Lecture 8: Thyroid Hormones

Thyroid Hormone Synthesis

• Hypothalamus releases TRH which triggers the APG thyrotropes to release TSH

• TSH acts on the thyroid gland, causing the follicle cells to synthesise and release thyroid hormones T3 and T4

   

Production of T3 and T4 in Follicular Cells

• Iodide is transported to the thyroid follicular lumen, where TPO and DuOx add iodide to the tyrosine residues on the thyroglobulin backbone.

• Tg is then endocytosed and proteolysed to form T3 and T4, which then exocytosed via a specific thyroid transporter

Iodide Balance

• Average intake – 400ug

  • Similar amount excreted urine

• Actively concentrated in thyroid, salivary, gastric, lacrimal, mammary glands and the choroid plexus

•  70-80ug taken up daily by the thyroid

• Total iodide content of thyroid: 7500ug

  • 1% released daily – 75% of this is secreted as thyroid hormone; the rest is secreted as free iodide

Biphasic Function of Iodide

• Low intake – rate of hormone synthesis directly related to availability 

• If intake exceeds 2mg/day – intraglandular iodide concentration causes the suppression of DuOx activity and NIS and TPO genes – blocks hormone biosynthesis

Thryroid Hormones

• Made up of 2 tyrosine rings

• 3 types

  • T4

  • T3

  • rT3

Thyroxine - T4

• Most common form of thyroid hormone (90%)

  • Least biologically active

  • Has 4 iodide molecules

• Insoluble in serum - transported in conjunction with binding proteins

  • 90ug produced each day

T3

• Most biologically active

  • Has 3 iodide molecules

• Makes up ~10% of total thyroid hormone produced

  • 8ug produced each day

rT3

• Less than 1% is secreted - is inactive

• Produced at low concentration

  • Iodide is missing from the bottom left ring, closest to the backbone

Binding Proteins

• Synthesised in the liver to transport T4 in serum

• 3 types

  • Thyroxine Binding Globulin

  • Transthyretin

  • Albumin

• The binding of thyroid hormones to proteins is low in affinity and dynamic

  • Exists in an equibrium

Binding Proteins + Liver Disease

• Leads to a loss in the effective T4 transport to peripheral tissues

  • Can lead to thyroid disease

Thyroxine Binding Globulin (TBG)

• Binds 70-75% of plasma T4

• Large circulating reservoir of T4

  • if iodide is depleted, circulating hormone is present

• Prevents loss in urine

• Affinity: High

• Binding Capacity: Low

  • binds tightly with a limited capacity

• Specificity: Binds T4 and T3

• Half-Life (t_1/2) : 5 days

Transthyretin (TTR)

• Binds 20% of plasma T4

• Important for delivery to CNS/ brain - must be converted to have an effect

• Affinity: low

• Binding Capacity: high

• Specificity: Only binds T4

• Half-Life (t_1/2) : 2-3 days

Albumin

• Ubiquitous protein in the blood

• Binds 5-10% of plasma T4

• Affinity: very low - hormone readily released at tissue

• Binding Capacity: very high (due to this ubiquitous expression in the blood)

• Specificity: Binds T4 and T3

  • Protein-bound T4: Protein-bound T3: 20:1

• Half-Life (t_1/2): 15 days

Transport of Thyroid Hormone

• Synthesis in the thyroid gland

• Most is bound to TBG, with a smaller amount bound to TTR and albumin

• Bound T4 transported the peripheral tissues in circulation

  • Bound hormone is inactive → must be released and converted to have an effect on target cells

• Free hormone is active and can interact with receptors

Structure of TBG

• Single polypeptide chain

• 20% CHO (Carbohydrate branches) by weight, molecular heterogeneity

• Stability and half-life are extended after T4 binding   

Factors Increasing Levels of TBG in Serum

• T4

• Oestrogens/Androgens      

• Doubles in concentration during pregnancy

  • Thyroid hormones are critical for foetal development

Factors Decreasing Levels of TBG in Serum

• Corticosteroids,

• Illness,

• Stress,

• Cirrhosis,

• Nephrotic disorders (kidney disease)

  • Excreted in the urine

Locations of T4

• Most is bound to binding proteins (99.96%)

  • small percentage is free (0.04%)

