JULIA CREIDER, PGY5ENDOCRINE & METABOLISM

Thyroid Hormone Synthesis, Secretion, Action, Receptors &

Antibodies

Objectives

At the end of this presentation, you will be able to understand: Regulation and feedback mechanisms of the

hypothalamic-pituitary-thyroid axis Synthesis of thyroid hormone Thyroid hormone secretion and transport Downstream action of thyroid hormone on tissues Role of thyroid antibodies

Hypothalamic-Pituitary-Thyroid Axis

Hypothalamic-Pituitary-Thyroid Axis

Thyrotropin-Releasing Hormone (TRH)

Location: synthesized in hypothalamus, stored in median eminence then transported via pituitary portal venous system

Function: controls synthesis/release of TSH from anterior pituitary

Negative Control: gene expression negatively regulated by T3 and T4, which also down regulate TRH receptors in pituitary

Hypothalamic-Pituitary-Thyroid Axis

Thyroid-Stimulating Hormone (TSH)

Structure: glycoprotein composed of alpha and beta subunit Beta subunit unique Alpha shared by LH, FSH,

hCG Shared subunit mediates

thyrotoxicosis – transient gestational thyrotoxicosis and hyperthyroidism associated with trophoblastic tumours

Function: controls thyroid cell growth and hormone production

TSH Regulation

Feedback T3 concentration in the hypothalamus within the

thyrotrophs cells regulates mRNA expression, TSH translation and T3/T4 release

TRH controls post-translational glycosylation and release of TSH

Inhibitors of TSH Somatostatin, dopamine, dopamine agonists, high dose

glucocorticoids

Thyroid Gland Basics

Thyroid gland is located in the neck

Function is to produce appropriate amounts of thyroid hormones

Gland is composed of closely packed, spherical units termed follicles

Thyroid Hormone Structure

Comprised of iodinated thyronines

Produced by follicular cells

Iodide is key structural component

Follicular Cell

Thyroid Hormone Synthesis

Requirements: Iodide Sodium-Iodide Transporter (NIS) Thyroglobulin (Tg) Thyroid peroxidase (TPO)

Iodide

Shellfish, kelp, dairy, some vegetablesDietary intake in North America is ~150-300

mcg daily largely due to iodinization of saltThyroid uptakes ~60-75 mcg daily

Iodine Autoregulation

Capacity of thyroid to modify its function to the availability of iodine, independent of pituitary TSH

Allows maintenance of adequate/appropriate thyroid hormone secretion is varying intake of iodine

Major adaptation to low iodide intake is preferential synthesis of T3 over T4

Iodide excess inhibits many thyroidal functions including: Iodide trapping Thyroglobulin iodination (Wolff-Chaikoff) Thyroid hormone release from the gland

Trapping – Active transport of iodide

Sodium-Iodide Symporter (NIS)

Location: basal membrane of follicular cell

Function: actively transports iodide from blood

Regulation: stimulated by TSH, suppressed by excess iodide

Trapping – Active transport of iodide

Pendrin Location: apical

membrane of follicular cell

Function: transports iodide into the membrane-colloid interface

Mutation: Pendred syndrome (goiter and congenital deafness)

Thyroglobulin

Large glycoprotein, composed of two subunits

Structure: Includes 140 tyrosyl residues, but only four tyrosyl sites are sterically orientated for effective hormonogenesis in each molecule

Role: serves in synthesis and storage of thyroid hormone

Regulation: TSH regulates the expression of the thyroglobulin gene

Thyroglobulin

Organification – Oxidation of iodide and Iodination of tyrosyl residues in thyroglobulin

Thyroid Perioxase (TPO)

Function: catalyzes both iodide oxidation and covalent linkage of iodine to the tyrosine residues of thyroglobulin

Regulation: gene expression stimulated by TSH

Inhibition: methimazole, carbimazole and propythiouracil

Coupling– Linking pairs of iodotyrosine molecules within the tyroglobulin

Pinocytosis and Proteolysis – Return of thyroglobulin into cell and release of T4/T3

Pinocytosis Colloid is

engulfed in vesicles and absorbed into cell

Proteolysis Lysosomes fuse

with vesicle Releases T4

and T3 and inactive peptides

Deiodination – Iodothyroxines within the cell to conserve and reuse iodine

MIT and DIT are deiodinated

Intrathyroidal deiodinases found in mitochondria and microsomes

Stimulation of the Thyrocyte

TSH Receptor Activated by TSH G protein-coupled

receptor Also binds

TSHR stimulating, blocking and neutral antibodies

LH hCG

Also expressed in osteoclasts, fibroblasts, adipocytes and retroorbital adipocytes and skin

