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Thyroid Gland Diagram • Butterfly-shaped and largest endocrine gland located in the anterior neck overlying the trachea just inferior to the larynx. • The thyroid gland has two lateral lobes connected by a medial tissue mass called the isthmus. • Thyroid gland composed of follicles which are concentric rings of cells with a lumen. • The walls of each follicle formed largely by cuboidal epithelial cells which produce the glycoprotein thyroglobulin. • Iodinated thyroglobulin is used to synthesize two thyroid hormones (thyroxin and triodothyronine) which are collectively called thyroid hormone. • Thyroxine (T4) and triodothyronine (T3) - thyroid hormone stimulates enzymes concerned with glucose oxidation and therefore increases basal metabolic rates and heat production (calorigenic effect) • Synthesis of thyroid hormone: Diagram  Thyroglobulin synthesis - TSH is secreted and sent to follicle cells in the thyroid. Thyroglobulin is synthesized on ribosomes and sent to golgi where sugar residues are attached. Thyroglobulin is then packed into vesicle, transported to the apex of the cell, and discharged into the lumen of the follicle.  Iodine attachment - cells accumulate iodides and convert them to iodine which attach to tyrosine aminoacids of thyroglobulin (results in monoiodotyrosines, MIT, and diiodotyrosines, DIT). Enzymes link iodotyrosines: MIT + DIT = T3 and DIT + DIT = T4. Hormones are still part of the colloid and need to be cleaved to be released.  Hormone Cleavage - follicle cells reclaim iodinated thyroglobulin by endocytosis and packaged in lysosomes - lysosomal enzymes cleave T3 and T4 which diffuse into the blood stream.  NOTE: some T4 is converted to T3 before secretion BUT most T3 is generated in the peripheral tissues • T3 and T4 are second messenger hormones Thyroid Hormones TRH (thyrotropin releasing hormone) made by the hypothalamus stimulates the release of TSH (thyroid stimulating hormone) from the anterior pituitary which regulates the production and release of thyroid hormones. The thyroid produces two major hormones which regulate metabolism. [Thyroid hormones: Foye 688-702.] Thyroid hormones have two major physiological effects. They increase protein synthesis in virtually every body tissue and increase oxygen consumption dependent upon Na+ -K+ ATPase (Na pump). Hyperthyroidism occurs seven to nine times more likely in women than in men. Hyperthyroidism often occurs between the ages of twenty and forty, while hypothyroidism often appears after the age of fifty. Certain types of thyroid tumors appear to be more prevalent in certain age groups. In the last few years, case studies of patients with hyperthyroidism in their youth have shown that, thirty years later, the same individuals have a greater tendency to develop hypothyroidism. Thyroid problems have a strong genetic component, and may "skip" generations. Family histories may be useful in suggesting the nature of thyroid problems at early stages. Prematurely gray hair, hair which starts to go gray before the age of thirty, is often a sign of thyroid dysfunction. Patchy hair loss, usually temporary, is often observed by persons with Graves' or Hashimoto's disease. A list of conditions related to the thyroid are instructive as to the importance of the thyroid in human disease.