4. Thyroid Hormone

Question 1

When does the thyroid gland begin to develop?

Answer 1

Day 24

Question 2

What does the thyroid gland originate from?

Answer 2

Proliferation of endodermal epithelial cells on median surface of developing pharyngeal floor

Question 3

What does the thyroid precursor develop from?

Answer 3

Thyroglossal duct

Question 4

What happens in relation to the thyroid gland by weeks 7-10?

Answer 4

Thyroglossal duct obliterates

Question 5

When is the thyroid gland fully developed?

Answer 5

10 weeks

Question 6

What is the term used to describe disorders of thyroid gland formation?

Answer 6

Thyroid dysgenesis

Question 7

What is an ectopic thyroid gland?

Answer 7

Thyroid gland doesn’t reach final position at front of neck

Question 8

What is the aetiology of a thyroglossal duct cyst?

Answer 8

Duct doesn’t atrophy

Question 9

Where can the parathyroid glands be found?

Answer 9

Posterior surface of the thyroid

Question 10

What is the function of parathyroid hormone?

Answer 10

Increases blood calcium levels

Question 11

What are parafollicular or c-cells?

Answer 11

Single cells in epithelial lining of follicle or in groups in connective tissue between follicles
Secrete calcitonin

Question 12

What is the function of calcitonin?

Answer 12

Lowers blood calcium levels

Question 13

Describe the structure of the thyroid gland

Answer 13

Divided into clusters of follicles by connective tissue

Follicles contain a thin layer of epithelial cells and colloid

Question 14

What does the synthesis of thyroid hormones require?

Answer 14

Thyroglobin

Iodine

Question 15

What type of molecule is thyroglobin?

Answer 15

Large glycoprotein

Question 16

Where is thyroglobulin made?

Answer 16

Epithelial cells surrounding follicle

Question 17

What stimulates the production of thyroglobulin?

Answer 17

TSH

Question 18

Describe thyroid hormone synthesis

Answer 18

1. Thyroglobulin secreted into follicle lumen
2. TSH causes cells to actively transport I- into cytosol
   I- is converted to active iodine
3. Iodine enters follicle lumen
   Attaches to tyrosine in colloid
   Forms DIT and MIT
4. Iodinated tyrosines link together to form T3 and T4
5. Thyroglobulin colloid is endocytosed
   Combined with lysosome
6. Lysosomal enzymes cleave T3 and T4 from thyroglobulin colloid
   Hormones diffuse from follicle cell into the bloodstream

Question 19

What is thyroperoxidase?

Answer 19

Enzyme on colloid facing membrane of epithelial ells

Question 20

What reactions does thyroperoxidase catalyse?

Answer 20

Ioidination of tyrosines on thyroglobulin

Coupling reactions

Question 21

How are T3 and T4 transported in the blood?

Answer 21

Bind to throxine-binding globulins or albumin

Question 22

What are the benefits of having T3 and T4 bound to transport proteins?

Answer 22

Stops them having unregulated effects
Protects from enzyme attack and renal excretion
Longer half life than other hormones

Question 23

How much iodine is contained in the average adult body?

Answer 23

20-50mg

Question 24

What percentage of the iodine in the body is contained in the thyroid?

Answer 24

60%

Question 25

What types of areas have low iodine in the soil?

Answer 25

Glaciated, mountainous, heavy rainfall

Question 26

What foods are sources of iodine?

Answer 26

Seafood, iodized salt, milk

Question 27

How does dietary iodine get into the bloodstream?

Answer 27

Absorbed in the GI tract Converted to iodide in the gut Circulates bound to serum proteins

Question 28

What transports iodide into follicular cells?

Answer 28

Na+/I- co-transporter

Question 29

What inhibits the production of TRH?

Answer 29

Intracellular T3 | Circulating T4

Question 30

What regulates TSH synthesis?

Answer 30

Increased by TRH Decreased by T3 Slightly higher during the night

Question 31

What are the functions of TSH?

Answer 31

1. Increases size and vascularity of the thyroid gland 2. Increases iodide transport into follicular cells 3. Increases production and iodination of thyroglobulin 4. Increases endocytosis of colloid from lumen

Question 32

What plasma proteins can thyroid hormones bind to?

Answer 32

Thyroxine binding globulin Thyroxine binding pre-albumin Albumin

Question 33

Which plasma protein has high affinity and low capacity for thyroid hormones?

Answer 33

TBG

Question 34

What is the type of receptor that thyroid hormones bind to?

Answer 34

Retinoid X Receptor