Thyroid Gland DR SYED SHAHID HABIB MBBS FCPS DSDM

Thyroid Gland Histology Acini / follicles ( active & inactive gland) Cells Thyroid cells – amine hormones- T3,T4, and rT3 C - cells (Para follicular cells) protein hormone – Calcitonin)

Thyroid Hormones Triiodothyronine (T 3 ) By Thyroid Follicular Cells Thyroxine or Tetraiodothyronine (T4) By Thyroid Follicular Cells Reverse Triiodothyronine - rT 3 By Thyroid Follicular Cells Calcitonin By Thyroid Para Follicular C Cells

SYNTHESIS ColloidBlood Thyroid Cell I-I- ATP Na I-I- I MIT Tyrosine NaI DIT Thyroglobulin Mol MITDIT T3T3 T4T4 Peroxidase TG Peroxi EPR Exo + + IODIDE TRAPPING OXIDATION OF IODIDE ION IODINATION OF TYROSINE COUPLING

SECRETION Colloid Blood Thyroid Cell DIT Thyroglobulin Mol MITDIT T3T3 T4T4 MITDIT T3T3 T4T4 Lyso pro MITDIT T3T3 T4T4 I Tyrosine Deiodinase I Endo TSH FROM ANTERIOR PITUITARY

Production of Thyroid Hormones Iodide (I - ) actively transported into the follicle and secreted into the colloid. Oxidized to iodine (I o ). Iodine attached to tyrosine within thyroglobulin chain. –Attachment of 1 iodine produces monoiodotyrosine (MIT). –Attachment of 2 iodines produces diiodotyrosine (DIT). MIT and DIT or 2 DIT molecules coupled together.

Production of Thyroid Hormones (continued) T 3 and T 4 produced. TSH stimulates pinocytosis into the follicular cell. –Enzymes hydrolyze T 3 and T 4 from thyroglobulin. Attached to TBG and released into blood.

Production of Thyroid Hormones (continued)

T3T3 T4T4 Normal Plasma level 1.2 – 3.1 n mol/L n mol/L Free 0.3 ng0.15 ug Bound 99.8%99.98% Binding Proteins Thyroxin Binding Globulin Duration of action 1-2 days4-6 days Site of formation/ Source Thyroid Cells + T4Thyroid Cells Potency 4-5 times More potentLess potent

Mechanism of Action T3,T4 – Genomic action – formation of new proteins TRH – Activation of Phospholipase-C System TSH – Activation of cAMP system

FEEDBACKCONTROL OF THYROID SECRETION T3 T4 PituitaryTSH HYPOTHALAMUS TRH

PHYSIOLOGICAL EFFECTS OF THYROID HORMONE  Regulation of metabolism – (O2 consumption)  Mental development  Normal growth  potentiate somatomedin  Normal secretion of GH.  Skeletal maturation  ossification of cartilage & growth of teeth  Normal menstrual cycle  Conversion of carotene to vitamin A.

Actions of T 3 Stimulates protein synthesis. Promotes maturation of nervous system. Stimulates rate of cellular respiration by: –Production of uncoupling proteins. –Increase active transport by Na + /K + pumps. –Lower cellular [ATP]. Increases metabolic heat. Increases metabolic rate. –Stimulates increased consumption of glucose, fatty acids and other molecules.

Effects of Hypersecretion Effects of hypersecretion are due to increased O 2 consumption Catabolism of carbo; proteins; & fats. Enzyme synthesis No. and size of mitochondria ATPase activity

Effects of Hypersecretion Caloregenic Effects Effects on CVS Effects on CNS Effects on GIT Effects on Sympathetic system Effects on Growth

Caloregenic effects It involves the effects on –Carbohydrate Metabolism –Protein Metabolism –Fat Metabolism Increased Catabolism

EFFECTS ON CVS Increases Heart rate Cardiac output Systolic pressure Pulse pressure Mean pressure

Effects on CNS  Rapid cerebration  Psychoneurotic tendency  Tremors  Time for stretch reflex is reduced  Decreased sleep

Table 11.8

GOITER

Features of Hyperthyroidism Increased –BMR, and Heat production –Heart rate – palpitation –Weight loss Diarrhea Exophthalmos & corneal ulcers GRAVE’S DISEASE

Features of Hypothyroidism Decreased BMR and Pulse rate Easy fatigability Slow reflexes Slow mentation Slow speech Increased weight Increased sleep Myxedema (water, chondriatin sulphate, and hyaluronic acid)

Cretinism