EN6 - Thyroid Gland 1

EN6 - Thyroid Gland 1

1. INTRODUCTION

   • Maintains metabolism for optimal normal function
   • Stimulate O2 consumption by cells
   • Regulate lipid & carbohydrate metabolism
   • Essential for normal growth & maturation

2. STRUCTURE

   • Develop from invagination of pharynx
   • Bilaterally symmetrical @ ventrolateral surface of trachea
   • 2 lobes connected by ISTHMUS (not obvious in cats & dogs)
   • Not normally palpable
   • Excellent blood suplly from cranial carotid a.
     • Drains via caudal thyroid veins into internal jugular veins
   • Rich sympathetic nerve supply

3. HISTOLOGY

   • Follicles & Interfollicular connective tissue
   • Capsule (connective tissue)
     • Surrounds thyroid gland
     • Sends smaller pieces of CT into gland - surrounds individual thyroid follicles
   • Lymphocyte patches sometimes seen @ thyroid & parathyroid glands
   • Thyroid follicles
     • = Simple EPT surrounding COLLOID (gel-like material)
   • Principal Cells
     • aka Thyroid Follicular Cells/Follicular EPT Cells
     • secrete Thyroid Hormones & Thyroglobulin
       • Thyroid hormones stored extracellularly as thyroglobulin
       • Thyroglobulin = a glycoprotein - main component of colloid
     • When gland activity increases...
       • Principal cell thickness increases
       • Less colloid stored
     • Squamous/Low Cuboidal
       • = Relatively inactive gland
     • Columnar
       • = Active gland
   • Parafollicular/ Clear/C Cells
     • Found @ EPT lining of follicle/In groups in CT between follicles
     • Large, Clear (don't stain w/ H&E)
     • Secrete CALCITONIN
       • Lowers Ca level in blood

4. CHEMISTRY OF THYROID HORMONES

   • Thyroid hormones = derivatives of TYROSINE bound covalently to IODINE
   • Number & position of iodine determine biological activity
     • Thyroxine (T4): 2 tyrosines linked tgt w/ 4 iodines
     • Tri0iodothyronine (T3): 2 tyrosines linked tgt w/ 3 iodines
     • Reverse T3: 2 tyrosines linked tgt w/ 3 iodines (Little/no biological activity)
   • Thyroglobulin
     • = A large glycoprotein from which T4 & T3 are synthesised
     • Part of colloid
     • Synthesised by principal cells
       • Secreted into follicle lumen which contains colloid
     • T4 & T3 remain bound to thyroglobulin until secreted
       • Colloid ingested by principal cells -> Peptide bonds hydrolysed
         • Free T3&T4 discharged into capillaries & central circulation
   • Synthesis of T3 & T4= Iodination & condensation of tyrosine

5. PRINCIPAL CELL FUNCTIONS

   • 1. Collect & transport iodine
   • 2. Synthesise thyroglobulin
   • 3. Remove thyroid hormones from thyroglobulin & secrete them into circurlation
   • Incorporation of Iodine
     • Iodine = essential for thyroid hormone synthesis
     • Ingested iodine converted to iodide
       • Absorbed (active transport so efficient uptake)
         • Principal cells avidly take up iodide by IODINE TRAPS @ outer plasma mem.
           • Inside cell: Iodide transported into follicle lumen w/ thyroglobulin
             • Iodide oxidised to iodine
   • Thyroid Peroxidase
     • Catalyses binding of iodine in principal cells to tyrosine
       • 1. Iodination of tyrosines (Oxidative reaction)
       • 2. Synthesis of T4 or T3 from two iodotyrosines (Coupling reaction)
         • T4:T3 ratio in thyroglobulin = 5:1
     • @ luminal surface of principal cells