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Cell biology > Thyroid gland & control of metabolism (lecture 27) > Flashcards

Thyroid gland & control of metabolism (lecture 27) Flashcards

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1
Q

Function of the thyroid gland

A

Plays an important role in regulating metabolism & body weight

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2
Q

What are the products of the thyroid?

A

T3 - Triiodothyronine
T4 - Tetraiodothyronine (thyroxine)
Calcitonin

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3
Q

What is the major product of the thyroid?

A

T4

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4
Q

What is the most active product of the thyroid?

A

T3

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5
Q

What does calcitonin do?

A

Involved in calcium homeostasis

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6
Q

Structure of the thyroid gland

A

Biggest endocrine tissue/gland in the body

Found in the neck – below adams apple, above sternal notch & infront of the trachea

2 lobes with a connecting isthmus

Rich blood supply

2-4 pairs of parathyroids imbedded in posterior of thyroid

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7
Q

What are the 2 lobes of the thyroid gland connected by?

A

Isthmus

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8
Q

Why does the thyroid gland need a rich blood supply?

A

To carry thyroid hormones away & deliver thyroid with iodine – key ingredient to making thyroid hormones

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9
Q

What do parathyroids do?

A

Produce parathyroid hormones

Important in controlling calcium & phosphate balance

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10
Q

What is the functional unit of the thyroid gland?

A

Follicles - follicular cells

Single layer of cells surrounding a pool of colloid

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11
Q

What happens in the colloid?

A

Production & storage of thyroid hormones

Relatively large store
Grows & shrinks all the time

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12
Q

What are C cells?

A

C cells secrete calcitonin

Generally bigger than colloid

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13
Q

How are thyroid hormones produced?

A

By iodination and coupling of tyrosine

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14
Q

What is thyroglobulin?

A

Tyrosine rich
Produced by follicular cells and moved into the colloid

Glycoprotein synthesised by follicular cells and released into the follicular lumen (colloid) by exocytosis

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15
Q

What happens as the follicular-colloid border?

A

Tyrosine residues within thyroglobulin are iodinated in the presence of the enzyme thyroperoxidase

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16
Q

What are T1 and T2?

A

Precursors
T1 = monoidotyrosine
T2 = diidotyrosine

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17
Q

What are T3 and T4?

A

Active hormones

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18
Q

How are T3 and T4 produced?

A

The coupling of T1 and T2 under the control of thyroperoxidase

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19
Q

When is tyrosine iodinated?

A

When in moves into the colloid

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20
Q

What happens in the iodination of tyrosine?

A

1 iodine added = monoidotyrosine

2 iodines added = diiodotyrosoine

thyroperoxidase enzyme needed

21
Q

What happens in the coupling of precursors?

A

Tetra T4 = composed of 2 di

Tri T3 = composed of a mono and a di

22
Q

Where is thyroglobulin produced from?

A

Follicular cells

23
Q

How is iodine transported into the follicular cells?

A

By sodium coupled iodine transporters

Uses the Na gradient – allows I to be taken out of the blood into the cell

24
Q

What is TPO?

A

Thyroperoxidase

25
Q

What does TPO do?

A

Catalyses iodination and coupling

26
Q

How does thyroglobulin enter the follicular cells?

A

By pinocytosis/endocytosis

27
Q

How happens when thyroglobulin has entered the follicular cell?

A

Fuses with lysosomes which contain proteases that break down thyroglobulin
When broken up it will release T1, T2, T3 & T4

28
Q

What happens to T1 and T2 in the follicular cell?

A

T1 and T2 are not active so no point in being moved into the blood

T1 and T2 in the cells are broken down again and iodine is recycled back into the colloid

29
Q

What happens to T3 and T4 in the follicular cell?

A

T3 & T4 are active so are moved into the blood – act as steroid hormones (lipophilic)

Have to be bound to a protein in the blood to be carried around as they are not soluble

Thyroid binding proteins - Don’t bind all of the T3 & T4

Free T3 and T4 is what binds to receptors and causes an effect

30
Q

Half life of T4

A

Around 6 days

31
Q

Half life of T3

A

Around 10 hours

Tissues can convert T4 to T3 when its needed –T4 acts as a reservoir

32
Q

What are the 3 major thyroid transporting proteins produced by the liver?

A

Thyroxine-binding globulin

Thyroxine-binding prealbumin

Albumin

33
Q

What does the hypothalamus do?

A

Decides how much releasing hormone (TRH) it releases into the anterior pituitary

34
Q

How does the TRH work?

A

If TRH is released it will act on thyrotrophs in the anterior pituitary & stimulate them to produce TSH (thyroid stimulating hormone)

TSH travels in the blood to thyroid gland, act on walls of follicular cells to increase thyroid hormone production

Negative feedback

35
Q

How does TSH effect the thyroid gland?

A 
Increase iodine uptake
Increase thyroglobulin synthesis
Increase iodination of thyroglobulin
Increase pinocytosis of colloid
Increase lysosomal activity
Increase in size of thyroid cells (cuboidal to columnar)
36
Q

Metabolism of thyroid hormones

A

Most plasma T3 is derived from peripheral metabolism of T4 produced by the thyroid.

This can be a “step up” or “step down” process

37
Q

What are the 3 enzymes involved in thyroid hormone metabolism?

A

1,5’-deiodinase
2,5’-deiodinase
3,5’-deiodinase

38
Q

What is 1,5’-deiodinase?

A

Enzyme involved in metabolism of thyroid hormones

Most abundant enzyme

Provides T3 to the plasma

Step up process

39
Q

What is 2,5’-deiodinase?

A

Enzyme involved in metabolism of thyroid hormones

Active in the brain and pituitary

Provides T3 in the CNS

Step up process

40
Q

What is 3,5’-deiodinase?

A

Enzyme involved in metabolism of thyroid hormones

Inactivates T4 by converting it to rT3 (reverse T3) - takes iodine group off at the wrong place

Most T4 is deactivated to rT3

Step down process

41
Q

Where are thyroid hormone receptors found?

A

In the nucleus of the cells
Receptor is bound to the DNA
When hormone isn’t bound, receptor blocks gene transcription

42
Q

What are the 4 main types of thyroid hormone receptors?

A

Alpha 1
Alpha 2
Beta 1
Beta 2

43
Q

Which thyroid hormone receptor doesn’t bind T3?

A

Alpha 2

44
Q

Do thyroid hormone receptors have a high affinity for T3 or T4?

A

T3

45
Q

What is hypothyroidism?

A

Underactive thyroid or low thyroid, is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone

46
Q

Symptoms of hypothyroidism

A
  • General tiredness and lethargy
  • Bradycardia
  • Mental slowness
  • Cold intolerance
  • Weight gain
  • Depression (in about 50% of cases)
  • Dry skin
  • Puffy hands and face
47
Q

What is hyperthyroidism?

A

Overactive thyroid

Occurs when your thyroid gland produces too much of the hormone thyroxine

Accelerates your bodies metabolism

48
Q

Symptoms of hyperthyroidism

A
  • Heat intolerance
  • Weight loss
  • Warm moist skin
  • Tachycardia (atria fibrillation and heart failure)
  • Fine tremor of fingers
49
Q

What is Graves disease?

A

Common cause of thyrotoxicosis (excess of thyroid hormone in the body)
Autoimmune condition in which auto-antibodies stimulate the TSH receptor

  • “Stare” - upper eyelid retraction
  • “Bulging eyes” – swelling of muscles in orbit pushing eyeball out
  • Goitre – enlarged thyroid due to over stimulation

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