Thyroid Physiology

Question 1

What hormones does the thyroid gland make?

Answer 1

thyroxine (T4) and triiodothyronine (T3) and calcitronin

Question 2

How is the thyroid visualized? to check for enlargement or nodules

Answer 2

Pt given radioactive iodine which is taken up by the thyroid

Question 3

Describe the cellular structure of thyroid tissue

Answer 3

Follicular cells surround colloid
Colloid contains thyroglobulin
Rich blood supply
Parafollicular cells that make calcitonin

Question 4

Briefly describe how thyroid hormone is synthesized

Answer 4

1. Na/I transporter takes up iodine
2. pendrin proves iodine into colloid/follicular lumen
3. Tg made in follicualr cell and transported out via secretory vesicles
4. Tg iodinated in colloid by thyroid peroxidase
5. endocytosis of iodinated Tg into follicular cell
6. cleavage of I-Tg into T4 and T3
7. MIT and DIT recycled

Question 5

anion exchanger on apical side of follicular cell that move I+ into the colloid /follicular lumen

Answer 5

Pendrin

Question 6

made in the follicular cell and packaged into vesicles

Answer 6

thyroglobulin

Question 7

coats luminal side of secretory vesicles containing Tg

Answer 7

thyroid peroxidase

Question 8

mediates binding of iodine to select tyrosine residues of thyroglobulin

Answer 8

thyroid peroxidase

Question 9

Internal rearrangement of 2 iodinated tyrosyl residues forms

Answer 9

iodothyronine (via thyroid peroxidase)

Question 10

Only ___% of the tyrosine residues on Tg are iodinated

Answer 10

~20%

Question 11

is endocytosed by follicular cell

Answer 11

Iodothyronine

Question 12

hydrolysis of iodothyronine forms …

Answer 12

T3 and T4

Question 13

What is the first thing to happen ti T3 and T4 once they enter the blood stream?

Answer 13

immediately bound by plasma proteins

Question 14

What are the plasma binding proteins for T3 and T4?

Answer 14

Thyroxine-binding globulin (TBG) > albumin and transthyretin (TTR)

Question 15

_____ will increase the amount if TBG in blood. How will the inc in TBG affect the levels of free T4?

Answer 15

Pregnancy, hepatitis, heroine abuse

will increase TOTAL T4 but free T4 will not change

Question 16

Enzyme that metabolizes T4 to T3? T4 to rT3?

Answer 16

T4 to T3: 5’/3’ mondeoiodinase

T4 to rT3: 5/3 mondeoiodinase

Question 17

difference between T3 and rT3?

Answer 17

rT3 is inactive because 2 Iodides are on the outer ring, and there needs to be 2 iodides on the inner ring for activity

Question 18

T0 aka

Answer 18

thyronine

Question 19

Enzymes that metabolize T3 –> T2 –> T1–>T0

Answer 19

5’/3’ mondeoiodinase

5/3 mondeoiodinase

Question 20

is primarily responsible for forming the T3 in the blood

Answer 20

type 1 5’/3’ mondeoiodinase

Question 21

is primarily responsible for forming the T3 that regulates the production of TH

Answer 21

type 2 5’/3’ mondeoiodinase

Question 22

Where is type 1 and 2 5’/3’ mondeoiodinase found?

Answer 22

1 = liver, kidney and thyroid (forms T3 in blood)
2= pituitary, CNS, placenta (forms T3 that regulates TH production)

Question 23

Describe the mechanism of TH when it reaches target cells

Answer 23

• transported into the cell
• Bind THR (thyroid hormone receptor)
• THR dimerizes with RXR (retinoid X receptor)
• Activate transcription (sometimes will inhibit transcription)

Question 24

Thyroid hormone that acts like_____ hormone because it binds to an intracell/nuclear receptor

Answer 24

steroid

Question 25

effect of hyper- and hypo- thyroidism on…

basal metabolic

Answer 25

inc - dec

Question 26

effect of hyper- and hypo- thyroidism on…

carb metabolism

Answer 26

inc - dec

Question 27

effect of hyper- and hypo- thyroidism on…

protein metabolism

Answer 27

inc - dec

Question 28

effect of hyper- and hypo- thyroidism on…

lipid metabolism

Answer 28

inc - dec

Question 29

effect of hyper- and hypo- thyroidism on…

thermogenesis

Answer 29

inc - dec

Question 30

effect of hyper- and hypo- thyroidism on…

blood glc levels

Answer 30

no change - no change

Question 31

effect of hyper- and hypo- thyroidism on…

serum cholesterol

Answer 31

dec - inc

Question 32

Futile cycle with hyperthyroidism

Answer 32

breakdown of lipids for carb generation in liver + protein metabolism to supply carb in liver opposes use of aa and glycerol from breakdown of protein an lipids for carbs in liver

*breaking down something just to re-build it in the same place (all in liver)

Question 33

describe the negative feedback regulation of TH production

Answer 33

inc T4 and T3 inhibits the release of TSH…

• Indirectly by decreasing the amount of thyrotrophin RH receptors on thyrotrophs
• Directly by downregulating the transcription of TSH (in thyrotrophs)

Question 34

Is there positive feedback for TH production? explain

Answer 34

yes,
Decrease in T3 and T4 activate the release of TSH by… (reverse of neg reg)

• Indirectly by increasing the amount of thyrotrophin RH receptors on thyrotrophs
• Directly by upregulating the transcription of TSH (in thyrotrophs)

Question 35

In __________ of hypothalamus, _______ RH is made → to ________ → activates ______ production in thyrotrophs → ________ activates T3 and T4 production

Answer 35

In arcuate nucleus of hypothalamus, thyrotrophin RH is made → to anterior pituitary → activates TSH production in thyrotrophs → TSH activates T3 and T4 production

Question 36

TSH exerts ________ effects on thyroid cells

Answer 36

growth factor
TSH aka thyrotrophin

also increases the activity of every step in production of TH…
•TSH increases activity of the Na/I symporter
•Stimulates the iodination of Tg in the colloid/follicular lumen
•Stimulates the conjugation of Iodinated tyr to make T3 and T4
•Increases the endocytosis of iodinated Tg
•Stimulates the proteolysis of iodinated Tg in lysoendosomes
•Increases the secretion of T4 and T3