The Thyroid Gland

Question 1

What does the thyroid gland weigh?

Answer 1

15-20g

Question 2

What are the 2 physiologically active forms of thyroid hormone?

Answer 2

T₃ (triiodothyronine)

T₄ (thyroxine)

Question 3

What is T₃ also known as?

Answer 3

Triiodothyronine

Question 4

What is T₄ also known as?

Answer 4

Thyroxine

Question 5

What are the 2 cell types of the thyroid gland structure?

Answer 5

C (clear cells)

Follicular cells

Question 6

What is the function of C cells?

Answer 6

Secrete calcitonin (calcium regulating hormone)

Question 7

What is the function of follicular cells?

Answer 7

Support thyroid hormone synthesis and surround hollow follicles

Question 8

What are thyroid follicle walls made of?

Answer 8

Follicular cells

Question 9

What is found inside thyroid follicles?

Answer 9

Colloid (sticky glycoprotein) matrix, contains 2 to 3 months supply of TH

Question 10

How long a supply of TH is contained in thyroid follicles?

Answer 10

2 to 3 months

Question 11

What do the follicle cells produce?

Answer 11

Enzymes that make thyroid hormones as well as thyroglobulin (large protein rich in tyrosine residues)

Question 12

What happens to the enzymes and thyroglobulin that is produced by the follicular cells?

Answer 12

Packaged into vesicles and exported form follicular cells into the colloid via the pendrin

Question 13

As well as producing enzymes and thyroglobulin, what else to follicular cells do?

Answer 13

Concentrate iodide form plasma and transport it into the colloid where it combines with the tyrosine residues to form the thyroid hormones

Question 14

Where is iodide and tyrosine derived from?

Answer 14

Diet

Question 15

What forms together to form the thyroid hormones?

Answer 15

Iodide and tyrosine residues

Question 16

Explain the process of iodide entering the follicular cell?

Answer 16

Enters follicular cell from plasma via Na⁺/I^- transporter (symport):

1) Coupling to Na enables follicular cells to take up iodide against a concentration gradient
2) Iodide is then transported into the colloid via the pendrin transport

Question 17

What transporter is used to transport iodide into colloid?

Answer 17

Pendrin transporter

Question 18

What is thyroid peroxidase also known as?

Answer 18

Thyroperoxidase

Question 19

Explain the process of iodide and tyrosine residues forming thyroid hormones?

Answer 19

Enzymes catalyse addition of iodide to tyrosine residues on the thyroglobulin molecule (process in which iodide loses an electron to become iodine)

Question 20

What is formed from the addition of one iodine to tyrosine?

Answer 20

MIT (monoiodotyrosine)

Question 21

What is formed from the addition of a second iodine to tyrosine (one iodine added to MIT)?

Answer 21

DIT (diiodotyrosine)

Question 22

From MIT/DIT, what forms T₃?

Answer 22

MIT + DIT

Question 23

From MIT/DIT, what forms T₄?

Answer 23

DIT + DIT

Question 24

What enzyme catalyses the formation of thyroid hormones and where is it located?

Answer 24

Thyroid peroxidase located on the apical membrane of follicular cells

Question 25

How much tyrosine and iodine forms T₄?

Answer 25

2 tyrosine and 4 iodine

Question 26

How much tyrosine and iodine forms T₃?

Answer 26

2 tyrosine and 3 iodine

Question 27

What is the thyroid glands response to TSH?

Answer 27

1) Portions of colloid are taken back up into follicular cells by endocytosis
2) Within the cell they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release the thyroid hormones
3) Both T3 and T4 are lipid soluble so pass across follicular membrane into plasma where the bind with plasma proteins, mainly thyroxine-binding globulin

Question 28

What is release of TH into the plasma under the influence of?

Answer 28

TSH released from pituitary

When not stimulated, TH are stored in colloid

Question 29

Does thyroxine binding globulin (TBG) have a greater affinity for T3 or T4?

Answer 29

T4, releasing it slowly into the plasma, partly accounting for the longer half life of T4

Question 30

What does TBG stand for?

Answer 30

Thyroxine binding globulin

Question 31

What is the half life of T3 and T4?

Answer 31

T3 - about 1 day

T4 - about 6 days

Question 32

Does all TH exert an inhibitory effect on TSH and TRH?

Answer 32

No, only free TH that is not bound to proteins

Question 33

Is most circulating TH T3 or T4?

Answer 33

Most is in the form of protein bound T4 (about 100nmole/L), while T3 is only about (2-3nmol/L)

Question 34

Is most binding of TH to TH receptors done by T3 or T4?

Answer 34

Although T4 is 50x greater in concentration than T3, 90% of TH binding to TH receptors inside cells is T3

This is because TH receptor has a higher affinity for T3 than T4 (T3 is more physiologically active)

Question 35

Does TH receptor have a greater affinity for T3 or T4?

Answer 35

T3

Question 36

T4 can be deiodinated to T3 by what enzymes?

Answer 36

Deiodinase enzymes

Question 37

What happens to circulating T4 that is not used?

Answer 37

50% is deiodinated in plasma, the rest is deiodinated inside target cells

Question 38

For regulation of thyroid hormone release, what things stimulate continuous secretion due to the release of TRH from hypothalamus?

Answer 38

Cold, exercise, pregnancy

Question 39

What are some functions of TH once it binds to receptors in target cells?

Question 40

For regulation of thyroid hormone release, what things are inhibitory to the continuous secretion of TH?

Answer 40

Glucocorticoids and somatostatin:

• somatostatin inhibits TSH (TH required for GH action)
• glucocorticoids inhibit TSH and conversion of T4 to T3