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Thyroid Factor

The Natural Thyroid Diet

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Figure 1-1. A calcified lingual thyroid (covered by saliva) is seen at the base of the tongue in a young infant.

Figure 1-2. The normal thyroid gland is composed of two lateral lobes connected via an isthmus. A, The pyramidal lobe, a remnant of the caudal end of the thyroglossal duct, extends cranially from the isthmus of the thyroid gland. Reproduced with permission from Greenspan FS, Gordon JS. Basic and clinical endocrinology. 5th ed. Norwalk (CT): Appleton and Lange; 1997. B, A cross-section view of the anterior portion of the neck at the level of T1, showing the thyroid relations. Reproduced with permission from Linder HH. Clinical anatomy. Norwalk (CT): Appleton and Lange; 1989.

several signal transduction pathways: the cyclic adenosine monophosphate-mediated activation of a protein kinase A system is responsible for increased hormone production and, to a lesser extent, increased growth, and the phospholipase C-mediated activation of the protein kinase C system is primarily responsible for cellular growth.

The follicular cells of the thyroid gland synthesize and secrete thyroxine (T4) and, to a lesser extent, triiodothyronine (T3), the more active hormone. T3 is predominantly produced by the extrathyroidal conversion of T4 to T3 (Figure 1-3) in the peripheral tissues by the action of 5'-deiodinase on T4. A TSH-stimulated thyroid gland secretes relatively more T3.

Normal thyroid hormone formation depends on normal levels of TSH and an adequate but not excessive supply of iodine (150 to 500 p,g per day). Iodine is absorbed in the gastrointestinal tract after being reduced to iodide. Thyroid cells normally take up about 25% of the absorbed iodide through a sodium-iodide symporter. Iodide is oxidized and incorporated into tyrosine molecules of thyroglobulin via a thyroid peroxidase enzyme, which also couples monoiodinated and deiodinated tyrosines to form T3 or T4 (Figure 1-4). Thyroglobulin is then exocytosed and stored as colloid. Endocytosis of colloid droplets followed by proteolysis of thyroglobulin leads to release of thyroid hormones into the capillaries.

Figure 1-3. Thyroid hormone synthesis in a thyroid follicle. DI = deiodinase; DIT = diiodotyrosine; H2O2 = hydrogen peroxide; I- = iodide ion; I0 = active I; MIT = monodiodotyrosine; T3 = triiodothyronine; T4 = thyroxine; Tg = thyroglobulin; TPO = thyroidal peroxidase; Tyr = tyrosine;. Reproduced with permission from Greenspan FS, Gordon JS. Basic and clinical endocrinology. 5th ed. Appleton and Lange; 1997.

Figure 1-4. Iodine metabolism: the values are representative of those that might be found in a healthy subject ingesting 500 |jg of iodine a day. The actual iodine intake varies considerably among different individuals. GI = gastrointestinal; T4 = thyroxine; T3 = triiodothyronine; I = iodine; ECF = extracellular fluid. Reproduced with permission from Greenspan FS, Gordon JS. Basic and clinical endocrinology. 5th ed. Appleton and Lange; 1997.

Figure 1-4. Iodine metabolism: the values are representative of those that might be found in a healthy subject ingesting 500 |jg of iodine a day. The actual iodine intake varies considerably among different individuals. GI = gastrointestinal; T4 = thyroxine; T3 = triiodothyronine; I = iodine; ECF = extracellular fluid. Reproduced with permission from Greenspan FS, Gordon JS. Basic and clinical endocrinology. 5th ed. Appleton and Lange; 1997.

Abnormalities of the thyroid gland develop because of disorders of thyroid function or because of hyperplastic or neoplastic growth.2

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