• Bound and free hormones travels to tissues and will be converted to T3 to exert and effect on tissue

Biological Activity of T3

• Feedback control (Axis) 

• Tissue action  

• Faecal excretion

Transport of T3 and T4 Into Cells

• Bound hormones can’t enter cells - no soluble in serum or lipophilic

  • Must pass through specific transported

• Free T3 and T4 can enter cells via specific transporters (MCT8, MCT10, OATP1c1)

• T3 biologically active

• T4 is inactive and must be converted by intracellular iodothyronine deiodinases

  • 3 types

Iodothyronine Deiodinases (DIO)

• Seleno-cystine containing enzymes

  • Selenium bound to cysteine      

• Selenium accepts the ‘spare’ iodide

• Type 1 and 2 form T3 -bioactive)

• Type 3 (& 1) form rT3 - INACTIVE

rT3

• Produced by DIO 3

• Alters the concentration of thyroid hormone that reaches the brain and tissues

Importance of DIO3 in Neonatal Development

• Amount of thyroid hormone that enters the foetal brain is critical to ensure correct brain development

  • Problematic if too much or too little enters the brain

• Enzyme ‘fine-tunes’ the concentration

Iodothryonine deiodinase 1 (DIO1)

• Expressed in the liver, kidney and muscle

  • Low-affinity enzyme capable of outer and inner ring deionisation of T4 - can form T3 or rT3

• Has good blood supply – conversion to T3 and then transported in the circulation

• Also found in thyroid – role in hormone regulation

• Produces most of and regulates the circulating T3

DIO 1 As A Scavenger Enzyme

• May remove iodide from sulfated thyroid hormones

Iodothryonine deiodinase 2 (DIO2)

• Predominates in areas of the CNS (glial cells), pituitary thyrotropes – important role in regulation

  • Intracellular localised in the ER

• Only T4 is delivered to the brain by TTR

• Controls intracellular T3 concentration

• Converts T3 in cells only

• Upregulated in the brain in hypothyroidism

DIO 2 Role in Regulation

• Acts as a thyroid axis sensor in thyrotropes – integrates total circulating T3 and T4

• Acts as a feedback signal to regulate TSH secretion

•  Generates circulating pool of T3 in euthyroid conditions

Iodothryonine deiodinase 3 (DIO3)

• Produces inactive rT3

• Prevents thyroid hormones access to specific tissues

• Can also inactive T3 to T2

• Upregulated in hyperthyroidism to blunt overproduction of T4

• Enzyme inactivated by starvation and injury

Thyroid Hormone Receptor

• Intracellular receptors → Found in nucleus

• 2 types: TRα and TRβ receptors

  • Coded by the 2 genes, THRA and THRB

• Form heterodimer with retinoid X receptor

       

THR A Gene

• Located on chromosome 17

• Encodes TRa - alternatively spliced to form 2 main isoforms

  • Only TRa1 binds to T3 - expressed in heart

THR B Gene

• Located on chromosome 3

• Encodes TRB1 and TRB2 - high affinity receptors for T3

  • TRB 1 expressed in brain, liver and kidneys

  • TRB2 expression is restricted to pituitary and hypothalamus

Consequence of T3 Bound to TRB 2

• Inhibits the expression of pre pro-TRH gene in the PVN of the hypothalamus and the B subunit TSH gene in thyrotropes

Function of TR-RXR Heterodimer

• Functions as a transcription factor      

• Binds to TRE (thyroid response element)  

  • Acts as a gene repressor/ activator            

• Increased gene transcription

• Can also inhibit gene transcription

Reason For T3 Being More Biologically Active

• Binding site on thyroid hormone receptor 15-fold ↑ affinity for T3 than T4

Effect of T3-TRE Binding

• Effects dependent on the location of the TRE, which gene it is and whether T3 increases or decreases the gene transcription

  • Increased transcription of growth hormone genes → Increased GH

  • Decreased production of PRL - binding of hormone to element occurs next to PRL gene

  • Decreased in a and B- subunits of TSH

Feedback Mechansim of T3

• Hormone acts on the thyrotropes to decrease TSH, by decreasing gene transcription of a- and B- subunits, following its binding to the receptor

Biological Activity of Thyroid Hormones (Overall)

• Thyroid gland secretes thyroid hormoens predominantly T4 (and T3)  

