The Thyroid Gland

Question 1

What are the types of cell in the Thyroid gland and their arrangement?

Answer 1

C cells – lie in between follicles

Follicular cells – surround the hollow follicles

Question 2

What are the main actions of TSH on thyroid cells?

Answer 2

TSH stimulates follicular cells to endocytose colloidal thyroglobulin.

Question 3

What are the steps by which T4 and T3 are formed and secreted into the ECF?

Answer 3

1. Iodines are added to Tyrosines to form MIT (1 iodine) and DIT (2 iodines)
2. MIT and DIT undergo reactions where:
   
   1. MIT + DIT = T3
   2. DIT + DIT = T4
3. Once T3 and T4 are formed, they bind to Thyroxine-binding globulin in-order to travel through the blood.
4. TSH activates thyroxine-binding globulin (TBG) to be endocytosed.
5. TBG has a higher affinity for T4, hence the longer half-life of T4.

Question 4

What is the T3/4 ratio in thyroid secretion? Conversion of T4 into T3 in target tissues?

Answer 4

1) 50 times more T4 in plasma than T3.
2) However, thyroid receptors have a much greater affinity for T3 than T4. This means that T4 is de-iodinated to T3 by deiodinase enzymes.

Question 5

Where are T3 receptors located?

Answer 5

T3 receptors are located inside the target cells and are called nuclear receptors.

Question 6

What are the mechanisms of action of T3 and what are the biological effects in humans?

Answer 6

1. T3 Mechanisms of action:
   
   1. Alter protein synthesis by changing transcription and translation
2. T3 Biological effects:
   
   1. Raises metabolic rate and promotes thermogenesis
   2. Increases hepatic gluconeogenesis
   3. Increase in proteolysis
   4. Increase in lipolysis
   5. Critical for growth (lack of TH results in retarded growth)
      
      1. Stimulates GH receptor expression
   6. Required for foetal brain development – deficiency can be caused by dietary iodine deficiency in mother during pregnancy.

Question 7

What is the relevance of dietary iodine and catabolism of T4 & T3?

Answer 7

1. Relevance of dietary iodine:
   
   1. Required during pregnancy to prevent TH deficiency in children.
   2. Dietary iodine is required to form Thyroid hormones
2. Catabolism of T4 & T3:
   
   1. T4 is converted to T3 within the target issue This conversion happens by the T4 losing 1 iodine ion.