Thyroid Physiology

Question 1

When does the thyroid develop during embryogenesis?

Answer 1

4th week

Question 2

Where does the thyroid originate from?

Answer 2

Tongue - endodermal thickening occurs forming the foramen caecum

Question 3

How do C cells become part of the thyroid?

Answer 3

As it comes in contact with the lower pharynx

Question 4

What is the final location of the thyroid gland, when does it settle here?

Answer 4

C5-T1 - by the 8th week

Question 5

What happens to the thyroglossal duct?

Answer 5

Most disintegrate

Question 6

What can happen as the thyroid migrates down?

Answer 6

Thyroglossal cysts can form

Question 7

How heavy is the thyroid gland?

Answer 7

Around 25g

Question 8

What chemical is required in order for the gland to secrete hormones?

Answer 8

Iodine

Question 9

What is the gland made up of?

Answer 9

Follicles surrounded by parafolicular cells

Question 10

Name the chemical produced by parafollicular cells

Answer 10

calcitonin

Question 11

Describe the pathway leading to thyroid hormone synthesis

Answer 11

1. Follicular cells produce thyroglobulin (tyrosine rich protein)
2. Iodide ions are actively transported into the follicular cell where they are concentrated
3. Enzyme thyroperoxidase converts iodide to iodine
4. Iodine & thyroglobulin move into the colloid
5. Iodine attaches to tyrosin residue

Question 12

Name the product if one iodine attaches

Answer 12

Monoiodotyrosin (MIT)

Question 13

Name the product if two iodines attach

Answer 13

Di-iodotyrosin (DIT)

Question 14

What are the two potential products from MIT and DIT?

Answer 14

MIT + DIT = triiodothyronine (T3)

DIT + DIT = thyroxine (T4)

Question 15

Which is more potent T3 or T4?

Answer 15

T3 - biologically active hormone

Question 16

How does the body signal to the thyroid gland to produce T3 & T4?

Answer 16

Hypothalamus releases TRH stimulating the pituitary which produces TSH that binds to thyroid leading to the production of T3 & T4

Question 17

How are thyroid hormones transported in the blood?

Answer 17

Bound to serum proteins or free in plasma

Question 18

Name three serum proteins thyroid hormones bind to

Answer 18

• thyroxine binding globulin
• thyroxine binding pre-albumin
• albumin

Question 19

In what form can thyroid hormones act on organs/tissues

Answer 19

Free in the plasma

Question 20

What effect does thyroid hormone have on the liver?

Answer 20

Increases glycogenolysis & gluconeogenesis

Question 21

What effect does thyroid hormone have on adipose tissue?

Answer 21

increased lipolysis

Question 22

What effect does thyroid hormone have on the lungs?

Answer 22

increased breathing rate

Question 23

What effect does thyroid hormone have on the heart?

Answer 23

increased heart rate & force of contraction

Question 24

Overall what is the effect of thyroid?

Answer 24

Increases metabolic rate

Question 25

How does thyroid increase metabolic rate?

Answer 25

• increase number/size of mitochondria
• increase oxygen use & ATP hydrolysis
• increase synthesis of enzymes

Question 26

How do thyroid hormones increase sympathomimetic action?

Answer 26

Increase sympathetic response by increasing number of receptors

Question 27

Name three types of de-iodinases & where are they found

Answer 27

D1 (liver & kidneys)
D2 (heart, muscle, CNS, fat, thyroid, pituitary)
D3 (foetal tissue & placenta, brain)

Question 28

What do D1 & D2 do?

Answer 28

Convert T4 - T3

Question 29

Describe the action of D3

Answer 29

Converts T3 into inactive T3 which is rapidly excreted