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Semester 2 MEH > L21: The Thyroid Gland > Flashcards

L21: The Thyroid Gland Flashcards

1
Q

Where is the thyroid gland located?

A

Anterior to trachea–> lower larynx and upper trachea
Inferior to the thyroid cartilage and anterior to the cricoid cartilage
Isthmus extends from 2nd to 3rd rings of the trachea

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2
Q

What is the macroscopic structure of the thyroid gland?

A

Two lobes connected by the isthmus–> butterfly shaped
Bow tie shape and location
Size 2-3cm and 15-20g making it one of the largest endocrine glands in the body
Parathyroid glands embedded on posterior surface

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3
Q

What is the microscopic structure of thyroid gland?

A

Follicular and parafollicular cells (C cells)
Follicular cells arranged in spheres called follicles
Connected by connected tissue
Within connective tissue–> parafollicular cells

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4
Q

What are the follicles?

A

Spheres
Follicular cells surrounding central lumen
Filled with colloid–> a deposit of thyroglobulin

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5
Q

What is the difference between the follicular cells and the parafollicular cells?

A

Follicular cells produce thyroid hormone

Parafollicular cells produce calcitonin

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6
Q

What are the thyroid hormones? What is the difference between them?

A 
Both made from 2 tyrosines with iodine at three or four position in the aromatic rings
T3--> Triiodothyronine
--> Monoiodotyrosine and diiodotyrosine 
T4--> Tetraiodothyronine (thyroxine)
--> diiodotyrosine and diiodotyrosine
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7
Q

What is the function of thyroglobulin?

A

Acts as scaffold on which the thyroid hormones are formed
Contains 134 tyrosines
Iodination–> addition of iodine
Coupling–> monoiodotyrosine added to diiodotyrosine or di added to di

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8
Q

What is thyroid peroxidase?

A

Enzyme that regulated 3 separate reactions involving iodide

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9
Q

What are the steps that thyroid peroxidase is involved in?

A

1) Oxidation–> iodide to iodine –> require H2O2
2) Addition of iodine to tyrosine acceptor residues on protein thyroglobulin
3) Coupling of MIT or DIT to generate thyroid hormones within the thyroglobulin protein

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10
Q

Where does iodine come from?

A

Diet

  • Diary products
  • Grains
  • Meats
  • Vegetables
  • Eggs
  • Iodized salts
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11
Q

How is iodine absorbed?

A

Reduced to iodide

Taken up by thyroid epithelial cells by sodium-iodide symporter or iodine trap

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12
Q

How is thyroid homrone synthesised?

A

1- Iodide is transported into epithelial cells up its concentration gradient using the Na+ gradient (Na+/Iodide symporter)
2- Thyroglobulin is synthesised in thyroid follicle cell from AA
3- Secretion (exocytosis) of thyroglobulin into the lumen of the follicle (colloid space)
4- Oxidation of iodide to produce iodinating species
5- Iodination of the side chains of tyrosine residue in thyroglobulin to form MIT (mono-iodotyrosine) or DIT (di-iodotyrosine)
6- Coupling of DIT with MIT/DIT to form T3/T4 respectively within thyroglobulin (1:10 ratio)
7- Thyroglobulin taken into thyroid follicle cell (epithelial cell lumen) by endocytosis/pinocytosis
8- Proteolytic cleavage of thyroglobulin occur to release T3 and T4 and these diffuse from the epithelial cells into the circulation

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13
Q

Where are thyroid hormones stored?

A

T3 and T4 stored extracellularly in the lumen of the follicles as part of thyroglobulin molecules
T3= 0.4 micromoles
T4= 6 micromoles

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14
Q

What is the most common form of thyroid hormone?

A

90% secreted us T4 however the majority is converted to T3 in liver and kidneys

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15
Q

How do thyroid hormones circulate around the body?

A

Some free T3 and T4 <1%

Bound to throxine-binding globulin

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16
Q

What control thyroid hormone secretion?

A

Under control of hypothalamus and AP
Hypothalamus TRH
Stimulates release of TSH from AP
Acts on follicular cells in the thyroid gland
Release T3 and T4 into the circulation
High levels of T3 and T4 negatively feedback on the TSH from the AP and TRH and hypothalamus inhibiting synthesis
Stress –> increases release
Fall in temperature –> increases release

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17
Q

What is TRH?

