RCSI Thyroid Hormone - Synthesis, Transport and Cellular Mechanism PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Thyroid Hormone - Synthesis, Transport and Cellular Mechanism Class Year 2 Course Biochemistry Lecturer Dr Jeevan Shetty Date 05/Jan/2025  LEARNING OUTCOMES Describe the structure and lo...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Thyroid Hormone - Synthesis, Transport and Cellular Mechanism Class Year 2 Course Biochemistry Lecturer Dr Jeevan Shetty Date 05/Jan/2025  LEARNING OUTCOMES Describe the structure and location of the thyroid Describe the origin of thyroid gland and its functional relationships Explain the mechanisms and control of thyroid hormone synthesis Outline the biochemical and clinical functions of thyroid hormones Learning Outcome #1 DESCRIBE THE STRUCTURE AND LOCATION OF THE THYROID  Thyroid Gland: Location and Structure hyoid bone thyroid cartilage cricothyroid membrane Cricoid cartilage Location: The thyroid gland is situated below the thyroid cartilage, behind the strap muscles. Structure: Comprises two lobes joined by an isthmus, located below the cricoid cartilage. Pyramidal lobe in ~50% adults; non-palpable, enlarges with disease.  Thyroid Gland & Follicles The functional unit of the thyroid gland is the thyroid follicle or acinus Thyroid follicle = Follicular (epithelial) cells + Lumen filled with colloid Capillaries: Deliver nutrients and transport hormones Sympathetic Innervation: Influences hormone synthesis/secretion Lymphatics: Drain excess fluid C-Cells (Parafollicular Cells): Produce calcitonin Structures surrounding the thyroid follicle  LO1 Thyroid Gland: Blood supply & Innervation Arterial Supply: Superior thyroid artery (from external carotid) Inferior thyroid artery (from thyrocervical trunk) Venous Drainage: Superior & middle thyroid veins → internal jugular vein Inferior thyroid vein → brachiocephalic vein Innervation: Inferior thyroid arteries Recurrent laryngeal nerve off thyrocervical trunk from subclavian (branch of vagus) Damage may cause vocal cord paralysis Emergency Airway: Schwartz's Principles of Surgery, 2014 Cricothyrotomy through the cricothyroid membrane Learning Outcome #2 DESCRIBE THE ORIGIN OF THYROID GLAND & ITS FUNCTIONAL RELATIONSHIPS  Thyroid Gland Origin - Embryology 1 The thyroid is the first endocrine gland to develop, on about 24th day of gestation (3rd week of gestation) – The thyroid gland arises from 1st pharyngeal arch – Thyroid precursor develops from thyroglossal duct, a diverticulum that appears in the floor of the pharynx – This grows and descends in the neck as an initially hollow structure which later solidifies and becomes bilobed connected by an isthmus  Thyroid Gland Origin - Embryology 2 3 Initial descent anterior to the pharyngeal gut - still connected to the The thyroglossal duct breaks down tongue by thyroglossal duct. Eventually it bifurcates into 2 lobes  Thyroid Gland Origin - Embryology Thyroglossal duct cyst 4 Thyroglossal duct cyst if duct does not atrophy, remnant manifests clinically as thyroglossal As gland descends, it forms its mature shape, by a 7-8th duct cyst week of gestation. Positioned to final location anterior to 50% present as midline trachea. cystic masses just below The thyroglossal duct obliterates entirely by 7-10 weeks level of hyoid bone Remnants of the duct may persist Ectopic thyroid tissue can arise along this path (next slide)  Ectopic Thyroid Gland Sites of normal & http://3.bp.blogspot.com/-g2bJ6X_Rbtg/T0ZGs48uOgI/AAAAAAAAFAw/kJKuXrO2p1g/s1600/lingual_1b.jpg ectopic thyroid along thyroglossal duct Lingual Thyroid 4-year-old girl with slowed growth Thyrotropin (TSH) 55 uIU/ml (nl 0.5-4.5) Total Throxine (T4) 3.5 mcg/dl (nl 5-12) Learning Outcome #3 EXPLAIN THE MECHANISMS AND CONTROL OF THYROID HORMONE SYNTHESIS  Thyroid Hormone: An Overview Thyroid hormones (T3 and T4) are synthesized from iodide (I-) and the tyrosine residues of thyroglobulin ‘Reverse T3’ (rT3) may