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