A

Thyrotopin releasing hormone
Tripeptide
Released from cells in dorsomedial nucleus of the hypothalamus

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18
Q

What is TSH?

A

Thyroid stimulating hormone
Released from AP
Glycoprotein
Two non-covalently lined subunits (alpha and beta)
–> alpha subunit also present in FSH and LH
Released at low amplitude pulses–> higher level attained during the night

19
Q

What does TSH stimulate?

A 
Interacts with receptors on follicle cells 
Stimulates all aspects of synthesis and secretion
Stimulates 
--> Iodide uptake
--> Iodide oxidation
--> Thyroglobulin synthesis
--> Thyroglobulin iodination
--> Colloid pinocytosis into cell
--> Proteolysis of thyroglobulin
--> Cell metabolism &amp; Growth
20
Q

What sort of receptor is the TSH receptor?

A

GPCP
G protein alpha s –> adenyl cyclase–> cAMP–> PKA–>
At high concentration G alpha q also activated

21
Q

What are the general actions of thyroid hormone?

A

Increase basal metabolic rate and heat production

  • most tissues except brain, spleen and testis
  • increase number and size of mitochondria
  • stimulate synthesis of enzymes in respiratory chain

Stimulation of metabolic pathways

  • catabolic pathways generally stimulated
  • Lipid metabolism - stimulate lipolysis and beta oxidation of fatty acids
  • Carbohydrate metabolism - insulin-dependent entry of glucose into cells and increases gluconeogenesis and glycogenolysis

Sympathomimetic effects
-increase target cell response to catecholamines (adrenaline and NA) –> increase receptor density on target cell

22
Q

What are the tissue specific effects?

A

Directly affect bone mineralisation

CVS–> increase responsiveness to catecholamines

  • -> increase CO–> via ↑ HR and force of contractility
  • -> increase peripheral vasodilation to carry extra heat to surface

Nervous system

  • -> important for development and adult function
  • -> increase myelination of nerves and development of neurones

Permissive roles in FSH and LH secretion
–> No thyroid hormone no ovulation

23
Q

What happens if a pregnant women is thyroid hormone deficient?

A

T3 and T4 required for development of cellular process, hyperplasia of neurones, myelination of nerve fibres
Absence from birth to puberty–> mental and physical retardation–> cretinism
Not correct within a few weeks of birth–> damage permanent

24
Q

How is thyroid hormone deficiency in adults detected?

A

Poor concentration
Poor memory
Lack of initiative

25
Q

What is the thyroid hormone receptor?

A

Member of large family of nuclear receptors
Hormone-activated transcription factors
Act by modulating gene expression

26
Q

How does the thyroid hormone receptor work?

A

Thyroid receptors bind to DNA in absence of thyroid and causes transcriptional repression
Hormone binding associated with conformational change in the receptor–> transcriptional activator

27
Q

What is the mechanism of action of T3 and T4?

A
  1. Cross plasma membrane–> lipid soluble but specific thyroid hormone transporters
  2. Thyroid hormone receptor–> pre-bound to specific sequences–> hormone response elements (HREs) in promoter region of thyroid hormone regulated genes –> dimer with retinoid X receptor
  3. Thyroid hormone binds to thyroid hormone receptor on DNA
  4. Binding relieves repression of gene transcription–> gene expressed (transcription)
  5. Translated to protein–> cellular response
28
Q

What are some of thyroid activated genes?

A

Phosphenolpyruvate carboxy kinase (PEPCK) (gluconeogenesis)
Ca2+ ATPase
Na+/ K+ ATPase
Cytochrome oxidase (ETC)
6-phosphogluconate dehydrogenase (Pentose phosphate pathway)

29
Q

How is T4 converted to T3?

A

Removal of 5’iodide
Important mechanism for regulating the amount of active hormones in cells
T3 4/10?x more active than T4
Removal of 3’iodide produces inactive reverse T3 (rT3)
can bind to the hormone receptor without stimuating them–> blocks the effect of T3

30
Q

What is Goitre?

