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LCRS - Endo and Neuro > Lecture 6 - The thyroid glands and Iodothyronines > Flashcards

Lecture 6 - The thyroid glands and Iodothyronines Flashcards

1
Q

Where is the thyroid located?

A

In the neck or trachea region

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2
Q

What are the key features of the thyroid gland?

A

Its formed of the right and left lobes - each with around 2 parathyroid lobes The middle triangle is composed of the lower part - isthmus and the upper part - pyramid Thyroid cells are arranged in follicles which are doughnut shaped - colloid in the centre ( thick, yellowish, proteinatious and gel-like substance) and follicular cells around it In between follicles are parafollicular cells which have endocrine functions

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3
Q

What is the endocrine function of paraventricular cells?

A

Ca2+ regulation

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4
Q

Describe the mechanism by which iodothyronines are synthesised

A

I- enters the follicular cells via the Na/I symporter and moves into I- into the colloid via a pendrian pump I- converted to I + Tg (thyroglobulin) via THYROID PEROXIDASE in presence of H2O2 This then forms either monoiodotyrosyl/diiodotyrosil MIT & DIT are the synthesised into T3 and T4 via coupling reaction using TPO and H2O2

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5
Q

How are T3 and T4 removed from the colloid?

A

TSH attaches to its receptor Stimulates endocytosis of T3 and T4 (colloid taken up and broken down by enzymes to form them) Then T3&4 filled vesicles are transported into the blood

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6
Q

What is the enzyme that causes iodination of I- into T3 and T4?

A

THYROID PEROXIDASE

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7
Q

Towards which cell membrane does the colloid face?

A

The apical membrane

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8
Q

What is the precursor molecule for iodothyronines?

A

Tyrosine

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9
Q

What are the steps in the synthesis of T3?

A

Tyrosine-> 3-monoiodotyrosine + 3,5-diiodotyrosine ->(coupling process) 3,5,3’-triiodothyronine (T3)

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10
Q

What are the steps in the synthesis of T4?

A

Tyrosine -> 2x 3,5-diiodotyrosine ->(coupling reaction) 3,5,3’,5’-tetraiodothyronine (T4/Thyroxine)

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11
Q

How are T3 and T4 transported in the blood?

A

Mostly bound to plasma proteins

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12
Q

Which plasma proteins is T3 transported with?

A

80% thyroid-binding/thyroxine globulin 15% albumin 2% pre-albumin 0.5% Bioactive component

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13
Q

What plasma proteins is T4 transported with?

A

70% thyroxine/thyroid-binding globulin 10% albumin 15% prealbumin 0.05% bioactive component

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14
Q

What is prealbumin now known as?

A

Transthyretin

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15
Q

Why do they need to be bound to proteins when transported in blood?

A

They are very easily taken up by cells, even if not target cells

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16
Q

Which of the plasma proteins that transport T3&4 is the most abundant and which is the least?

A

Thyroxine binding globulin most abundant Albumin least abundant (in this case)

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17
Q

What is a latent period?

A

Time taken for biological effect to occur after molecule (e.g. T3/4) in blood

18
Q

What is the latent period for T3/4?

A

T3 around 12h T4 around 72h

19
Q

What are the biological half lives of T3/4?

A

T3 around 2 days T4 around 7-9 days NB: longer than any other hormone

20
Q

What is the main hormone produced by thyroid gland?

A

T4

21
Q

What is the main use of T4?

A

It is deiodinated to T3 - acts as a source of T3 as T3 is MORE BIOACTIVE

22
Q

Which is the more bioactive component between T3 and T4?

A

T3

23
Q

What is another process that T4 can undergo?

A

It can be deiodinated in a different position leading to production of biologically INACTIVE molecule rT3

24
Q

What does the thyroid do under starvation?

A

Synthesises more rT3 than T3/4 as the thyroid doesn’t need to be wasting energy, so it is ‘shut down’

25
Q

What is the basal metabolic rate?

A

The level of metabolic activity happening in the body when inactive

26
Q

What are the main actions of iodothyronines?

A

Increase basal metabolic rate (in most peripheral tissues -> calorigenesis) Increase protein, carbs and fat metabolism (important in normal growth and development)

27
Q

What is the relationship between metabolism and weight in a hypERthyroid patient?

A

Weight decreases Increase CATAbolic metabolism

28
Q

What is the relationship between metabolism and weight in a hypOthyroid patient?

A

Weight increases Increase ANAbolic metabolism

29
Q

When do iodothyronines play an essential role and why?

A

During fetal growth and development Lack of IT results in CRETINISM - both mental and physical growth reduced (if untreated during 1st months)

30
Q

How are IT levels tested in newborns?

A

Heel-prick test - measures IT levels in blood

31
Q

What other actions do IT take part in?

A

Potentiate some of the actions of catecholamines (tachycardia, glycogenolysis, lipolysis) Interact with other endocrine systems Effects on CNS Increases Vit A synthesis

32
Q

What is a sign of hypothyroidism?

A

Yellow skin - but NOT yellow sclera - due to carotene build up in blood

33
Q

What is the mechanism of action of IT?

A

T4 and T3 enter the cell T4 -> into T3 (more biologically active) ***T3 and T4(less) increase protein synthesis, attaching to thyronine receptor (nucleus) increasing transcription*** T3 might stimulate membrane transport mechanisms and metabolic activity in mitochondrion

34
Q

Summarise the thyroid production method

A

Hypothalamus -> TRH (Thyrotrophin) Adenohypophysis -> TSH (THYROTROPHIN - Thyroid stimulating hormone) Thyroid follicles -> T3 and T4

35
Q

What are the inhibitory molecules in the negative feedback loop for IT?

A

SOMATOSTATIN Glucocorticoids Increase in T3 and T4 Increase in Thyrotrophin (TSH) Inorganic iodide (Wolff-Chaikoff effect)

36
Q

What are the stimulatory molecules in the negative feedback loop for IT?

A

Oestrogens THYROTROPHIN RELEASING HORMONE

37
Q

What cells in the adenohypophysis produce thyrotrophin?

A

Thyrotroph cells

38
Q

How is iodothyronine production regulated in the negative feedback loop?

A
39
Q

What is thyrostimulin?

A

2 unit glycoprotein

40
Q

Where is thyrostimulin found?

A

Anterior pituitary (inc. heart, adipose, testis, ovary)

41
Q

What does thyrostimulin bind to?

A

Thyrotrophin (TSH) receptor

LCRS - Endo and Neuro (22 decks)

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