Thyroid Gland Physiology

Question 1

hypothalamus secretes

Answer 1

thyrotropin releasing hormone

Question 2

thyrotropin releasing hormone stimulates

Answer 2

the anterior pituitary

Question 3

the anteriorly pituitary secretes

Answer 3

thyroid stimulating hormone

Question 4

thyroid stimulating hormone stimulates

Answer 4

the thyroid gland

Question 5

the thyroid gland secretes

Answer 5

triiodothyronine (T3) and levothyroxine (T4) and calcitonin

Question 6

thyroid gland is made up of

Answer 6

follicles

Question 7

the follicle is made up of

Answer 7

follicular cells which enclose a central colloid

Question 8

the colloid consists of

Answer 8

spheres filled with thyroglobulin

Question 9

surrounding the follicles

Answer 9

are parafollicular c cells which secrete calcitonin

Question 10

iodide (I-) is

Answer 10

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

Question 11

iodide in the colloid is then converted

Answer 11

to iodine (I2) using the enzyme thyroid peroxidase

Question 12

the thyroglobulin within the colloid contains

Answer 12

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

Question 13

this generated either a

Answer 13

singly or doubly iodinated species of tyrosine

Question 14

if one iodine is added you get

Answer 14

MIT (monoiodotyrosine)

Question 15

if two iodines are added you get

Answer 15

DIT (diiodotyrosine)

Question 16

thyroid peroxidase then

Answer 16

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

Question 17

T3 is made up of a combination of

Answer 17

one MIT and one DIT residue

Question 18

T4 is made up of a combination

Answer 18

of two DIT residues

Question 19

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

Answer 19

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

Question 20

The T3 and T4 produced in colloid

Answer 20

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)

Question 21

T3 is the

Answer 21

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

Question 22

T3 and T4 hormones in the circulation are

Answer 22

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

Question 23

plasma proteins for T3 and T4

Answer 23

• 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

Question 24

unbound T3 and T4

Answer 24

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

Question 25

bound T3 and T4

Answer 25

are biologically inactive

Question 26

bound T3 and T4 then reach

Answer 26

there target cells and are take up through a MCT transporter

Question 27

within there target cell

Answer 27

T4 is covered to T3 by a deiodinase enzyme

Question 28

there are 2 classes od deiodinase enzyme

Answer 28

- 5 prime 3 prime deiodinases (5'/3') - 5 non prime 3 non prime deiodinases (5/3)

Question 29

5'/3' deiodinase coverts

Answer 29

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

Question 30

5/3 deiodinase converts

Answer 30

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

Question 31

what is reverse T3

Answer 31

inactive form of T3

Question 32

how many types of 5 prime 3 prime deiodinases are there

Answer 32

3

Question 33

type 1 deiodinase (D1)

Answer 33

found in the liver and the kidneys

Question 34

type 2 deiodinase (D2)

Answer 34

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

Question 35

type 3 deiodinase (D3)

Answer 35

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

Question 36

after deiodination the T3 then

Answer 36

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

Question 37

the whole process either

Answer 37

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

Question 38

there are different

Answer 38

thyroid hormone receptors isoformes present in different tissues

Question 39

types of thyroid hormone receptors

Answer 39

- thyroid hormone receptor alpha 1 and alpha 2 - thyroid hormone recepto beta 1 and beta 2

Question 40

where is thyroid hormone receptor alpha expressed

Answer 40

predominantly in the heart, intestine, bone and skeletal muscles

Question 41

where is thyroid hormone receptor beta predominantly expressed

Answer 41

brain, liver, hypothalamus and pituitary

Question 42

resistance to thyroid hormone alpha caused by

Answer 42

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

Question 43

resistance to thyroid hormone alpha causes

Answer 43

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

Question 44

symptoms of resistance to thyroid hormone alpha

Answer 44

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

Question 45

resistance to thyroid hormone beta caused by

Answer 45

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

Question 46

resistance to thyroid hormone beta causes

Answer 46

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

Question 47

resistance to thyroid hormone receptor beta is more

Answer 47

common than resoatcne to thyroid hormone receptor alpha

Question 48

presentation of resistance to thyroid hormone beta

Answer 48

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

Question 49

physiological affects of thyroid hormone

Answer 49

- increases basal metabolic rate - increases thermogenesis - increases carbohydrate metabolism - increases lipid and protein metabolism

Question 50

how does thyroid hormone increase basal metabolic rate

Answer 50

- increases the number and the size of mitochondria - increases oxygen used and rate of ATP hydrolysis - increases synthesis of repsiratory chain enzymes

Question 51

how does thyroid hormone increase thermogenesis

Answer 51

30% of temperature regulation is due to thyroid hormone thermogenesis

Question 52

how does thyroid hormone increase carbohydrate metabolism

Answer 52

- increases blood glucose due to stimulation of glycogenolysis and gluconeogenesis - increases insulin dependant glucose uptake into cells

Question 53

how does thyroid hormone increase lipid and protein metabolism

Answer 53

- mobilises fat from adipose tissue and increases fatty oxidation in tissues - increases protein synthesis

Question 54

the production and secretion of what required thyroid hormone

Answer 54

growth hormone releasing hormone and glucocorticoid induced growth hormone releasing hormone

Question 55

what else requires thyroid hormones

Answer 55

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

Question 56

if hypothyroidism is untreated during pregnancy what happens

Answer 56

the neonate has a marked reduction in IQ

Question 57

thyroid hormones are required for normal

Answer 57

central nervous system activity

Question 58

thyroid hormones increase the

Answer 58

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