provided the original work is properly cited.

José María Fernández-Santos, Jesús Morillo-Bernal,

**2. Histological organization of the thyroid gland** 

Additional information is available at the end of the chapter

**Synthesis by C Cells** 

http://dx.doi.org/10.5772/46178

subject of the present review.

activity (Figure 1).

**1. Introduction** 
