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R - Endocrine Physiology (Megan) > Lecture 5 - Thyroid gland and hormones > Flashcards

Lecture 5 - Thyroid gland and hormones Flashcards

1
Q

How much does the thyroid gland weigh?

A

15-20g.

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2
Q

When is the thyroid palpable?

A

During dysfunction.

When the thyroid is underactive and hyperactive.

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3
Q

What hormones are made by the thyroid gland?

A

T4 - tetraiodothyronine - thyroxine.

T3 - triiodothronine

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4
Q

Which vertebral level is the thyroid in line with?

A

C5-T1.

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5
Q

Which cell types are in the thyroid gland?

A
  • C (clear cells) - secrete calcitonin.

- Follicular cells - support thyroid hormone synthesis.

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6
Q

What do follicular cells surround?

A

Colloid - a sticky glycoprotein matrix.

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7
Q

How much thyroid hormones are kept inside follicular cells?

A

2-3 months worth.

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8
Q

What are follicular cells?

A

simple epithelium - can be columnar, cuboidal or squamous depending on how active the cell is and how much colloid it is surrounding.

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9
Q

What are follicular cells?

A
  • spherical cells that support thyroid hormone synthesis.
  • they manufacture the enzymes that make thyroid hormone
  • they also make thyroglobulin (rich in tyrosine residues)
  • they concentrate iodide
  • All of these go into the colloid
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10
Q

Where does the body get iodide and tyrosine?

A

From the diet.

If we are deficient in either one - we become deficient in thyroid hormone.

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11
Q

What combines to create the precursor thyroid hormones?

A

Tyrosine residues (from thyroglobulin) and iodide. Inside the colloid.

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12
Q

How does iodide enter the cell against its concentration gradient?

A

Na/I symport transporter.

Symport (both ions move in the same direction.

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13
Q

What is the name of the enzymes created in the follicular cells called?

A

Thyroid peroxidase/ thyroperoxidase.

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14
Q

what are the prehormones created in the colloid?

A

Monoiodotyrosine.

Diiodotyrosine.

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15
Q

What are the active hormones called created in the colloid?

A

Triiodothyronine - T3.

Tetraiodothyronine - T4 - thyroxine.

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16
Q

How are the active hormones created from the prehormones?

A

By conjugation.

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17
Q

How are the prehormones created?

A

By addition of iodine to the tyrosine residues.

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18
Q

What type of hormones are thyroid hormones?

A

Amine hormones. But they behave like steroids hormones.

Lipophilic and hydrophobic.

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19
Q

Where are thyroid hormone receptors located?

A

INSIDE the cell - due to being lipophilic it easily crosses the cell membrane.

20
Q

How long is the half life of T3?

A

1 day.

21
Q

How long is the half life of T4?

A

6 days.

22
Q

What binding protein does T3 and T4 bind to?

A

Thyroxine-binding globulin.

23
Q

What thyroid hormone does thyroxine-binding hormone have a higher affinity for?

A

T4 - this gives it a longer half life. This means there is more T4 in the plasma present than T3.

24
Q

What hormone triggers the release of thyroid hormones into the plasma by follicular cells?

A

TSH - thyroid stimulating hormone from the anterior pituitary.

25
Q

What triggers the release of TSH by the anterior pituitary?

A

Thyotropin releasing hormone from the hypothalamus.

26
Q

How is T3 and T4 released to create their physiologically active form?

A

Proteolytic enzymes chop off the thyroglobulin once they are exocytosed from the follicular cells into the plasma.

27
Q

Where are the thyroid hormones stored until needed?

A

In the colloid in their inactive form (bound to thyroglobulin).

28
Q

How much circulating T3 and T4 is bound to plasma proteins?

A

99.8%.

29
Q

How much circulating T3 and T4 is unbound and free in the plasma?

A

0.2%.

30
Q

What thyroid hormone is in highest concentration in the plasma?

A

T4 - because it has the highest affinity to plasma proteins.

31
Q

How much more T4 is in the plasma than T3?

A

50x more (free and bound).

32
Q

what is the thyroid hormone that binds to intracellular receptors most?

A

T3 - 90%

TH receptor has a higher affinity to T3 making it more physiologically active.

33
Q

Which thyroid hormone is more physiologically active?

A

T3 - TH receptors have a higher affinity for it.

34
Q

How is T4 converted into T3?

A

It it deionised by deionising enzymes in the plasma and in the cells.
T3 is the main thyroid hormone that binds to TH receptors inside of the cell but most of that has been derived from T4.

35
Q

Where are deionising enzymes found?

A

In the plasma and inside cells which have TH receptors.

36
Q

What thyroid hormone is given to hypothyroid patients?

A

T4 because it has a longer half life.

Not T3 because the body can deionise T4 to create T3.

37
Q

What is stimulatory on the hypothalamus releasing TRH?

A

Cold
exercise
Pregnancy

38
Q

What is inhibitory on release of TSH from the anterior pituitary?

A

Glucocorticoids (also inhibits the conversion of T4 to T3)

somatostatin (GHIH)

39
Q

Where does TH bind to its receptors?

A

Inside the cell. They migrate to the nucleus and alter protein synthesis.

40
Q

what is the function of thyroid hormone?

A
  • Raises metabolic rate (ALL metabolic reactions - anabolic and catabolic - no change in ATP).
  • ^this PRODUCES HEAT.
  • increases hepatic gluconeogenesis (no effect on BG if pancreases secretes enough insulin)
  • net increase in proteolysis
  • net increase in lipolysis
  • permissive effect on GH
  • vital for brain development (in utero and out)
41
Q

What is a common condition of hyperthyroidism?

A

Graves disease

42
Q

What controls the negative feedback mechanism of the anterior pituitary and hypothalamus?

A

The increase in thyroid hormones.

NOT the physiological effect they have in the body.

43
Q

What are symptoms of hyperthyroidism?

A
  • HEAT INTOLERANCE
  • Enlarged thyroid gland (2-3x)
  • increase in metabolic rate and heat production (Weight loss and feeling too hot)
  • increased protein catabolism (muscle weakness and weight loss)
  • altered nervous system function (hyper excitable reflexes and psychological disturbances)
  • elevated Cardio function (Increased HR, high output, high contractile force - can lead to cardiac failure)
44
Q

What are symptoms or hypothyroidism?

A
  • decreased metabolic rate and heat production (weight gain and feeling cold)
  • disrupted protein synthesis (brittle nails and thin skin)
  • altered nervous system (slow speech, fatigue, slow reflexes)
  • reduced cardio function (slow HR and weak pulse).
45
Q

How much iodine does our body need?

A

50mg per year.

46
Q

What is enlargement of the thyroid gland known as?

A

Goitre

R - Endocrine Physiology (Megan) (10 decks)

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