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MNSR > MNSR 42 - Thyroid Structure Hormones and Diseases > Flashcards

MNSR 42 - Thyroid Structure Hormones and Diseases Flashcards

1
Q

describe where you can find thyroid gland

A

• Attached to the larynx

–Downgrowth from the floor of the pharynx

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2
Q

average weight of thyroid gland in adults

A

25 grams

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3
Q

describe the thyroid gland

A

highly vascularised - greater blood supply than kidney

Bi-lobed structure w/ one lobe on either side of the trachea (butterfly shaped)

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4
Q

what do thyroid glands consist of

A

follicles

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5
Q

what does each follicle consist of?

A

fluid filled space w/ jelly like substance from glycoprotein - thyroglobulin = colloid
surrounded by follicular cells

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6
Q

what does colloid consist of

A

thyroglobulin, - a precursor protein to the thyroid hormones

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7
Q

describe follicular cells

A

highly vascularized connective tissue with lightly stained follicular
cells

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8
Q

what holds follicular cells together

A

Held together by stroma - connective tissue supporting the follicles

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9
Q

thyroid gland regulation

A 
Hypothalamus releases TRH
Stimulates release of TSH from anterior
pituitary
Promotes release of thyroid hormones
tri-iodothyronine (T3) and thyronine (T4)
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10
Q

effects of tri-iodothyronine (T3) and thyronine (T4) when released in the body

A

positive effects on target organs
negative feedback effects on
hypothalamus and pituitary

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11
Q

what substance is essential for production of thyroid hormones

A

iodine

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12
Q

what feature is found outside membrane of follicular cells

A

iodine pump

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13
Q

what controls iodine uptake

A

Uptake of iodine partially under the control of the pituitary hormone TSH

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14
Q

how much iodine needs to be ingested in the body daily?

How much iodine does iodine store

A

150µg/day is sufficient, the thyroid is very economical in its use and
recycles much iodine
• The thyroid stores about 4,000µg of iodine in a variety of forms

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15
Q

what does TSH stimulate

A

TSH stimulates follicular cells to increase uptake of iodide from the
bloodstream. Follicular cells then pump the iodide into the follicular
spaces

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16
Q

how is iodide oxidised

A

• Iodide (I- ) is oxidised by the thyroid peroxidase (TPO) enzyme to form
iodine (I2 )

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17
Q

production of tri-iodothyronine and thyroxine

A
  1. endoplasmic reticulum and golgi complex of the thyroid follicular cells produce tyrosine containing thyroglobulin(Tg) - this is transported by exocytosis to the colloid
  2. Iodide in the blood is sent into colloid symporters using active transporters (Na+ and I- enter the colloid)
  3. Iodide is oxidised by active form by thyroperoxidase (TPO) in luminal membrane and exits through luminal channel to center colloid.
  4. Active iodide attach to tyrosine within thyroglobulin catalysed by TPO to make monoiodotyrosine(MIT)
  5. When 2 iodides attach to tyrosine makes Di-iodotyrosine (DIT)
  6. 1 MIT + 1 DIT = Trio-iodothyronine
    1DIT + 1DIT = thyroxine
  7. thyroid follicular cells engulfed portion of thyrglobunlin (Tg) containing colloid by phagocytosis
  8. Lysosomes attack englufed vesicle and splite the iodine products
  9. T3 and 4 diffuse into blood
  10. MIT and DIT deiodinated and free iodide is recycled
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18
Q

Thyroglobulin (Tg)

A

Precursor to thyroid hormones
• Produced within the follicular cells
• Forms a homodimer (two Tg molecules bound together)
• Each thyroglobulin molecule contains 140 tyrosine amino acids

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19
Q

why does T4 thyroxine form more readily

A

Peroxidase is much more efficient at combining two DIT residues and thus generation of T4 occurs much more readily
1DIT + 1DIT = thyroxine/T4
The thyroid gland primarily
produces T4 rather than T3.

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20
Q

what must be stimulated for thyroglobulin to be moved back into follicle cells

A

Thyroglobulin remains within the colloid until TSH stimulates endocytosis of colloid back into the follicle cells

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21
Q

how can T3 and T4 be released into blood stream

A

A protease cleaves off T3 and T4, and the hormones can now be released into the bloodstream

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22
Q

which thyroid hormone is more potent

A

T3

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23
Q

half life of thyroxine/T4

A

6 days

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24
Q

half life of T3

A

1.3 days

25
Q

what happens to thyroid hormone when it enters the cells

A

most of the thyroxine is de-iodinated to T3, (80%),
or reverse T3 (20%) which is
biologically inactive

26
Q

thyroid hormone ratio released into blood stream

A 
Hormones released into the
bloodstream in a 10:1 ratio:
90% Thyroxine (T4)
10% Tri-iodothyronine
(T3)
27
Q

what proteins do T3 and T4 bind to

A

serum protein
globulin and albumin - becomes inactive and serve as reservoir
only free T4 and T3 can enter target cells

28
Q

what type of proteins have a higher affinity for T3 and T4

A

carrier proteins
the proportion of T3 in the plasma in an unbound (or “free”) state is
about ten times greater than the amount of free T4.

