7 The Thyroid And Iodothyronines Flashcards Preview

Y1 LCRS Endocrinology > 7 The Thyroid And Iodothyronines > Flashcards

Flashcards in 7 The Thyroid And Iodothyronines Deck (19)
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1
Q

Q: What is the thyroid gland anatomy? Shape?What is included in its structure? (2) Full on?

A

A: at top of trachea in neck

Bi-lobed

ISTHMUS = the bit of tissue that joins together the two lobes

PYARMID = the bit of tissue on the top of the isthmus which is present in some people

Thyroid is full of follicles

2
Q

Q: Describe the structure of thyroid follicles. (3)

A

A: Follicular cells = the cells lining the follicle

Colloid = proteinaceous substance found inside the follicles

Parafollicular Cells = found between the follicles and they also provide hormones

3
Q

Q: What produces iodothyronines?

A

A: follicular cells

4
Q

Q: How are iodothyronines made? Include diagram.

A

A: anterior pit produces TSH and TSH binds to TSHR -> lots of effects (Thyrotrophin Receptors are found on the serosal/basal membrane of follicular cells)

1 sodium-iodide (negative 1 iodine ion) pump= stimulated -> iodide pumped into cells and then out the cell via pendrin pumps (IODIDE HAS BEEN MOVED FROM BLOOD TO THE COLLOID)

2 nucleus stimulated -> thyroglobulin (TG) produced -> moved to colloid and remains associated with the apical membrane

3 thyroid peroxidase (TPO) enzyme is stimulated and in the presence of hydrogen peroxide it converts iodide into a reactive iodine form which is very short lived (at apical membrane)

4 TSH keeps stimulating TPO which allows COUPLING REACTIONS to take place where Triiodothyronines (T3) and Tetraiodothyronines (T4) can be formed

The thyroid hormones are produced on the apical membrane

5 TSH then stimulates lysosomes to move towards the apical membrane and stimulates the uptake of colloid by the apical membrane (colloid is a massive store of iodothyronines)

6 The colloid that has been internalised then fuses with the lysosome and the enzymes break down the protein, liberating T3 and T4

7 T3 and T4 move out in to the blood

5
Q

Q: What does having an overactive thyroid mean? Treatment?

A

A: secreting too much thyroxine

given carbimazole

6
Q

Q: How do you get T3 and T4?

A

A: MONOIODOTYROSINE (MIT) is where you add 1 iodine and DIIODOTYROSINE (DIT) is where you add 2 iodines

then get coupling reactions

DIT + DIT = T4 aka thyroxine
DIT + MIT = T3

7
Q

Q: How are T3 and T4 transported around the body?

A

A: mainly bound to plasma proteins

a) thyroid-binding globulin: TBG (70-80%) = specific to T3/4
b) albumin (10-15%)
c) prealbumin (aka transthyretin)

very little is unbound

8
Q

Q: What percentage of T3 and T4 are unbound? What does this mean?

A

A: T3- 0.5%
T4- 0.05%

bioactive form

9
Q

Q: What’s the main hormone product of the thyroid gland? What happens to it in target tissue?

A

A: T4

deiodinated to T3 (bioactive form) in target tissue

(some T3 is made but most is made as a result of T4 deiodination)

can be deiodinated in different position to get reverse T3 (inactive)

10
Q

Q: Draw simple diagrams for thyrosine, T3, reverse T3 and T4.

A

A: DIAGRAM

11
Q

Q: What is the mechanism for thyroid hormone action?

A

A: Iodothyronines taken up by cells via transporter

any t4 that enters is converted to t3

some t3 that enters causes non-nuclear actions on ion channels

t3 main effect is binding to THR in the nucleus where receptor-hormone complex affects transcription of various genes and protein synthesis eg hormones/receptors

t3 stimulates metabolic activity so has a lot of effects on the mitochondria

IMPORTANT: Iodothyronines have a GENOMIC ACTION

12
Q

Q: What is important to remember about iodothyronine action?

A

A: action is genomic

13
Q

Q: What are the actions of TH?

A

A: fetal growth and development
untreated congenital hypothyroidism: cretinism

TH and TSH measured in new born infant’s heel-prick test

increase basal metabolic rate (-> lose weight)
protein, carbohydrate and fat metabolism
potentiate actions of catecholamines (eg tachycardia, lipolysis)
effects on the GI eg diarrhea, CNS, reproductive system

14
Q

Q: What is the latent period for T3 and T4?

A

A: t3- around 12hrs

t4- around 72hrs

15
Q

Q: What is the half life of T3 and T4?

A

A: t4- 7-9 days

t3- 3 days

16
Q

Q: Why is half life and latent period relevant?

A

A: may have to withdraw treatment eg if patient is having a scan

17
Q

Q: Explain how TH production is controlled.

A

A: effects in anterior pituitary to thyrotroph cells
somatostatin and glucocorticoids decrease
oestrogen and TRH increases

oestrogen and glucocorticoids can also affect TSH

TSH (thyrotrophin) is produced

affects thyroid -> produce T4 and T3
iodide (wolff-chaikoff effect) decreases

negative feedback from T4 and T3

  • direct -ve effect on the thyrotroph cell in anterior pituitary
  • indirect -ve effect on the hypothalamus
18
Q

Q: What is the wolff-chaikoff effect?

A

A: too much I- (iodide) inhibits thyroid hormone synthesis

19
Q

Q: What is thyroglobulin? Contains? What can happen to them? Where?

Options? (2)

A

A: Thyroglobulin is a long array of amino acids which have a certain number of TYROSYL RESIDUES which become iodinated by the reactive iodine

in colloid

tyrosyls can be iodinated in one or two positions to form MONOIODOTYROSINE (MIT) and DIIODOTYROSINE (DIT)