1. Functional Unit and Structure of thyroid gland:
The thyroid gland is composed of follicles, each consisting of a single layer of epithelial cells surrounding a cavity filled with colloid containing thyroglobulin. This structure is the functional unit of the thyroid, responsible for the synthesis, storage, and secretion of thyroid hormones.
2. Synthesis of Thyroid Hormones:
Thyroglobulin, a large glycoprotein, is synthesized, glycosylated, and secreted into the follicle lumen, where iodination of its tyrosine residues occurs. This process involves the uptake of plasma iodide by the follicle cells, oxidation of iodide, and iodination of tyrosine residues of thyroglobulin, facilitated by thyroperoxidase.
3. Storage and Secretion:
The iodinated thyroglobulin serves as a significant store of thyroid hormones within the gland. The thyroglobulin molecule is taken up into the follicle cell by endocytosis, where proteolytic enzymes act on thyroglobulin, releasing T4 and T3 to be secreted into the plasma. The surplus monoiodotyrosine (MIT) and di-iodotyrosine (DIT) are scavenged by the cell, and the iodide is removed enzymatically and reused.
4. Regulation of Thyroid Function:
Thyroid function is regulated by Thyrotrophin-releasing hormone (TRH) from the hypothalamus and thyroid-stimulating hormone (TSH) from the anterior pituitary gland. TSH controls all aspects of thyroid hormone synthesis, including the synthesis and secretion of thyroglobulin, the generation of H2O2, the iodination of tyrosine, and the actual secretion of T3 and T4. The production of TSH is regulated by a negative feedback effect of thyroid hormones on the anterior pituitary gland.
5. Actions of Thyroid Hormones:
Thyroid hormones, particularly T3, produce a general increase in the metabolism of carbohydrates, fats, and proteins, regulating these processes in most tissues. They are crucial for normal growth and development, influencing growth hormone production and potentiating its effects on its target tissues. They also play a significant role in the response to a cold environment, augmenting cardiac rate and output and increasing the tendency to dysrhythmias such as atrial fibrillation.
6. Transport and Metabolism:
Thyroid hormones are transported in the blood, mainly bound to thyroxine-binding globulin (TBG). They are metabolized in their target tissues by deiodination, deamination, decarboxylation, and conjugation with glucuronic and sulfuric acids. The liver is a major site of metabolism, and the hormones are excreted partly in the bile and partly in the urine.
With its intricate physiology, the thyroid gland is central to metabolic processes, growth, and development in the human body. Its functional units, the follicles, are meticulously regulated to synthesize, store, and secrete thyroid hormones, ensuring the proper functioning of various physiological processes crucial for maintaining metabolic equilibrium in the body.