Effects of TSH on the Thyrocyte

Changes thyrocyte morphology Induces pseudopods at the follicular cell-colloid border,

accelerating thyroglobulin resorption, which increases thyroid hormone release

Cell growth Individual cells increase in size and vascularity

Iodine metabolism TSH stimulates all phases of iodide metabolism (uptake,

transport, secretion), increased NIS expressionThyroglobulin and TPO mRNA expression

Results in increased incorporation of iodide into MIT, DIT, T3, and T4

Summary

Thyroid Hormone Transport

Both T3 and T4 are poorly water solubleCirculate bound to plasma proteins

Three major transport proteins: Thyroxine-binding globulin (TBG) Transthyretin Albumin

Thyroxine-Binding Globulin (TBG)

Liver-derived glycoprotein

Each TBG molecule has a single binding site for T4 or T3

Carries about 70% of circulating thyroid hormones (high binding affinity for T4 and T3)

Conditions Affecting Binding of TBG

Congenital TBG deficiency, X-linked recessive – low total T4/T3 but free

hormone levels are normal and euthyroid Congenital TBG excess – elevated total T4/T3, normal fT4/fT3 and

euthyroidPhysiologic

Pregnancy/estrogen – increase acid content of TBG, decrease metabolic clearance and elevated TBG levels

Pathophysiologic Estrogen secreting tumours, OCP, acute heaptitis Systemic illness – decrease TBG due to cleavage by leukocyte

protease and decrease binding affinity, lowers hormone concentrations

Drugs Salicylates, high dose phenytoin, furosemide – bind TBG, displace

T4/T3

Transthyretin

Formerly known as: thyroxine-binding prealbumin

Binds 10% of circulating T4Affinity for T4 is 10-fold greater than for T3Expressed in the choroid plexusDissociation of T4 and T3 from transthyretin is

rapidConditions Affecting Binding

Congenital increased affinity – elevated total T4 but normal fT4

Ectopic production within pancreatic and hepatic tumours

Albumin

Binds T4 and T3 with lesser affinity, but high plasma concentration

15% of circulating T4 and T3Rapid thyroid hormone dissociation, makes it

major source of free hormone to tissuesConditions Affecting Binding

Hypoalbuminia – low total levels but normal free Familial dysalbuminemic hyperthroxiemia – higher

affinity for T4, elevated total T4 but normal free T4

Metabolism of Thyroid Hormones

Most of plasma pool of T3 (80%) is derived from: 5’-deiodination of T4

in the liver, kidney and skeletal muscle

Diodination of the inner ring of T4 (5-deiodination) produces reverse T3 (metabolically inert)

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Metabolism of Thyroid Hormones

Three deiodinases enzymes catalyze reaction: D1 D2 D3

Permit local tissue and cellular modulation of thyroid hormone action

Deiodination

D1 Location: most

abundant form, found predominantly in liver, kidney and lesser extent in thyroid gland, skeletal and heart muscle, and other tissues

Function: major converter of T4 to T3

Inhibition: PTU (not methimazole), amiodarone and iodinated radiocontrast dye

Deiodination

D2 Location: expressed

in the brain and pituitary gland, where it maintains constant levels of intracellular T3 in the CNS

Function: maintenance of the level of intracellular T3 and its neuronal cellular functions

D2 is very sensitive to circulating T4

Deiodination

D3 Location:

chorionic membranes of the placenta and glial cells in the CNS

Function: inactivates T4 by converting it to rT3 and it inactivates T3 by converting it to 3,3’-T2

Elevated in hyperthyroidism and decreased in hypothyroidism

Transport through Cell Membranes

Originally thought to be primarily passiveSeveral specific thyroid hormone transport

proteins identified: MCT8 MCT10 Organic anion transporting polypeptide 1C1

(OATP1C1) – predominately in the brain, transports T4 preferentially

Most cells, 90% of T3 is in cytosol except pituitary, where 50% of T3 is in the nucleus

Mechanism of Thyroid Hormone Action

Genomic Actions T3 interacts with its nuclear receptors to regulate

gene activity T3 binds to a specific nuclear thyroid hormone

receptor (TR), which in turns bind to DNA at specific sequences called thyroid hormone response elements (TREs)

T3 has a 15-fold higher binding affinity for TRs than T4

There are tissue-specific preferences in expression of the various TRs (difference expression in hypothalamus vs kidney, liver, brain and heart)

