Anatomy,Histology,nerve supply,blood supply and synthesis of Thyroid Hormone
The thyroid gland is an endocrine gland located in the front of the neck, below the Adam's apple. It is responsible for producing and releasing thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), which regulate various processes in the body, including metabolism, growth, and development.
Anatomy:
The thyroid gland consists of two lobes, connected by a bridge of tissue called the isthmus. It is situated anteriorly to the trachea, just below the larynx. The gland is typically shaped like a butterfly, with the lobes wrapping around the trachea.
Histology:
Histologically, the thyroid gland is composed of many spherical structures called thyroid follicles. These follicles are lined with a layer of epithelial cells known as follicular cells. The follicular cells produce and store the thyroid hormones within the follicles. Interspersed between the follicles are parafollicular or C cells, which secrete the hormone calcitonin.
Nerve Supply:
The thyroid gland is innervated by the autonomic nervous system. Sympathetic innervation arises from the superior cervical ganglion, while parasympathetic innervation is provided by the vagus nerve (cranial nerve X). However, the autonomic nervous system's role in regulating thyroid function is minimal compared to the hormonal control.
Blood Supply:
The thyroid gland receives its blood supply from the superior thyroid arteries, which arise from the external carotid arteries, and the inferior thyroid arteries, which branch from the thyrocervical trunk. These arteries deliver oxygenated blood to the gland. The venous drainage is achieved through the superior, middle, and inferior thyroid veins, which eventually drain into the internal jugular vein.
Synthesis of Thyroid Hormone:
The synthesis of thyroid hormone occurs within the follicular cells of the thyroid gland. The process involves several steps:
1. Uptake of iodide: Iodide ions (I-) are actively transported from the bloodstream into the follicular cells by a specific transporter known as the sodium-iodide symporter (NIS).
2. Oxidation of iodide: Within the follicular cells, iodide is oxidized to iodine (I2) by the enzyme thyroid peroxidase (TPO).
3. Formation of thyroglobulin: Thyroglobulin, a large glycoprotein synthesized by the follicular cells, is secreted into the colloid within the follicles.
4. Iodination of thyroglobulin: Iodine is added to the tyrosine residues of thyroglobulin, resulting in the formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT).
5. Coupling of MIT and DIT: MIT and DIT molecules combine to form T3 (one MIT and one DIT) or T4 (two DIT molecules).
6. Storage and release: The thyroid hormones T3 and T4 are stored within the colloid of the follicles. When stimulated, the follicular cells reabsorb the colloid and release T3 and T4 into the bloodstream.
7. Transport and conversion: In the bloodstream, thyroid hormones bind to carrier proteins (such as thyroxine-binding globulin, TBG) for transport to target tissues. T4 is the main hormone released, but it is converted to the more active T3 in peripheral tissues through the removal of one iodine atom.
Once released into the bloodstream, thyroid hormones exert their effects on various target organs throughout the body, regulating metabolism, growth, and development.