be found in significant amounts; this form is biologically inactive  WHAT IS THYROID HORMONE? Critical hormone for brain development, skeletal function and growth in infants Regulates metabolic activity in all tissues except brain/spleen/testes – increases basal metabolic rate and oxygen consumption – heat production by stimulation of Na+-K+ ATPase Affects function of virtually all tissues  THYROID HORMONE SYNTHESIS - BUILDING BLOCKS Iodine Thyroglobulin Tyrosine IODINE: Micronutrient in food (seafood, dairy products, grains & vegetables grown in soil with iodine) Recommended daily intake (IOM) Kids: ~ 90-130 mcg Adults: 150 mcg Pregnant women: 220 mcg Lactating women: 290 mcg Iodized salt first available 1924 in Michigan 1500 mg iodized table salt (1/4 tsp) has 114 mcg iodine  ENDEMIC GOITER Central Africa In areas in which the soil is deficient in iodide, hypothyroidism is common. The thyroid gland enlarges (forms a goitre) to produce more thyroid hormone. Prevention of iodine deficiency- Himalayas Worldwide, table salt (NaCl) enriched with iodine (iodized salt) CRETINISM IN ENDEMIC SEVERE IODINE DEFICIENCY Thyroid hormone necessary for fetal & postnatal brain development and skeletal maturation Iodine deficiency during pregnancy in BOTH mom AND baby → at birth, coarse facial features, umbilical hernia, large fontanelles, macroglossia Continued iodine deficiency postnatally → severely stunted physical growth and cognitive and neurologic development  THYROID HORMONE SYNTHESIS - BUILDING BLOCKS IODINE THYROGLOBULIN: large glycoprotein made by rough ER of thyroid follicular epithelial cells, stored in vesicles and exocytosed into colloid Composed of two subunits Contain TYROSINE residues sterically oriented for thyroid hormone production  Synthesis of Thyroid Hormones Steps in thyroid hormone synthesis Iodide trapping Oxidation/Organification Coupling Endocytosis Proteolysis Transport Action on receptors  SYNTHESIS OF THYROID HORMONES 1 1. Iodide Trapping: 2 Basal membrane of thyroid follicular endothelial cell has a system for active transport of iodide from bloodstream against gradient: sodium-iodide symporter (Na+/K+ ATPase) Stimulated by TSH Inhibitory anions: perchlorate (ClO4-) is used clinically as an inhibitor of iodide uptake in some situations 2. Pendrin Transports I- out into colloid Pendred syndrome: one cause of congenital hypothyroidism NIS = So diu m-Iod ide symp orte r; TP O- thyro id pe roxi dase ; DUOX -du al oxid ase Kogai et al. 20 12 Pharmacology & Therapeutics  SYNTHESIS OF THYROID HORMONES 3. Oxidation/Organification: 3a I- is converted to I2 in the presence of H2O2. catalyzed by an enzyme called thyroid peroxidase (TPO) I2 combines with tyrosine residues of thyroglobulin (Catalyzed by TPO) : (aka “iodination”) 3a 3b 3b Tyrosines may be monoiodinated (MIT) or diodinated (DIT) 4 4. Coupling: Iodinium Ion Formation of thyronines: –MIT + DIT → T3 –DIT + DIT → T4 Iodinated thyroglobulin is stored as colloid Colloid stores amount to 2-3 months supply of NIS = So diu m-Iod ide symp orte r; TP O- thyro id pe roxi dase ; DUOX -du al oxid ase thyroid hormone Kogai et al. 20 12 Pharmacology & Therapeutics  STEP 2: OXIDATION/ORGANIFICATION Oxidation of iodide (I-) to Iodine (I2): apical membrane of the follicular cell I- is converted to I2 in the presence of H2O2 catalyzed by an enzyme called thyroid peroxidase (TPO) : (aka “iodination”) I2 combines with tyrosine residues of thyroglobulin Catalyzed by TPO Formed are one of two products: NH2 monoiodotyrosine (MIT) → 1 iodine diiodotyrosine (DIT) → 2 iodines STEP 3: COUPLING OF IODOTYROSYL RESIDUES MAKES T4 AND T3 NH Either two molecules of DIT 2 combine (T4) OR one DIT and one MIT combine (T3) DIT (2)+DIT (2)=T4 (4 iodines) MIT(1)+DIT (2)=T3 (3 iodines) x2 Catalyzed by TPO T4, T3 and uncoupled DIT/MIT stay bound to thyroglobulin and are stored in the lumen as colloid T3 has 3 iodines, T4 has 4  SECRETION OF THYROID HORMONES 5. Endocytosis: