S10 L1 - Thyroid Gland

Question 1

• *Thyroid gland**
• Location
• Structure
• How to feel the thyroid gland?
• Relationship to parathyorid same gland?

Answer 1

Location:
Lies against and around front larynx and trachea, below the thyroid cartilage (Adam’s apple). Isthmus extends from 2nd to 3rd rings of trachea.
Structure:
2 lobes, joined by isthmus
​- How to feel the thyroid gland?
You can feel the thryoid cartilage but CAN’T feel the gland normally, can only feel it if it is enlarged
Relationship to parathyorid same gland?
They are distinct glands

Question 2

• *Embryological development of thyroid gland**
• Describe the process

Answer 2

1. Epithelial proliferation of the floor of the phranyx at the base of the tongue
2. Migrates to position: First descends as diverticulum through the thyroglossal duct and migrates downwards passing in front of the hyoid bone
3. Degeneration: The thyroglossal duct subsequently degenerates
4. The now detached thyroid then continues to its final position of the next two weeks

Question 3

• *Histology of thyroid tissue**
• What is the structure of thyroid tissue
• What does this look like histologically
• *Histology of parathyroid tissue**
• What is the structure of parathyroid tissue
• What does this look like histologically

Answer 3

Histology of thyroid tissue
What is the structure of thyroid tissue:
- Thyroid follicular cells - produce thyroglobin
- Thyroid parafollicular cells - produce calcitonin
- Colloid - strore of thyroglobin
What does this look like histologically:
pic

Histology of parathyroid tissue
What is the structure of parathyroid tissue:
- Principal cells (also called chief cells) - produce parathyroid hormone
​What does this look like histologically:
pic

Question 4

Thyroid hormones

• How are T₃ and T₄ made (equation) and full names of these hormones
• Role of thyroglobulin
• Role of thyroid peroxidase

Answer 4

How are T₃ and T₄ made (equation):
MIT + DIT -> T₃
(monoiodotyrosine + diiodotyrosine -> triiodothyronine)
DIT + DIT -> T₄
(diiodotyrosine + diiodotyrosine -> tetraiodothryonine, also called thyroxine)
Role of thyroglobulin:
acts as a scaffold for iodination and coupling
see ​pic
Role of thyroid peroxidase:
Mmebrane bound enzyme that regulates 3 seperate reactions involving iodide -
1. Oxidation: of iodide (I^-) to iodine (I), requires presence of H₂O2
2. Iodination: Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin
3. Coupling: of MIT and DIT to generate thyroid hormones within the thyroglobulin protein

Question 5

Production and Regulation of Thyroid hormones

• Describe the process
• Describe briefly what the thyroid hormone is usually secreted and why (Not the mechanism of how it is secreted - later flashcard), and how it is transported

Answer 5

Describe the process: (also see pic)

1. Thyroglobulin is synthesised in the follicular cells and secreted through exocytosis into the lumen of the thyroid follicle.
2. Dietary iodine must be reduced from I to I^- to be absorbed in the small intestine
3. Iodide is transported through the blood into the thyroid epithelial cells through the 2Na⁺:I^-transporter, against its concentration gradient, and is secreted into the lumen of thethyroid follicle.
4. Iodide undergoes oxidation to produce an iodinating species, catalysed by thyroid peroxidase in the presence of H₂O₂.
5. Side chains of tyrosine found in the thyroglobulin are iodinated to form either mono- or diiodotyrosine (MIT or DIT), catalysed by thyroid peroxidase.
6. MIT and DIT then couple to formT₃and T₄ within the thyroglobulin, catalysed by thyroid peroxidase:
   
   • MIT + DIT → T₃
   • DIT + DIT → T₄
7. Thyroglobulin, with the newly formed thyroid hormones attached to its backbone, is brought back into the thyroid follicular cells via pinocytosis.
8. Thyroglobulin is broken down and the thyroid hormones are released. Any uncoupled iodine residues are recycled.

• *Describe briefly how it is secreted and transported**
• 90% of thyroid hormone secreted is T₄.
• However, T₃ is four times as biologically active, meaning most T₄is converted to T₃ by the liver and kidneys.
• Thyroid hormones are hydrophobic, so both T₃ and T₄ are transported in the blood bound to thyroxine-binding globulin.

Question 6

• *Dietary iodine**
• Which food sources
• How much does the thyroid gland contain of the body’s iodine?

Recap - Which thyroid hormone is usually secreted? What happens to this hormone? Why does this happen? How are T₃ and T₄ transported?

Answer 6

Which food sources:
Fish, dairy, grains, meat, cows milk
How much does the thyroid gland contain of the body’s iodine?
90-95%

• *Recap - Which thyroid hormone is usually secreted? What happens to this hormone? Why does this happen? How are T₃ and T₄ transported?**
• 90% of thyroid hormone secreted is T₄
• Biological activity of T₃ is 4 times that of T₄
• Most of T₄ is converted to T₃ in the liver and kidneys
• T₃ and T₄ are transported in blood bound to the protein thyroxine-binding globulin

Question 7

• *TSH**
• Type of hormone
• Structure of this hormone, with similaries to which hormones?

What is the trigger for thyroid hormone release?

Answer 7

TSH
- Type of hormone
Glycoprotein
- Structure of this hormone, with similaries to which hormones?
Glycoprotein composed of 2 non-covalently bound subunit (alpha and beta). The alpha subunit is also present in FSH and LH. Beta subunit provides unique biological acitivity.

What is the trigger for thyroid hormone release?
pic

Question 8

How does TSH cause the cellular responses seen in the last flashcard (i.e. how does it stimulate the processes, with what mechanism?)

Answer 8

Question 9

HPA of thyroid hormones

Answer 9

Question 10

• *General actions of thyroid hormone**
• 3 main actions
• Tissue specific effects

Answer 10

3 main actions:
pic

Tissue specific effects:
Cardiovascular system:
Increases heart’s responsiveness to catecholamines:
This causes:
- Increased cardiac output (increase in heart rate and increase in force of contraction)
- Increased Peripheral vasodilation to carry extra heat to body surface
Nervous system:
Essential for both development and adult function
- Increased Myelination of nerves & development of neurons.

Question 11

• *Thyroid Hormone Receptors**
• Type of receptor
• How do these function?

Answer 11

• Members of the large family of nuclear receptors
• Function as hormone-activated transcription factors
• Act by modulating gene expression
• Thyroid hormone receptors bind DNA in the absence of hormone, usually leading to transcriptional repression.
• Hormone binding is associated with a conformational change in the receptor that causes it to function as a transcriptional activator

Question 12

Normal plasma levels of thyroid hormones (don’t need to learn values)

Answer 12