Published on July 8, 2007
ENDOCRINE SYSTEM 2 R. Kalliecharan
Thyroid Gland • Location--- below Larynx, has lateral lobes and isthmus (and maybe pyramidal lobe) • Embryology--- endoderm (cephalic part of digestive tract • Histology---- composed of thyroid follicles • ---are spherical, lumen is filled • with colloid—a gelatinous material • and lined by simple epithelium • - from simple squamous to simple columnar
Cells of Thyroid Follicles • 1) Thyroid Follicular cells ---secrete Thyroxine (T4) & Triiodothyronine (T3) • ---hormones are stored in colloid • 2) Parafollicular or C cells --- secrete calcitonin which acts on osteoclasts by • a) Inhibiting bone resorption b)↑ Ca++absorption by bones • c)↓ Ca++ and PO4– in blood • • High levels of Ca increases calcitonin and ↓ PTH • Note : In humans, calcitonin appears to play a minimal role in Ca regulation
Thyroid Follicular Cells • A) TSH from Pit binds to receptors on basal surface of follicular cells and triggers synthesis and secretion • 1) Thyroglobulin(TGB) syn via RER and Golgi is released into lumen of follicle • 2) Iodide (I-) is actively transported from capillaries by a Na/I symporter on basal surface into cytosol
•3) 2 (I-) is oxidized to Iodine (I2) by thyroid Peroxidase on apical membrane •4) Iodine in lumen binds to tyrosine of TGB to form T1 , T 2 , T3 and T4
• 4) T1 = monoiodotyrosine • T2 = diiodotyrosine • T1 and T2 = T3 (triiodothyronine ) • T2 and T2 = T4 ( thyroxine or tetraiodothyronine ) • 5) Secretion--- pinocytosis of TGB and lysosomal digestion. T1 , T2 ,T3 and T4 are released. T1 and T2 are reused in cell. T3 and T4 enter blood capillary
Control of Thyroid Hormone Secretion • 1) By level of iodine---diet low in iodine hinders syn of hormones causing hypothyroidism. Thyroid hypertrophy results from ↑ TSH secretion . This disorder is called iodine deficiency goiter. • 2) By negative feedback system
Disorders • Cretinism results from hyposecretion of thyroid hormones during fetal life or infancy— dwarfism and mental retardation • Graves Disease results from thyroid hyperfunction due to immunological dysfunction. Antibodies binds to TSH –receptors on thyroid cells and mimics TSH
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