The Thyroid Gland - anatomy and function

Question 1

Where is the thyroid gland located?

Answer 1

Lies against and around the front of larynx and trachea

Below thyroid cartilage
Isthmus extends from 2nd to 3rd rings of trachea

Question 2

What joins the 2 thyroid lobes?

Answer 2

The isthmus

Question 3

Is the thyroid the first endocrine gland to develop?

Answer 3

Yes !

Question 4

Describe the embryological development of the thyroid

Answer 4

At approx 3-4 weeks gestation, thyroid gland appears as an epithelial proliferation at base of tongue.

As the thyroid descends, it remains attached to the tongue via the thyroglossal duct. It passes infront of the hyoid bone on its journey.

The duct should subsequently degenerate

Question 5

The thyroid tissue is made up of 2 cell types…

Answer 5

Follicular cells

Parafollicular cells

Question 6

Describe a thyroid follicle

Answer 6

Thyroid follicles are comprised of follicular cells arranged in spheres with central colloid.

Colloid is extracellular even though it is inside the follicle

Question 7

What do follicular cells produce?

Answer 7

Thyroid hormones

Question 8

What do the parafollicular cells produce?

Answer 8

Calcitonin

Question 9

What does the colloid store?

Answer 9

Thyroglobulin

Question 10

What are thyroid hormones comprised of?

Answer 10

2 tyrosines linked together with iodine at 3 or 4 positions on the aromatic rings

T3 = triiodothyronine 
T4 = tetraiodothyronine (thyroxine)

Question 11

What is T4 called?

Answer 11

Thyroxine

Question 12

What does thyroglobulin act as?

Answer 12

A scaffold on which thyroid hormones are formed - they contain many tyrosines which are used to synthesise thyroid hormones

Question 13

What is the role of thyroid peroxidase in thyroid hormone synthesis ?

Answer 13

Oxidation of iodide to iodine

Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin

Coupling MIT or DIT to generate thyroid hormones within the thyroglobulin protein

Question 14

The thyroid gland contains what percentage of all iodine in the body?

Answer 14

90-95%

Question 15

Dietary iodine is reduced to iodide before absorption principally in..

Answer 15

Small intestine

Question 16

Iodide is taken up from blood by thyroid epithelial cells, which have what symporter?

Answer 16

Sodium- iodide symporter (iodine trap)

Question 17

What are some common sources of dietary iodine?

Answer 17

Dairy products
Grains
Meat
Vegetables 
Eggs
Iodized salt

Question 18

90% of thyroid hormone secreted is T4 or T3?

Answer 18

T4

Question 19

Is the biological activity of T3 or T4 greater?

Answer 19

T3

Question 20

T3 and 4 are transported in the blood bound to the protein…

Answer 20

Thyroxine binding globulin

If bound it is not biologically active

Question 21

How is thyroid hormone secretion regulated?

Answer 21

Via negative feedback

Hypothalamus releases thyrotropin releasing hormone (TRH) , which acts on the anterior pituitary to release thyroid stimulating hormone (TSH).

TSH acts on the thyroid to increase release of thyroid hormones

Question 22

Thyroid hormones effect virtually every cell in body. They have 2 broad interconnected responses…

Answer 22

Effect cellular differentiation and development

Effect metabolic pathways

Question 23

TSH is what type of hormone?

Answer 23

Glycoprotein

Question 24

In most tissues, thyroid hormones stimulate the metabolic rate by…

Answer 24

Increasing number and size of mitochondria

Stimulating the synthesis of enzymes in the respiratory chain

Question 25

Do thyroid hormones generally stimulate catabolic or anabolic pathways?

Answer 25

Catabolic e.g lipolysis, beta oxidation of fatty acids

Also stimulates entry of glucose into cells, gluconeogenesis and glycogenolysis

Question 26

What are the sympathomimetic effects of thyroid hormones?

Answer 26

Increase target cell response to catecholamines - by increasing receptor number on target cells

Question 27

How do thyroid hormones effect CVS?

Answer 27

Increase heart’s responsiveness to catecholamines - increase CO
Increase in HR and force of contraction

Increase peripheral vasodilation to carry extra heat to body surface

Question 28

How do thyroid hormones affect nervous system?

Answer 28

Increase myelination of nerves and development of neurons

Question 29

What type of receptors are thyroid hormone receptors?

Answer 29

Nuclear receptors - act by modulating gene expression

Question 30

When thyroid hormone enters nucleus of cell and binds to thyroid hormone receptor on DNA, what happens?

Answer 30

Binding relieves repression of gene transcription and the gene is now expressed.

Expression of new protein mediates the effects of thyroid hormone - triggers cellular response

Question 31

What is the arterial supply to the thyroid from?

Answer 31

Superior and inferior thyroid arteries

Question 32

What is the superior thyroid artery a branch of?

Answer 32

External carotid artery

Question 33

What is the inferior thyroid artery a branch of?

Answer 33

Subclavian > thyrocervical trunk > inferior thyroid artery

Question 34

What is venous drainage of the thyroid via?

Answer 34

Superior
Middle
Inferior thyroid vein

Question 35

Once T4 enters the cell, what happens?

Answer 35

Mostly converted to T3

Question 36

Thyroid hormones increase basal metabolic rate. What does this mean in simple terms?

Answer 36

The cell burns more energy

Question 37

How do thyroid hormones affect the bones?

Answer 37

Increase resorption - thins out bones