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endocrine > Physiology 5 - Thyroid > Flashcards

Physiology 5 - Thyroid Flashcards

1
Q

List the cell types of the thyroid?

A
  • C (clear) cells (Parafollicular cells)

- Follicular Cells

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2
Q

What do C cells do?

A

Release calcitonin

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3
Q

Describe the structure of follicular cells?

A

Cuboidal cells that make the wall of thyroid follicles

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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

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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

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6
Q

What is thyroglobulin?

A

Glycoprotein containing many tyrosine residues used to make thyroid hormones

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7
Q

What enzyme catalyses thyroid hormone synthesis?

A

Thyroid Peroxidase

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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

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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

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10
Q

What do MIT and DIT stand for?

A

Monoiodotyrosine
Diiodotyrosine

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

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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

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12
Q

What plasma proteins bind T3/4?

A

Mainly Thyroxine-binding Globulin (TBG)

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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

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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

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15
Q

In what form is most plasma thyroid hormone?

A

Protein bound T4

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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)
24
Q

How is TH essential for growth?

A
  • Stimulates anaerobic processes
  • Stimulates growth hormone receptors
  • Essential for foetal brain development (though dysfunction could be related to the mothers lack of ion)
25
Q

Which are the main sources of Ion in a diet?

A

-Seaweed, milk, fish.

26
Q

what are the follicles?

A

Spherical cells whose centre is made up of colloid= sticky glycoprotein.
They contain about 2-3 months supply of thyroid hormone
Surrounded by a wall of follicular cells

endocrine (25 decks)

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