Endocrinology 6 - The Thyroid Gland and the Iodothyronines

Question 1

Where is the thyroid located?

Answer 1

At the top of the trachea

Question 2

What is the Isthmus?

Answer 2

The tissue that joins together the 2 lobes of the thyroid.

Question 3

What is the pyramid?

Answer 3

It is a tissue located on the top of the isthmus, which is present in some people. Clinically important as a thyroid tumour may be found here.

Question 4

Describe the basic structure of the thyroid

Answer 4

It is bi-lobed and full of follicles.

Question 5

What is the colloid?

Answer 5

The proteinaceous substance found inside the follicles. It is a massive store of iodothyronines.

Question 6

What are Follicular cells?

Answer 6

The cells lining the follicle.

Question 7

What are Parafollicular cells?

Answer 7

Found between the follicles and they also provide hormones.

Question 8

What are the 6 things that TSH does when it binds to the TSHR?

Answer 8

1. Stimulates the iodine pump to pump iodine into the cell.
2. Stimulates iodine to be pumped into the colloid via pendrin pumps.
3. Stimulates the synthesis of Thyroglobulin (TG).
4. Stimulates Thyroid Peroxidase (TPO).
5. Allows coupling reactions to form T3 and T4.
6. Stimulates lysosomes to move towards the apical membrane and stimulates uptake of colloid.

Question 9

How is iodine pumped into the colloid from the cell?

Answer 9

It’s is pumped through Pendrin pumps.

Question 10

What does TPO do?

Answer 10

It converts iodine into a radioactive iodine form (very short lived) in the presence of hydrogen peroxide.

Question 11

What is iodination?

Answer 11

It is when iodine is added to TG using TPO to form MIT/DIT in the colloid.

Question 12

What does the coupling reaction involve?

Answer 12

It involves combining two TG molecules to form T3 or T4 using TPO.

Question 13

What is the difference between T3 and T4?

Answer 13

T3 is formed when a MIT is combined with a DIT whereas T4 involves two DIT’s.

Question 14

What is the difference between MIT and DIT?

Answer 14

MIT is when only 1 Iodine is added (at position 3) to tyrosine whereas DIT is when 2 Iodine’s are added (position 3 and 5).

Question 15

How is T3 and T4 transported in the blood?

Answer 15

They are bound to plasma proteins.

Question 16

What are the 3 types of globulins that bind to T3 and T4?

Answer 16

1. TBG - specific to T3 + T4.
2. Albumin - is not specific, loosely binds to Ts + T4.
3. Prealbumin - binds to T4 but very little T3.

Question 17

What is the main hormone product of the thyroid?

Answer 17

T4.

Question 18

Is T4 or T3 more active? Which one has a shorter latent period and shorter half life?

Answer 18

1. T3 is more active.
2. Has a shorter latent period (12 hours compared to 72 hours)
3. Shorter half life (2 days compared to 7-9 days).

Question 19

How is T4 converted to T3?

Answer 19

T4 is converted to T3 is a process called Deiodination. This is the removal of one iodine from T4 and occurs in the target tissue.

Question 20

What is reverse T3 (rT3)? In what kind of situations would you want rT3?

Answer 20

1. It is when T4 is deiodinated at a different position which produces a biologically inactive T3.
2. During times where you would want a reduced metabolism (e.g. starvation).

Question 21

What are the main actions of Iodothyronines? (7)

Answer 21

1. Increase Basal Metabolism.
2. Increase protein, lipid and carbohydrate metabolism.
3. Important for foetal growth + development.
4. Enhance effects of Catecholamines.
5. Interact with other endocrine systems.
6. Effects on CNS.
7. Increase vitamin C synthesis from retinal.

Question 22

What is a mechanism of action of iodothyronine?

Answer 22

1. Enters target cell readily and binds to intracellular receptors.
2. Receptor-hormone complex will have effect in nucleus that will lead to new proteins being made.
3. It can also affect membrane transport pumps.
4. Has effects on the mitochondria - stimulates metabolic activity.

Question 23

How is iodothyronine production stimulated?

Answer 23

1. Neurones in hypothalamus release TRH into primary capillary plexus.
2. TRH passes down through portal to adenohypophysis and bind to membrane receptors.
3. Stimulates thyrotrophs to produce TSH.
4. TSH binds to receptors in thyroid and stimulates T3 + T4 production.

Question 24

How is iodothyronine production decreased? (5)

Answer 24

1. T3 and T4 have negative feedback effects on both the hypothalamus and the pituitary.
2. Wolff-Chaikoff effect - a large concentration of Iodide inhibits T3 + T4 production.
3. Somatostatin.
4. Thyrotrophin has auto-negative feedback.
5. Glucocorticoids.

Question 25

Where are thyrotrophin receptors found?

Answer 25

Basolateral membrane of the follicular cells.

Question 26

What is thyroglobulin?

Answer 26

Is a long array of amino acids which have a number of tyrosyl residues which can be iodinated by the reactive iodine.

Question 27

How is T3 and T4 taken into the cells from the colloid?

Answer 27

1. TSH stimulates lysosomes to move to apical membrane + stimulates uptake of colloid by apical membrane.
2. The internalized colloid fuses with lysosome and enzymes break down protein to liberate T3 and T4.
3. T3 and T4 move into blood.

Question 28

Are the bioactive iodothyronines bound to plasma proteins?

Answer 28

No, they are free molecules.

Question 29

What can a lack of iodothyronines lead to?

Answer 29

Cretinism.

Question 30

What kind of action do iodothyronines have?

Answer 30

Genomic action

Question 31

What increases iodothyronine production?

Answer 31

1. Thyroid releasing hormone.

2. Oestrogens.