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Endocrinology - Rachel Doohan Y5 > Thyroid Gland Physiology > Flashcards

Thyroid Gland Physiology Flashcards

1
Q

hypothalamus secretes

A

thyrotropin releasing hormone

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2
Q

thyrotropin releasing hormone stimulates

A

the anterior pituitary

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3
Q

the anteriorly pituitary secretes

A

thyroid stimulating hormone

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4
Q

thyroid stimulating hormone stimulates

A

the thyroid gland

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5
Q

the thyroid gland secretes

A

triiodothyronine (T3) and levothyroxine (T4) and calcitonin

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6
Q

thyroid gland is made up of

A

follicles

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7
Q

the follicle is made up of

A

follicular cells which enclose a central colloid

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8
Q

the colloid consists of

A

spheres filled with thyroglobulin

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9
Q

surrounding the follicles

A

are parafollicular c cells which secrete calcitonin

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10
Q

iodide (I-) is

A

actively transported through the follicular cells through the iodine transporter and into the colloid

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11
Q

iodide in the colloid is then converted

A

to iodine (I2) using the enzyme thyroid peroxidase

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12
Q

the thyroglobulin within the colloid contains

A

tyrosine amino acid residues, thyroid peroxidase enzyme then adds I2 onto the tyrosine amino acid residues

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13
Q

this generated either a

A

singly or doubly iodinated species of tyrosine

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14
Q

if one iodine is added you get

A

MIT (monoiodotyrosine)

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15
Q

if two iodines are added you get

A

DIT (diiodotyrosine)

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16
Q

thyroid peroxidase then

A

combines MIT and DIT through a process called coupling to generate T3 or T4

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17
Q

T3 is made up of a combination of

A

one MIT and one DIT residue

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18
Q

T4 is made up of a combination

A

of two DIT residues

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19
Q

why is 90% of the thyroid hormones produced T4 and only 10% T3

A

because thyroid peroxidase enzyme is much more efficient at combining two DIT residues than a MIT and DIT residue

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20
Q

The T3 and T4 produced in colloid

A

is bound to the thyroglobulin and stored in the follicular cells via pinocytosis so when TSH acts on the thyroid they can be released into the circulation (by cleavage form thyroglobulin)

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21
Q

T3 is the

A

major biologically active hormone and is 4x more potent than T4 (thyroxine)

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22
Q

T3 and T4 hormones in the circulation are

A

hydrophobic and lipohilic so they travel in the blood bound to plasma proteins

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23
Q

plasma proteins for T3 and T4

A
  • 70% of thyroid hormones are bound to thyroxine binding globulin (TBG)
  • 20% of thyroid hormones are bound to transthyterin (TTR)
  • 5% is bound to albumin
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24
Q

unbound T3 and T4

A

are the only biologically active thyroid hormones and are known as free T3 and free T4

25
Q

bound T3 and T4

A

are biologically inactive

26
Q

bound T3 and T4 then reach

A

there target cells and are take up through a MCT transporter

27
Q

within there target cell

A

T4 is covered to T3 by a deiodinase enzyme

28
Q

there are 2 classes od deiodinase enzyme

A
  • 5 prime 3 prime deiodinases (5’/3’)
  • 5 non prime 3 non prime deiodinases (5/3)
29
Q

5’/3’ deiodinase coverts

A

T4 to T3 by removing an iodine molecule from the outer tyrosine molecule

30
Q

5/3 deiodinase converts

A

T4 to reverse T3 (rT3) by removing an iodine molecules from the inner tyrosine molecule

31
Q

what is reverse T3

A

inactive form of T3

32
Q

how many types of 5 prime 3 prime deiodinases are there

A

3

33
Q

type 1 deiodinase (D1)

A

found in the liver and the kidneys

34
Q

type 2 deiodinase (D2)

A

found in the heart, skeletal muscle, CNS, fat, thyroid and pituitary gland

35
Q

type 3 deiodinase (D3)

A

found in fatal tissue, placenta and the brain (except in the pituitary)

36
Q

after deiodination the T3 then

A

binds to the thyroid hormone receptor within the nucleus to form the thyroid hormone complex together with retinoid receptor X then binds to the thyroid response element on the promotor region of target genes and influences transcription of the downstream gene

37
Q

the whole process either

A

activates or represses the transcription of genes that regulate basal metabolic rate and development

38
Q

there are different

A

thyroid hormone receptors isoformes present in different tissues

39
Q

types of thyroid hormone receptors

A
  • thyroid hormone receptor alpha 1 and alpha 2
  • thyroid hormone recepto beta 1 and beta 2
40
Q

where is thyroid hormone receptor alpha expressed

A

predominantly in the heart, intestine, bone and skeletal muscles

41
Q

where is thyroid hormone receptor beta predominantly expressed

A

brain, liver, hypothalamus and pituitary

42
Q

resistance to thyroid hormone alpha caused by

A

mutation in THRA leasing to defective signalling through the THR alpha receptor

43
Q

resistance to thyroid hormone alpha causes

A

tissue specific hypothyroidism but near normal thyroid function tests because the feedback loop is normal

44
Q

symptoms of resistance to thyroid hormone alpha

A

delayed bone development, chronic constipation, impairs neural and cerebellum development and bradycardia

45
Q

resistance to thyroid hormone beta caused by

A

mutation in THRB leading to defective signalling through the THR beta receptor

46
Q

resistance to thyroid hormone beta causes

A

elevated T3 and T4 and TSH levels because the THRB receptor is expressed in the pituitary and hypothalamus

47
Q

resistance to thyroid hormone receptor beta is more

A

common than resoatcne to thyroid hormone receptor alpha

48
Q

presentation of resistance to thyroid hormone beta

A

goitre, abnormal cochlear development, affects colour vision, tahcycardia, impaired neural development

49
Q

physiological affects of thyroid hormone

A
  • increases basal metabolic rate
  • increases thermogenesis
  • increases carbohydrate metabolism
  • increases lipid and protein metabolism
50
Q

how does thyroid hormone increase basal metabolic rate

A
  • increases the number and the size of mitochondria
  • increases oxygen used and rate of ATP hydrolysis
  • increases synthesis of repsiratory chain enzymes
51
Q

how does thyroid hormone increase thermogenesis

A

30% of temperature regulation is due to thyroid hormone thermogenesis

52
Q

how does thyroid hormone increase carbohydrate metabolism

A
  • increases blood glucose due to stimulation of glycogenolysis and gluconeogenesis
  • increases insulin dependant glucose uptake into cells
53
Q

how does thyroid hormone increase lipid and protein metabolism

A
  • mobilises fat from adipose tissue and increases fatty oxidation in tissues
  • increases protein synthesis
54
Q

the production and secretion of what required thyroid hormone

A

growth hormone releasing hormone and glucocorticoid induced growth hormone releasing hormone

55
Q

what else requires thyroid hormones

A

development of foetal brain, as myelinogenesis and axonal growth require thyroid hormone

56
Q

if hypothyroidism is untreated during pregnancy what happens

A

the neonate has a marked reduction in IQ

57
Q

thyroid hormones are required for normal

A

central nervous system activity

58
Q

thyroid hormones increase the

A

number of receptors for adrenaline and noradrenaline which increases the force and rate of cardiac contraction

Endocrinology - Rachel Doohan Y5 (28 decks)

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