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...