• Human serum has a high concentration of T4 binding proteins which cause a high circulating level of protein-bound T4; TBG, TTR, albumin

  • Different affinities and capacities      

• Only free T3 and free T4 are biologically active     

• T3 derived from the peripheral conversion of T4 to T3 by iodothyronine deiodinases DIO1 and DIO2

  • DIO3 = rT3 - inactive      

• T3 and T4 bind to nuclear hormone receptors (TRE) to alter gene transcription in target cells

  • increase/ decrease gene expression

5 Biological Actions Of Thyroid Hormones

• Control of basal metabolic rate

• Growth Regulation

• Foetal development

• Cardiovascular Effects

• Musculoskeletal Effects

Effect of Age on Metabolic Rate

• Rate decreases with age due to actions of thyroid hormone

Action of Thyroid Hormones on Basal Metabolic Rate

• Proteins involved in metabolism will be influenced by T3 - affect the expression of proteins involved in process

  • Increases the expression of Na+/K+/ATPase

  • Increased expression of mitochondrial respiratory enzymes by thyroid hormone actions  

  • Increased expression of other enzymes and proteins involved in metabolism

Effects of Increased Metabolism:

• Increase O₂ consumption and demand for aerobic metabolism

  • Supply increased by increased Cardiac output and ventilation rate

• More substrates required

  • Increased food intake, mobilisation of body fat, CHO, carbs to drive rate

    • Reduced muscle mass and adipose tissue

• Increases in waste products e.g. CO₂ and urea

• Thermogenesis, sweating and water loss

  • thyroid hormone causes increase in core temp

  • water loss from breathing - increased ventilation

Consequences of Hyperthyroidism

• High metabolic rate

• Tremor

• Muscle wastage            

  • Tissues used as a substrate to drive metabolism

• Most common endocrine disorder in cats

  • Weight loss despite normal or increased appetite

Growth Regulating Role of Thyroid Hormone

• The effect of hormone is often synergistic with other hormones e.g. GH in early development

  • Affects most bodily functions and exerts effects on all organs and tissues throughout life

Deficiency of Thyroid Hormone

• Lead to abnormal growth, development, reproduction, behaviour, metabolism

  • dependent on where the person is in development

• Arrest of bone elongation; delayed bone maturation

• Reduction in growth hormone secretion – due to the lack of stimulation of the growth hormone gene by the thyroid hormone

2 Fold Role of Thyroid Hormones

• Production of GH via transcriptional control of thyroid hormone

• Direct systemic actions

Role of Thyroid Hormones in Foetal Development

• Key role in developing neural and skeletal development

  • DIO3 and TTRP important in the transport

Loss of T4 in Foetus

• This leads to irreversible intellectual disability and dwarfism

Effect of Iodide Deficiency in Foetal Development

• Miscarriage, congenital abnormalities, EIDS

Effect of Iodide Deficiency in Neonatal Development

• Neonatal goitre and hypothyroidism (difficult to detect)

Effect of Iodide Deficiency in Childhood

• Goitre, impaired mental function and delayed physical function

Cardiovascular Effects of T3

• Effects mediated directly, indirectly, or through enhanced responsiveness to catecholamine

• Increases:-

  • cardiac contraction and output

  • heart rate

  • oxygen supply to tissues

  • CO2 removal from tissue

  • Increased total blood volume - activation of RAAS (increased renal Na+ reabsorption)

Direct Effects of Thyroid Hormones on Cardiovascular System

• ↑ Myocardial Ca2+ uptake – stronger contraction

• ↑ expression of stronger α-myosin heavy chain and ↓ β 

  • 2 different effects on gene transcription  

• ↑expression of RYR in SR

  • Involved in heart muscle contraction - shortens relaxation time (diastole)

Indirect Effects of Thyroid Hormones on Cardiovascular System

•  ↑Metabolism

• Thermogenesis Vasodilation  

• ↑ sensitivity to catecholamines – stress hormones

T3 Musculoskeletal Effects

• A potent stimulatory effect on bone turnover, increasing both formation and resorption

  • increases linear bone growth after birth

  • Increases the rate of muscle relaxation

• Excess causes muscle breakdown and tremors  

• Normal skeletal muscle function requires T3

  • Smaller skull in response to iodide deficiency