Non-genomic Actions Actions mediated by T3 and T4 occur with certain

enzymes

Mechanism of Thyroid Hormone Action

Thyroid Hormone Action

Fetal Development Iodide is found in

thyroid tissue and pituitary TSH appear in the fetus at ~11 weeks

High placental content of D3 inactivates most maternal T3/T4

After 15-18 weeks, the fetus controls most of its own thyroidal secretion

Thyroid Hormone Action

Metabolism Effects Increases basal metabolic rate Increases oxygen consumption and heat production by

stimulation of Na-K-ATPase in all tissues Stimulates mitochondriogenesis, augmenting the

cell’s oxidative capacityLipids and Carbohydrate Metabolism

Increases hepatic gluconeogenesis and glycogenlysis and intestinal glucose absorption

Increase cholesterol synthesis and degradation (largely due to an increase in hepatic LDL receptors, accelerating LDL clearance)

Thyroid Hormone Action

Cardiovascular System Lowers peripheral vascular resistance Positive inotropic and chronotropic effects with

heightened adrenergic sensitivity Increased cardiac output Increases the rate of myocardial diastolic relaxation Increases the rate of depolarization and repolarization

of SA node increasing heart rate

Thyroid Hormone Action

Pulmonary System Maintains the ventilatory responses to hypoxia and

hypocapnia in the brain stem respiratory centerSkeletal System

Stimulates bone turnover and increases bone resorption

Gastrointestinal System Promotes gut motility

Hematopoeitic Effects Increased cellular demand for oxygen in

hyperthyroidism leads to increased production of EPO and erythropoiesis

Increases the oxygen dissociation from hemoglobin

Thyroid Hormone Action

Neuromuscular System Changes in speed of muscle contraction and

relaxation Fine distal hand tremor in hyperthyroidism Hyperthyroidism, there is increased protein turnover

and loss in skeletal muscleReproductive System

Regulates synthesis of pituitary hormones, stimulates GH production and inhibits TSH

Low levels can cause delayed puberty by impairing GnRH secretion

Thyroid Antibodies

Hallmark of autoimmune thyroid disorders

Antibodies to: Thyroglobulin (Tg) Thyroid peroxidase (TPO) Stimulate TSH-receptor (TSHR)

Anti-TPO and Anti-Tg

Polyclonal antibodies, IgG class Develop in response to thyroid injury No thought to be disease causing Contribute to disease mechanism

Complement fixing cytotoxicity or T cell activation Correlates well with thyroidal damage and

lymphocytic infiltration

Anti-TPO and Anti-Tg

Almost all patients with autoimmune thyroiditis are associated with anti-TPO and anti-Tg May be present in Graves as well

Anti-TPO has a higher affinity and occurs in higher concentrations

More common in patients: Sporadic goiter Multinodular goiter Isolated thyroid nodules and cancer

Not necessary for evaluation of thyroid function May be helpful to predict progression of subclinical

hypothyroidism

TSHR Antibodies

Presence favours an autoimmune cause for hyperthyroidism

Not sensitive or specific, need to interpret as part of clinical scenario

Usually are stimulating antibodies that compete with TSH for binding to its specific receptor site Can be stimulating, inhibitory or neutral

Anti-Tg and Anti-TPO are also detectable in 50-90% of patients with Graves

Take Home Messages

Requirements for thyroid hormone synthesis: Sodium-Iodide Transporter (NIS) Iodine Thyroglobulin (Tg) Thyroid peroxidase (TPO)

Steps for thyroid hormone synthesis: Iodine trapping via NIS Oxidation and organification of Tg by TPO Coupling pairs of MIT or DIT within the Tg molecule to

form T3/T4 Deiodination of MIT/DIT to conserve iodine Release of T3/T4 into circulation

Take Home Messages

T3 and T4 circulate bound to: thyroxine-binding globulin, transthyretin and albumin

T4 undergoes 5’-deiodination to form T3

T3 acts on the cell nucleus to regulate gene expression and protein synthesis

Thyroid hormone has target actions in most tissues

Take Home Messages

Tightly regulated through hypothalamic-pituitary-thyroid axis

TSH-Ab is most specific for Graves’ disease and is the mechanism for disease

Anti-TPO and Anti-Tg are evidence of thyroidal injury but are nonspecific for diagnostic purposes

References

Williams Textbook of Endocrinology 12th edition

Greenspan’s Basic and Clinical EndocrinologyEndocrine Physiology. Lange.UpToDateBoron WF (2003). Medical Physiology: A

Cellular and Molecular Approach. Elsevier/Saunders. P. 1300.

Questions