thyroid axis + physiology

Question 1

where is TRH released from

Answer 1

hypothalamus

Question 2

what does TRH do, what does it stand for

Answer 2

thyrotropin releasing hormone

stimulates the anterior pituitary to produce TSH

Question 3

where is TSH released from

Answer 3

anterior pituitary

Question 4

what does TSH stand for, what does it do

Answer 4

thyroid stimulating hormone

stimulates the thyroid gland to produce T4 and T3

Question 5

what does TRH act on

Answer 5

anterior pituitary

Question 6

what does TSH act on

Answer 6

thyroid

Question 7

what does high TSH suggest

Answer 7

hypothyroidism

Question 8

what does low TSH suggest

Answer 8

hyperthyroidism

Question 9

when are TSH levels unreliable

Answer 9

during acute illness or if there is pituitary disease present

Question 10

where is T4 and T3 produced

Answer 10

colloid thyroglobulins which are located within the follicular cells of the thyroid

Question 11

what is T4

Answer 11

thyroxine

Question 12

what is T3

Answer 12

triiodothyronine

Question 13

what percentage of all thyroid hormones does T4 and T3 make up

Answer 13

T3 = 10%
T4 = 90%

Question 14

which is the more active thyroid hormone, how much more active? why?

Answer 14

T3

x4 more active than T4 as it remains mostly free within the blood

Question 15

what happens to T4 in body cells

Answer 15

it is converted to T3

Question 16

what is T4 made up of

Answer 16

DIT + DIT

Question 17

what is T3 made up of

Answer 17

DIT + MIT

Question 18

how does low temperature affect thyroid hormones, in who does this happen

Answer 18

low temp = high TRH

in children and babies

Question 19

how does stress affect thyroid hormones

Answer 19

stress = decreased TSH & TRH

Question 20

how does the circadian rhythm affect thyroid hormones

Answer 20

night = high levels of hormone
morning = low levels of hormone

Question 21

what does deiodinase enzymes do

Answer 21

adds or removes an iodine atom from the thyroid hormones

Question 22

how many types of deiodinase enzymes are there

Answer 22

3

Question 23

where is type 1 deiodinase enzyme found

Answer 23

liver & kidney

Question 24

where is type 2 deiodinase enzymes found

Answer 24

everywhere! - heart, CNS, fat, brain pituitary

Question 25

what does type 2 deiodinase enzyme do

Answer 25

turns T4 to T3

Question 26

where is type 3 deiodinase enzyme found

Answer 26

placenta, foetal tissues, brain except from hypothalamus)

Question 27

what does type 3 deiodinase enzyme do

Answer 27

inactivated T3 and T4

Question 28

where is the thyroid gland normally located

Answer 28

C5/6-T1 | along the thyroglossal duct

Question 29

what does the thyroid develop from, how does it descend

Answer 29

evagination of the pharyngeal epithelium that descends from the foramen caecum to the normal location

Question 30

what are the abnormalities thatch occur during the descent of the thyroid, what do they result in

Answer 30

failure to descend = lingual thyroid excessive descent = retrosternal location in the mediastinum thyroglossal duct cyst

Question 31

what is the thyroid composed of

Answer 31

follicles surround by flat/cuboidal follicular epithelial cells

Question 32

what makes up the thyroid follicles

Answer 32

dense, pink material that contains thyroglobulin | occasionally there are some C cells

Question 33

what are C cells, what do they secrete

Answer 33

C cells = parafollicular cells | they secrete calcitonin

Question 34

how are T3 and T4 synthesised

Answer 34

iodine is take up by the follicle cells where it attaches to tyrosine residues on thyroglobulin to form MIT and DIT. these then couple to form T3 and T4

Question 35

attachment if iodine to thyroglobulin residues are inhibited by what drugs

Answer 35

carbimazole | propylthiouracil

Question 36

how does T3 & T4 travel in the blood, why?

Answer 36

they are hydrophilic so must bind to plasma proteins to travel in the blood

Question 37

give examples of plasma binding proteins and their percentages

Answer 37

thyroxine binding globulin (TBG) = 70% thyroxine binding prealbumin (TBPA) = 20% albumin = 5%

Question 38

what hormone does thyroxine binding globulin (TBG) bind to more strongly

Answer 38

T4

Question 39

when do thyroxine binding globulin (TBG) levels naturally increase

Answer 39

pregnancy | on the contraceptive pill

Question 40

what is the effect of thyroid hormones on basal metabolic rate, how does this happen

Answer 40

INCREASES basal metabolic rate by: increasing size + number of mitochondria increasing O2 use and rate of ATP hydrolysis increasing synthesis of respiratory chain enzymes

Question 41

what is the effect of thyroid hormones on thermogenesis

Answer 41

INCREASE

Question 42

what is the effect of thyroid hormones on carbohydrate metabolism

Answer 42

INCREASES: stimulation of glycogenolysis and gluconeogenesis = increase blood glucose insulin-dependent glucose uptake

Question 43

what is the effect of thyroid hormones on lipid metabolism

Answer 43

mobilise fats from adipose tissue | increases fatty axis oxidation in tissues

Question 44

what is the effect of thyroid hormones on protein metabolism

Answer 44

INCREASES protein synthesis

Question 45

what is the effect of thyroid hormones on  permissive symoathomimetic action

Answer 45

increase responsiveness to adrenaline and noradrenaline by increasing the number of receptors cardiovascular responsiveness is increased