Ithyroid Follicles

A. Thyroid follicles are hounded by follicular cells and parafollicular cells.

B. They are filled with a colloid that consists of iodinated thyroglobulin.

II. FOLLICULAR CELLS (Figure 21-1). These cells:

A. Contain thyroid-stimulating hormone (TSH) receptors

B. Synthesize thyroglobulin

C. Secrete thyroglobulin into the follicular lumen

D. Take up iodide (I-) from the blood using a Na+-1~ cotransporter and transport it to the follicular lumen

E. Oxidize iodide (2I~ + H207—>I7) using the enzyme thyroid peroxidase and iodinate tyrosine residues in thyroglobulin, thereby forming monoiodotyrosine (MIT) and di-iodotyrosine (DIT), which are then coupled to form triiodothyronine (T5) and thyroxine (T4)

F. Are stimulated by TSH to begin endocytosis of iodinated thyroglobulin

G. Break down iodinated thyroglobulin into MIT, DIT, T5, and T4 through lysosomal degradation

H. Deiodinatc MIT and DIT using the enzyme deiodinase to recycle iodide (I~); and secrete Tj and T.( into the bloodstream, which then circulate bound to thyroid-binding globulin (TBG). T4, which is converted by the liver and kidney into the functionally more potent T5, accounts for 90% of the thyroid output; T, accounts for 10% of the thyroid output.

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