29
Q

how many micro grams of T3 and T4 are delivered to tissue each day and explain

A

in circulation some of the thyroxine (T4) is de-iodinated to T3, so the
final tissue delivery is about 90 µgrams/day of thyroxine, to 40-60
µgrams/day of T3

30
Q

effects of thyroid hormone in the body

A

. Increase Basal metabolic rate

  1. Promote growth and maturation
  2. Specific physiological effects
31
Q

effects of thyroid hormone on metabolic rate

A

increase metabolic rate

32
Q

what specifically in metabolism does thyroid hormone effect

A

• Increased O2 consumption
• Increase in protein synthesis:
RNA synthesis increases leading to greater production of ribosomes and mRNA
The levels of metabolically important enzymes increase
Mitochondria increase in numbers resulting in greater
ATP levels

33
Q

how does thyroid impact growth and maturation in children

A

have a synergistic effect

with IGF-1 (insulin-like growth factor) on the growth of bone

34
Q

how can low levels of thyroid hormone affect children

A

– Low levels of hormone cause poor growth and retarded sexual
development at puberty

35
Q

how can high levels of thyroid hormone affect children

A

Paradoxically high levels in a young person will cause rapid
precocious growth at first, but this will stop at a younger age
because of early closure of the epiphyses

36
Q

physiological effect of thyroid hormone

A

Stimulates carbohydrate and fat metabolism - higher levels of lipids in the liver and higher blood cardiac output to keep pace with the raised metabolic
rate resulting in increased systolic pulse pressure
• Increased IT secretion and motility -> diarrhoea
• CNS effects include nervousness, irritability and muscle tremor –> tiredness and inability to sleep

37
Q

hypothyroidism

A

This is the state where the thyroid is underactive and producing insufficient levels of
hormone
– This will affect children and adults differently
– Often caused by low iodine in the diet

38
Q

symptoms of hypothyroidism in children

A

if left untreated it leads to Cretinism
– Poor growth, dwarf stature, mental
retardation, general adult effects.

39
Q

decreased thyroid activity before and after birth can risk what

A

risks severe mental retardation due to the failure of development of the CNS

40
Q

prevalance of hypothyroidism

A

1: 4,000 births

41
Q

give another name for hypothyroidism as an adult

A

Myxedema

42
Q

symptoms of myxedema

A 
Thickened dry skin
• Hair loss 
Decreased mental ability
Goitre
muscle stiffness
43
Q

treatment of hypothyroidism

A

Primary:
• Thyroid hormone
– Secondary:
• Alleviate symptoms

44
Q

prevalence of hypothyroidism in adults

A

0.01 - 0.08% population

– 80% of sufferers are female, commonly between 30 to 60 years

45
Q

causes of hypothyroidism in adults

A

Iodine deficiency
• Hashimoto’s thyroiditis
– Autoimmune
• Antibodies to thyroid peroxidase and/or thyroglobulin
• Self-destruction of cells of the thyroid
• Yo-Yo dieting
• Overactive adrenal gland

46
Q

hyperthyroidism

A

Thyroid hormone levels may be 5 to 15 times higher than normal values

47
Q

symptoms of hyperthyroidism

A

Hypertension, raised body temperature,
weight loss, hyperactivity, goitre.
– May be accompanied by a protrusion of
the eyeballs called exophthalmos

48
Q

major causes of hyperthyroidism

A

Results from an overactive thyroid
– Graves’ Disease (70-80% etiology)
– Toxic multi-nodular goitre (enlarged and contains multiple thyroid nodules)
– Toxic thyroid adenoma (hyper-functioning follicle due to adenoma)

49
Q

prevalence of graves disease

A

1.0% population
– Found more frequently in
women
– Female:male ratio 10:1

50
Q

causes of grave’s disease

A

– Autoantibodies to TSH
receptors on the thyroid gland
– Antibodies stimulate receptor - interfere with stimulation of thyroid gland by TSH - only stimulated by Thyroid stimulating immunoglobulin
– Increase secretion of T3 and T4

51
Q

goitre

A

Enlargement of the thyroid gland

52
Q

when can goitre occur

A

Hypothyroidism
Hyperthyrooidism
Normal Thyroid

53
Q

describe the condition exopthalmos

A

Bulging of the eye anteriorly out of the orbit

Retro-orbital oedema and fat deposition

54
Q

what is exopthalmos a symptom of

A

symptom of grave’s disease not hyperthyroidism

55
Q

how to diagnose hyperthyroidism

A 
Blood tests
Measuring the level of TSH in the blood
Measuring specific antibodies
Anti-TSH-receptor antibodies  
Iodine 123 uptake
56
Q

treatment of hyperthyroidism - radioiodine

A

Radioiodine (131 I) treatment
Thyroid’s natural ability to concentrate iodine
131I decays to 131Xe
Releases radioactive β-particles which kill colloid tissue

57
Q

treatment of hyperthyroidism surgery

A

Partial or full thyroidectomy

58
Q

treatment of - thyrostatics

A

Pharmacological agents
Prevent generation of iodinated thyroglobulin
Prevent release of T3 and T4

MNSR (17 decks)

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