20. Thyroid hormones (T3/T4) Flashcards Preview

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Flashcards in 20. Thyroid hormones (T3/T4) Deck (40):
1

thyroid hormones defintion

iodine containging hormones that control the body's metabolic rate

2

thyroid hormones source

1. follicles of thyroid
2. Most T3 formed in target tissues (by 5' deiodinase)

3

T3 functions

MNEMONICS : 4B's
1. Brain maturation
2. Bone growth
3. β- adrenerfic effects
4. increased metabolic rate

4

T3 function - bone growth

synergic with GH

5

T3 function - β adrenergic effects

increased β1 receptors in heart ---> increased CO.HR. SV. contractility

6

effect of thyroid hormones in heart

increased β1 receptors in heart ---> increased CO.HR. SV. contractility

7

T3 functions - Basal metabolic rate

increased BMI ---> increased
a. glycogenolysis
b. gluconeogenesis
c. lipolysis
d. increased Na/K pump activity ---> increased O2 concumption, RR, body temperature

8

T3 function ---> increased Na/K pump activity ---> ....

increased O2 consumption, RR, body temperature

9

TBG

thyroxine binding globulin

10

TBG - function

it binds most T3/T4 in blood

11

T3/T4 - active state

only free hormones ( not binded to TBG) are active

12

TBG - increased (situations )

estrogen (pregnancy or OCP use)

13

TBG - decreased situations )

1. hepatic failure
2. steroids

14

major thyroid product

T4

15

T3 production

T4 converted to T3 in peripheral tissue by 5' deiodinase

16

T3 vs T4 ( according action)

T3 binds nuclear receptor with GREATER affinity than T4

17

thyroid peroxidase

enzyme responsible for oxidation nd organificattion of iodide as well coupling of monoidotyrosine ( MIT) and di- iodityrosine (DIT)

18

thyroid peroxidase inhibitors

1. Propylthiouracil
2. Methimazole

19

Propylthioutacil Methimazole - mechanism of action

-both inhibit thyroid peroxidase
PTU also inhibits both peroxidase and 5' - deiodinase

20

5'- deidodinase inhibitor

Propylthiouracil

21

TRH action - on thyroid hormone axis

it stimulates TSH (pituitary)

22

thyroid production - negative feedback

- free T3/T4 to anterio pituitary , decrease sensitivity to TRH and to hypothalamus
-somatostatin decreases TSH
-dopamine decreases TSH

23

free T3/T4 decrease TSH production by binding to anterior pituitary and...

decrease sensitivity to TRH

24

Wold- Chaikoff effect

excess iodine temporarily inhibits thyroid peroxidase ---> decreased iodine organification --> decreased T3/T4 production

25

Iodine uptake inhibitors

anions
1. perchlorate
2. pertechnetate
3. thiocyanate

26

T4--> T3 - enzyme and location

5' deiodinase in peripheral tissue

27

thyroidglobulin is composed by

tyrisine

28

thyroid hormones - organification? pxoidation ? coupling ? ( and enzyme?)

- organification: TG +I2 (peroxidase)
-oxidation : I ---> I2 (peroxidase)
-couplinh : MIT + DIT

29

thyroid hormones - organification

endocytosis-->
1. coupling reaction (thyroid peroxidase ) and secretion of T3/T4
2. formation of tyrosine by MIT and DIT ( by deiodinase) and form again thyrohlobulin

30

• What plasma protein binds T3/T4 in blood? Is bound thyroid hormone active? What increases or decreases levels of the binding protein?

Thyroxine-binding globulin (TBG); only free hormone is active; liver failure/steroids decrease TBG, estrogen (pregnancy/OCPs) increases TBG

31

• Where are T3 and T4 produced? What element is required for their production? What is their main function?

T4 and some T3 are formed in thyroid follicles, but most T3 is formed in target tissues; iodine; control metabolic rate

32

What are the four main functions of T3?

4 B's =Brain maturation, Bone growth, β-adrenergic effects in heart (increased CO, HR, SV, contractility), increased Basal metabolic rate

33

What are the catabolic effects of thyroid hormones?

Increased glycogenolysis, gluconeogenesis, lipolysis

34

• Routine neonatal screening shows low thyroid hormone levels. What organ system's development are you concerned about? What is this called?

The central nervous system, because thyroid hormones are needed for central nervous system maturation; cretinism

35

Hypothalamus releases ____, stimulating pituitary to release ____, inciting T3/T4 production. ____ provides negative feedback to pituitary.

Thyrotropin-releasing hormone (TRH); thyroid-stimulating hormone (TSH); T3

36

From which large precursor protein is thyroid hormone derived?

Thyroglobulin

37

The major hormone product of the thyroid is ____ (T4/T3); ____ (T4/T3) has greater affinity for nuclear thyroid hormone receptors.

T4; T3

38

Both propylthiouracil and methimazole decrease formation of monoiodotyrosine and diiodotyrosine. How do the drugs differ?

Both inhibit peroxidase, but propylthiouracil also inhibits 5′-deiodinase (which converts T4 to T3 in peripheral tissue)

39

A 50-year-old woman has fatigue, cold intolerance, and weight gain. She has been adding iodized salt to food. Explain the pathophysiology.

Wolff-Chaikoff effect: Temporary inhibition of peroxidase by extra iodine, reducing iodine organification and T3/T4 (hypothyroidism)

40

A pregnant woman's lab studies show an increase in total T4. Should she be alarmed?

Not necessarily; total T4 level increases in pregnancy because of increased circulating TBG, but free T4 should remain normal