A

Enlargement of the thyroid gland
Accompany either hypo- or hyperthyroidism
Thyroid gland overstimulated

31
Q

What is hypothyroidism? What are the causes of it?

A

Low thyroid levels

  1. Failure of thyroid gland
  2. TSH or TRH deficiency
  3. Inadequate dietary supply of iodine
  4. Radioactive iodine
  5. Autoimmunuty
  6. Post surgery
  7. Congential
  8. Anti-thyroid drugs
32
Q

What are the symptoms associated with hypothyroidism?

A 
Obesity
Lethargy
Intolerance to cold
Bradycardia
Dry skin
Alopecia
Hoarse voice
Constipation
Slow reflexes
33
Q

What would you expect a blood test for a hypothyroid patient to show? Why?

A

Low T3
Low T4
Elevated TSH
No negative feedback from T3/4 so TSH increases

34
Q

What can hypothyroidism cause in infants and adults?

A

Cretinism –> infants

  • -> Dwarfed stature
  • -> Mental deficiency
  • -> Poor bone development
  • -> Slow pulse
  • -> Muscle weakness
  • -> GI disturbances

Myxedema

  • -> Thick puffy skin
  • -> Muscle weakness
  • -> Slow speech
  • -> Mental deterioration
  • -> Intolerance to cold
35
Q

What is the most common form of hypothyroidism? What causes it? How is it treated?

A

Hashimoto’s disease
Autoimmune–> destruction of thyroid follicles or production of antibody that blocks the TSH receptor on follicles
Treatment–> oral thyroid hormones
–> T4 used since longer half life
–> Normally 50-220 micrograms/day in single dose

36
Q

What is hyperthyroidism? What are the causes?

A 
High thyroid levels 
1 - Autoimmune Graves disease
2 - Toxic multinodular goitre 
3 - Solitary toxic adenoma 
4 - Excessive T4 (or T3) therapy 
5 - Drugs- amiodarone 
6 - Ectopic thyroid tissue
37
Q

What are the symptoms of hyperthyroidism?

A 
Goitre*
Bulging eyes*
Weight loss
Irritability
Heat intolerance, sweating, warm vasodilated hands
Tachycardia (noticeable heart beat), often irregular
Fatigue, weakness
Increased bowel movements
- increased appetite
Possible tremor of outstretched hands
Hyper-reflexive
Breathlessness
Loss of libido
Sweating and tremor
* not always
38
Q

What is the most common form of hyperthyroidism? What causes it? How is it treated?

A

Graves’ disease
Autoimmune disease
Production of thyroid stimulating immunoglobulin (TSI)
Continuously stimulates thyroid hormone secretion outside normal negative feedback control
Antithyroid drugs

39
Q

What are the symptoms associated with Graves’ disease?

A 
Increase in BMR
Excessive sweating 
Decrease in body weight 
Muscle weakness
Heart palpitations 
Bulging eyes (not in every case)
40
Q

What would you expect a blood test result of hyperthyroidism to show? Why?

A

High T3
High T4
Low TSH
T3/4 negative feedback on hypothalamus and anterior pituitary resulting in decreased levels of TSH
However TSI acting on TSH receptor causing T3 and T4 to continuously be produced

41
Q

What is thyroid scintingraphy used for? How does it work?

A

Used to show thyroid tissue
Usually in thyroid gland but could be ectopic
Would show Goitre, nodules etc…
Patient injected with Technetium 99m (99mTc)–> isotope scanning with gamma camera

42
Q

Why is Technetium a commonly used isotope? What else is it used for?

A

Biological half life- 1 day
Radiation exposure low
Bone scan, myocardial perfusion and brain imaging

43
Q

What are antithyroid drugs?

A

Treat over active thyroid
Carbimazole–> most commonly used
Pro-drug–> converted to methimazole in body
Prevents thyroid peroxidase from coupling and iodinating tyrosine on thyroglobulin

44
Q

How long does it take for anti thyroid drugs to have an effect? Why?

A

2-3 months

Lots of stored thyroid hormone in the follicles

Semester 2 MEH (23 decks)

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