Prior to secretion, colloid is 6 pinocytosed into follicular cell 6. Proteolysis: 5 Lysosomal proteases digest thyroglobulin and release T3 and T4 (MIT and DIT are degraded and the iodine re-used) T4 and T3 diffuse across the basolateral membrane into capillaries and enter the circulation  2 1 3 4 5 6 TH SYNTHESIS IN FOLLICULAR ENDOTHELIAL CELL  TRANSPORT OF THYROID HORMONES Thyroid hormones are hydrophobic: they need carrier proteins to travel in bloodstream T4 is bound more tightly to all three significant serum binding proteins Affinity of serum proteins for thyroid hormones: thyroxine binding globulin (TBG) > thyroxine-binding prealbumin (transthyretin) (TBPA) > serum albumin Only 0.015% of circulating T4 and 0.33% of T3 are in protein-free form  THYROID HORMONE CIRCULATES BOUND TO SERUM PROTEINS Thyroxine Triiodothyronine (T4) (T3) >99.95% bound >99.5% bound Free Hormone 0.04% 0.5% Thyroxine- 70% 77% binding globulin (TBG) Transthyretin 10% 8% Albumin 20% 15% Free hormone is the active and regulated horm  CONVERSION OF IODOTHYRONINES The thyroid gland secretes mostly T4 (90%) whose concentration in plasma is around 100 nmol/L T3 is 2 to 10 times more biologically active than T4 Total serum T3 is about 2% that of T4 (2nM vs 100 nM) ½-life T4 5-7 d T3 1-3 d rT3 5h  CONVERSION OF IODOTHYRONINES T4 = a prohormone for T3. The peripheral tissues, especially the liver and kidney, deiodinate T4 to produce circulating T3 (by In target tissues, deiodinase enzyme) T3 = 4x more T4 is converted to either more active potent than T4 T3 or inactive T3 Up to 80% of T3 in circulation is obtained this way (only 20% directly from thyroid) T4 converted to T3 by outer-ring monodeiodinase T4 converted to rT3 by inner-ring monodeiodinase It has been suggested that T4 acts as a ‘prohormone’ with most of the effects mediated by T 3 after deiodination. When the cell has sufficient T3, it switches to the inner-ring enzyme and makes the inactive rT3 Thyroid is Under Hypothalamic/Pituitary Control Negative feedback loop hypothalam TRH us TSH pituitar y 20 T3 % T3 T4 80 % Extrathyroidal Deiodination of T4  REGULATION OF THYROID HORMONE FACTORS CONTROLLING THYROID HORMONE SYNTHESIS Hypothalamus TRH synthesis and release Somatostatin (SS) synthesis and release Anterior pituitary gland TSH synthesis and release Availability of Iodide Integrity of Thyroid gland Tissue conversion of T4 →T3  THYROID STIMULATING HORMONE (TSH) A glycoprotein, synthesised in anterior pituitary Secretion: in low amplitude pulses, with slightly higher levels at night Effects: – Stimulates thyroid hormone synthesis – Increases size and vascularity of thyroid gland Increased levels of thyroid hormones have a negative feedback effect on TSH release from the pituitary – Eg an increase to 1.75 times normal thyroid hormone levels causes TSH levels to fall essentially to zero Decreased levels of thyroid hormone cause an increase in TSH release – This explains the effect of iodine deficiency in goitre  THYROID RESPONSE TO TSH Increase in cAMP, Ca2+ → calmodulin activity, increased protein kinase activity Increased colloid uptake Increased liberation of T3 and T4 Increased production of thyroglobulin Increased uptake of iodine Increased iodination of thyroglobulin Increased size and activity of thyroid cells Increased number of thyroid cells (growth – too much TSH leads to hypertrophy) INHIBITORS (DRUGS AND GOITROGENS) OF THYROID HORMONE SYNTHESIS LEARNING OUTCOME #4 OUTLINE THE BIOCHEMICAL AND CLINICAL FUNCTIONS OF THYROID HORMONES  MECHANISM OF ACTION OF THYROID HORMONE T4 converted to T3 T3 binds to thyroid receptor that complexes with RXR. This complex binds to the thyroid response element of DNA, which through both the addition of a coactivator and the release of a co-repressor begins transcription The proteins that are then synthesized mediate the various cellular response RXR: retinoid X receptor associated with T3 TR: thyroid hormone receptor  EFFECTS - METABOLIC