Thyroid gland hormones

Question 1

What is formed when iodine is attached to tyrosine?

Answer 1

mono-iodotyrosine (MIT)

di-iodotyrosine (DIT)

Question 2

What is T3 and T4 made of?

Answer 2

MIT + DIT = T3

DIT +DIT = T4

Question 3

What if the order is reversed in T3?

Answer 3

DIT +MIT = reverse tri-iodothyronine, rT3

binds to the receptor but doesn’t activate it = antagonist at T3 receptors

Question 4

What are the 2 sides of a follicular cell? Which way do they face?

Answer 4

basal (faces blood) and apical/luminal (faces lumen/colloid)

Question 5

How does the body get iodine usually? How does it get into the follicular cell? What is this process called? Active or passive transport?

Answer 5

diet - seafood and salt
through sodium-iodide symporter through basal side
electroneutral (no net electric charge)
active uptake (trapping)

Question 6

How does iodine move from the follicular cell to the colloid? What enzyme oxidises the iodine? What does this form?

Answer 6

moves across via pendrin

oxidation of iodide via THYROID PEROXIDASE (TPO)

Question 7

What is thyroglobulin?

Answer 7

a large glycoprotein with many tyrosine molecules where thyroid hormones are synthesised

Question 8

Where is thyroglobulin (Tg) made?

Answer 8

in follicular cells

Question 9

How is thyroglobulin transported from the follicular cells to the colloid?

Answer 9

exocytosis

Question 10

What happens in the organification process?

Answer 10

iodination of tyrosine molecules on Tg -> MIT and DIT

Question 11

What enzyme catalyzes the steps of forming thyroid hormones in the follicle? What are the 3 steps?

Answer 11

thyroid perosidase (TPO)

• oxidation of iodide
• organification
• coupling of iodotyrosines -> T3,T4

Question 12

What happens to the T3 and T4 once it’s been synthesised? How long can supplies of thyroid hormone last

Answer 12

stored of ioninated Tg in the colloid

several weeks

Question 13

How are thyroid hormones released into the blood?

Answer 13

Follicular cells pinch off bits of colloid then lysosomes added to give proteases which clean off the hormone to leave T3 or T4

Question 14

By what process is thyroglobulin/Thyroid hormones taken into the follicular cells?

Answer 14

Tg taken up by endocytosis

Fusion with lysosomes results in proteolysis and release of T3 and T4 into plasma

Question 15

What can T3/T4 inhibit? What can TSH inhibit?

Answer 15

T3/T4 can inhibit hypothalamus and anterior pituitary

TSH inhibits hypothalamus

Question 16

What is the implication of TSH as a trophic hormone? What steps does TSH stimulate?

Answer 16

TSH maintains integrity of thyroid gland - if in excess, increases size and numbers of thyroid follicular cells
acts on every step (of TH production and release)

Question 17

What can cold stimulate in neonates? What about in adults?

Answer 17

TRH (thyroid hormones)

negligible in adults, unless chronic exposure

Question 18

What does stress do in terms of thyroid hormone synthesis?

Answer 18

Inhibits

Question 19

what are the 3 proteins by which T3 and T4 can be bound to?

Answer 19

Thyroxine-binding globulin (TBG)
Transthyretin
Albumin

Question 20

T3/T4 only work when bound to a protein T/F?

Answer 20

FALSE - protein bound hormones cannot act biologically

Question 21

How can we measure thyroid function?

Answer 21

Measurement of free T4

Question 22

Of T3/T4 and rT3, rank them in order of shortest to longest half-life

Answer 22

rT3
T3
T4

Question 23

What is made in greater amounts? T3 or T4? Which is more potent? Which is the prohormone?

Answer 23

greater amounts of T4 made
T3 is more potent
T4 is the prohormone (T3 is the active hormone)

Question 24

T4 still has some action by itself T/F?

Answer 24

TRUE

Question 25

What enzymes work on T3/T4 when in target cells?

Answer 25

deiodinase enzymes

Question 26

What 2 things does T4 form?

Answer 26

T3 or rT3

Question 27

What are the deiodinase enzymes and what do they do?

Answer 27

D1 and D3

converts T4 into either T3 or rT3

Question 28

What are the 3 actions of thyroid hormones?

Answer 28

Increase met - CHOS, proteins, fats -> higher O2 consumption, heat production and increase BMR

Stimulate growth and development (esp CNS)

synergistic with actions of SNS and catecholamines (TH up-regulate expression of beta-adrenoceptors)

Question 29

At a molecular level, explain the physiological effects of T3/T4

Answer 29

increased met in mitochondria

Question 30

What are some scenarios where T4 is converted into rT3 instead?

Answer 30

liver/kidney disease, acute stress, drugs, diet (low protein, starvation)

causes lower metabolic rate