Thyroid

Question 1

What are the cells in the thyroid and how are they arranged?

Answer 1

Each follicle consists of a single layer of follicular cells surrounding a central protein rich colloid.
C cells between follices

Question 2

What do C cells do?

Answer 2

Synthesise & secrete calcitonin

Question 3

How is synthesis of thyroid hormones stimulated?

Answer 3

TSH acts at follicular cell receptors & 2nd messenger cAMP mediates active transport of iodide & synthesis or thyroglobulin

Question 4

Is T3 or T4 most active?

Answer 4

T3 is most active form, but shorter half-life.

Question 5

Which is the main thyroid hormone synthesised?

Answer 5

90% production is T4, which is converted to T3 when reaches cellular level

Question 6

How are thyroid hormones synthesised?

Answer 6

1. Iodide uptake into follicular cells by Na+ iodide symport
2. TG secreted hrough apical membrane into follicular lumen
3. Thyroid peroxidase iodonates tyrosine residues in TG

Question 7

How are T3 + T4 formed?

Answer 7

Tyrosine + 1I = MIT
Tyrosine + 2I = DIT

MIT + DIT = T3
DIT + DIT = T4

Question 8

How are thyroid hormones secreted?

Answer 8

1. TG, T3 + T4 move towards follicular membrane
2. Follicular cells close to capillaries carrying TSH in blood
3. TSH uses GPCR with cAMP to activate protein kinases
4. Stimulates thyroid hormones to enter follicular cell by phagocytosis
5. In follicular cell - TG, T3 +T4 bind to lysosome, which breaks it down (TG recycled back into colloid)
6. T3 + T4 are hydrophobic so they diffuse into the bloodstream

Question 9

What proteins do thyroid hormones bind to in the blood?

Answer 9

Globulin & albumin