Physiology 5 - Thyroid Flashcards

1
Q

List the cell types of the thyroid?

A
  • C (clear) cells (Parafollicular cells)

- Follicular Cells

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What do C cells do?

A

Release calcitonin

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Describe the structure of follicular cells?

A

Cuboidal cells that make the wall of thyroid follicles, containing colloid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What do Follicular cells do?

A

They produce Thyroglobulin (precursor to thyroid hormones) and the enzymes that make them.

They also concentrate iodide into the colloid for use in making thyroid hormones

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

How do follicular cells concentrate iodide in the colloid?

A

Recieve it from the plasma through a Na+/I- symporter using Na+ conc. gradient.

Pump it through to colloid through Pendrin Transporters

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is thyroglobulin?

A

Glycoprotein containing many tyrosine residues used to make thyroid hormones

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What enzyme catalyses thyroid hormone synthesis?

A

Thyroid Peroxidase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What thyroid hormones are made in the colloid?

A

T3 - Triiodothyronine
T4 - Thyroxine

Both are amine hormones made from tyrosine, hence they’re lipophilic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Describe the process of Thyroid hormone synthesis in the colloid?

A

1) iodide oxidised to iodine
2) Iodine added to Tyrosine Residues in thyroglobulin
(1 iodine = MIT, 2 iodine = DIT)
3) MIT + DIT = T3 or DIT + DIT = T4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What do MIT and DIT stand for?

A

Monoiodotyrosine
Diiodotyrosine

Tyrosine-iodine complexes which can be combined to form thyroid hormones

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What happens to transport the thyroid hormones out the colloid and cleave them into active forms?

A

Thyroid Stimulating Hormone triggers:

1) Endocytosis of colloid
2) Proteolytic enzymes then cut thyroglobulin to release THs
3) T3/4 diffuse through to plasma
4) most bind to plasma proteins

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What plasma proteins bind T3/4?

A

Mainly Thyroxine-binding Globulin (TBG)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Summary of TH production:

A

Iodine combined with tyrosine residues by Tyrosine Peroxidase in the Colloid.

TSH triggers cleaving and release of THs into plasma

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Whats the two main differences between T3 & T4?

A

T4 has a greater affinity for TBG, hence it binds better so has a longer half life (6 vs 1 day).

T3 has a greater affinity for Thyroid Hormone Receptors so is more physiologically active

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

In what form is most plasma thyroid hormone?

A

Protein bound T4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What happens to T4 prior to action on Thyroid hormone receptors?

A

Its deiodinated to T3 by deiodinase enzymes.
Either in plasma or target cells.

The degree of deiodination can change based on need for Active THs

17
Q

What factors regulate Thyroid Hormones?

A
  • Free Hormone (-ve feedback)
  • Cold/Exercise/Pregnancy
  • Glucocorticoids
  • Somatostatin
18
Q

How do Free Thyroid Hormones regulate themselves?

A

By -ve feedback

Free T3 or T4 inhibits TSH release from adenophysis and TRH from hypothalamus

19
Q

How does cold, exercise and pregnancy affect thyroid hormones?

A

They all stimulate TRH release from the hypothalamus

20
Q

How do glucocorticoids affect thyroid hormones?

A

Inhibit TSH release & T4 deiodination

21
Q

How does somatostatin affect thyroid hormones?

A

Inhibits TSH release

22
Q

Where are T3 receptors found?

A

Nuclei

23
Q

What does T3 do?

A
  • Raises metabolic rate
  • Promotes Thermogenesis
  • Promotes gluconeogenesis
  • Promotes Proteolysis
  • Promotes Lipolysis
  • Stimulates GH receptor expression (permissive)
  • Foetal Brain development
  • Sensitizes Beta receptors to epinephrine (Permissive)