Stimulation of carbohydrate metabolism – Increased glucose uptake; enhanced glycolysis; enhanced gluconeogenesis; increased insulin secretion Stimulation of fat metabolism – ↑ lipolysis: By increased mobilization of lipids from adipose → decreased fat stores, increased plasma free FA, increased β-oxidation; decreased plasma cholesterol, phospholipids and TAG. Note: Increased plasma cholesterol in hypothyroidism associated with atherosclerosis Action of Thyroid Hormone Other metabolic functions: The binding of TH to the receptor results in the coordinated and targeted gene expression of proteins ↑ number of mitochondria, ↑ Na+/K+ ATPase, ↑ oxygen consumption, ↑ protein synthesis and ↑ BMR  EFFECTS - SYSTEMIC Cardiovascular system Increased blood flow and cardiac output (Due to higher O2 demand?) Increased heart rate (higher excitability – synergystic effect with cathecolamines) Increased heart strength (in case of slightly elevated thyroid hormone. In higher excess, increased gluconeogenesis results in degradation of proteins) Respiratory system Increased respiration CNS Critical for normal CNS neuronal development Enhances wakefulness and alertness Enhances memory and learning capacity Required for normal ‘emotional tone’ Increase speed and amplitude of peripheral nerve reflexes  EFFECTS – GROWTH & DEVELOPMENT via GH, stimulate growth of somatotrophs of AP Direct- structural proteins of mitochondria Increase growth and maturation of bone Increase tooth development and eruption Increase growth and maturation of epidermis, hair follicles and nails Increase rate and force of skeletal muscle contraction Fetal CNS development (general body growth in childhood)  SUMMARY OF ACTIONS Category Specific Action Development of Inhibit nerve cell replication. Stimulate growth of nerve cell central nervous bodies. Stimulate branching of dendrites. Stimulate rate of system axon myelinization. Body growth Stimulate expression of gene for growth hormone in somatotrophs. Stimulate synthesis of many structural and enzymatic proteins. Promote calcification of bones Basal energy Regulate basal rates of oxidative phosphorylation, body economy of the body heat production, and oxygen consumption (thermogenic effect) Intermediary Stimulate synthetic and degradative pathways of metabolism carbohydrate, lipid, and protein metabolism. TSH secretion Inhibit TSH secretion by decreasing sensitivity of thyrotrophs to TRH  Tests of Thyroid Function Tests Normal Range What is Measured Thyroid Stimulating 0.5-4.5mU/L Pituitary secretion of Hormone (TSH) TSH in response to T4/T3 feedback Free Thyroxine (FT4) 0.8-1.8 ng/dL Free unbound T4 Free Triiodothyronine 2.3-4.2 pg/mL Free unbound T3 (FT3) Total Thyroxine (T4) 5-12 mcg/dL Bound+free Total Triiodothyronine (T3) 60-180 ng/dL Bound+free  Green box-- Normal ranges T4 and TS LOG-LINEAR INVERSE RELATIONSHI P BETWEEN TSH AND T4 Wide variation of individual “set points” Specific TSH:T4 set point for each individual Spencer J Clin Endocrinol Metab Feb 1990  TSH ASSAY IS THE OPTIMAL SCREENING TEST IN AMBULATORY HEALTHY PATIENTS TSH LOW NORMAL HIGH 4.5mU/L HYPERTHYROID EUTHYROID HYPOTHYROID  Disorder TSH T4 T3 FT4 Primary hypothyroidism ↑ ↓ N or ↓ ↓ Graves' disease (auto-immune, ↓ ↑ ↑ ↑ mimicking of TSH stimulation) TSH deficiency (secondary N or ↓ ↓ ↓ ↓ hypothyroidism) MCQ Which of the following is responsible for iodide activation & iodination reaction: A. Pendrin B. TPO C. TSH D. Lysosomes MCQ In a case of primary hypothyroidism, which of the following changes are expected in the Hypothalamic- Pituitary-Thyroid axis: A. High TRH, High TSH, High T4 B. Low TRH, Low TSH, Low T4 C. High TRH, High TSH, Low T4 D. Low TRH, High TSH, Low T4  CASE PRESENTATION A 46-YEAR-OLD WOMAN PRESENTED WITH A 3-MONTH HISTORY OF MALAISE AND MYALGIA. SHE COMPLAINED OF DIARRHOEA AND HAD LOST 8KG IN WEIGHT DURING THIS TIME Plasma Units Reference range Total T4 256 nmol/L 62 – 160 Free T4 34.6 pmol/L 9.4 – 25.0 TSH