Thyroid-Physiology by GerardLum


									                                                                                                                                         | Medicine
Thyroid Physiology

Anatomy                                                                               Control of Thyroid Function

2 Lobes united by Isthmus (anterior to 2nd , 3rd Tracheal cartilage)
Blood Supply
       Superior Thyroid Artery                      Inferior Thyroid Artery
External Carotid Artery                   Thyrocervical Trunk
Closely related to                        Closely related to
External Branch of Superior Laryngeal     Recurrent Laryngeal Nerve                   Thyroid Hormone Actions
Nerve                                                                                 ↑ O2 Cons umption
Venous                                                                                ↑ Mitochondrial size, number, enzymes
Superior Thyroid v.     Middle Thyroid v.                Inferior Thyroid v.          CVS
Superior Poles          Lateral Poles           Inferior Poles                        ↑ HR, SV, CO
Empty into IJV                                  Empty into Brachiocephalic v.         Respiratory
                                                                                      ↑ RR
Histology                                                                             Renal
                                                                                      ↑ Blood Flow, GFR
                                                                                      O2 Carrying Capacity
                                                                                      ↑ RBC mass
                                                                                      ↑ 2,3-DPG (Shift Hb-O2 curve to Right)
                                                                                      Protein Metabolism
                                                                                      Stimulate cellular uptake of a.a., Protein Synthesis
                                                                                      Lipid Metabolism
                                                                                      ↑ Plasma levels of FFA
                                                                                                   Hypothyroidism                           Hyperthyroidism
                                                                                        ↓ LDL Receptors                           ↑ LDL Receptors
                                                                                        ↓ Cholesterol Excre on                    ↑ Cholesterol Excre on
                                                                                        ↑ Circula ng Cholesterol                  ↓ Circula ng Cholesterol
                                                                                      CHO Metabolism
                                                                                      ↑ Rate of Intestinal Glucose Absorption, Uptake of Glucose
                                                                                      Vitamin Metabolism
                                                                                      Hepatic conversion of Carotene → Vitamin A (in Liver)
                                                                                      In Hypothyroidism – Accummulation of Carotene (yellow discoloration)
                                                                                      Growth, Tissue Development
                                                                                      ↑ Growth, Maturation Bone
                                                                                      Nervous System
                                                                                      Normal CNS Neuronal development
                                                                                      Enhance Wakefulness, Alertness
                                                                                      Enhance Memory, Learning Capacity
                                                                                      Required for Normal Emotional Tone
                                                                                      ↑ Speed, Amplitude of Peripheral Nerve Reflexes
                                                                                        Female                                    Male
                                                                                        Follicular Development                    Spermatogenesis
                                                                                        Maintain Pregnancy
Follicular Epithelium                                                                 Skin Changes
    Follicular Cells (Principal cells)          Parafollicular Cells (C Cells)                     Hyperthyroidism                          Hypothyroidism
Produces Thyroid Hormones                 Secrete Calcitonin (Ca2+ regulation)          Warm, Moist Skin                          Dry, Thick (non pitting edema),
                                          Pale staining                                 Extra Heat Production (due to             Scaly Skin
•     Thyroxine (T4)
•     Triiodothyronine (T3)               Larger than Follicular cells                  excess thyroid hormones produced )        Accumulation of proteins,
     Inactive                Active       Solitary cells or small clusters of cells                                               polysaccharides, hyaluronic acid,
Squamous               Low Columnar                                                                                               chondroitin sulphuric acid
                                                                                                                                  Promotes H2O retention
                                                                                                                                  | Medicine

Thyroid Hormone Synthesis                                                          Metabolism of Thyroid Hormone
                 Uptake of Iodide (I-) in exchange for Na+                         Normally Deiodinated in Liver, Kidneys, other tissues
                           (stimulated by TSH)                                     Of the Circulating T4
                                                                                   •     33% converted to T3
                   Oxidation of Iodide to for m Active Iodine
                                                                                   •     45% converted to rT3
               (by H2O2)(catalyzed by Thyroid Peroxidase, TPO)
                                        ↓                                                             T3                                     T4
                          Active Transport of Iodine                               Acts ↑ Rapidly (3-5X ↑ Potent)           Metabolically Inert
                     across apical surface of Follicular cell                      Greater Affinity to Nuclear receptors    Prohormone
                                        ↓                                          than T4
         Active Iodine + Tyrosine Residue of Thyroglobulin Molecules
                           Mono-iodotyrosines (MIT)                                Inhibition of 5’ Deiodi nase
                            Di-Iodotyrosines (DIT)
                 Uptake of Thyroglobulin into lumen of Follicle
                    Lining of Iodinated Tyrosine Residues

1% of Stored Colloid are removed daily
100% of Stored Colloid are removed when gland is very active (depleted)

                                                                                   Results in Plasma
                                                                                   •    ↓ T3
                                                                                   •    ↑ rT3

Secretion of Thyroid Hormones
Under influence of TSH, colloid droplets (consisting of thyroid hormone s within
thyroglobulin molecules) are taken back up into follicular cells by pinocytosis
Fusion of colloid droplets with lysosomes causes hydrolysis of thyroglobulin
Release T3, T4
About 10% of T4 undergo mono-dei odination to T3 before it is secreted
The released Iodine is reutilized
On average (per day)
80 ug of T4 are secreted
13 ug of T3 are secreted

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