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Endocrine System > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

Describe the anatomy of the thyroid gland

A

Shaped like a butterfly, lies across trachea at base of larynx, one of larger endocrine glands, (15-20g), but neither visible nor palpable in health.

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2
Q

What is the function of the thyroid gland?

A

Synthesises the thyroid hormones of which there are two physiologically active forms: T3 (triiodothyronine) and T4 (thyroxine).

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3
Q

What are the two cell types found in the thyroid gland?

A
  • C (clear) cells

* Follicular cells

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4
Q

What is the function of C (clear) cells?

A

Secrete calcitonin (Ca regulating hormone)

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5
Q

What is the function of follicular cells?

A
  • Support TH synthesis by making the enzymes that make TH.
  • Forms thyroglobulin, a large protein rich in tyrosin residues

Both are packaged into vesicles and transported to the colloid

• Concentrate iodide from the plasma and trnasport it into the colloid where it combines with tyrosine residues to form TH

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6
Q

Describe the structure of the thyroid gland

A

Contains follicles which are filled with colloid in the centre and are each surrounded by follicular cells.

In between each follicle are C cells.

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7
Q

What are thyroid follicles?

A

Spherical structures whose walls are made of follicular cells. Centre of follicle filled with colloid (sticky glycoprotein matrix). Contain 2-3 mths supply of TH.

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8
Q

How does the body receive tyrosine and iodide?

A

Derived from diet

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9
Q

How does iodide travel from the plasma into the follicular cells?

A

Via a Na+/I- transporter (symport). The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient.

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10
Q

What type of hormone is thyroid hormone?

A

Amine hormone and so is derived from tyrosine

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11
Q

How does iodide travel from the follicular cells into the colloid?

A

Via the pendrin transporter

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12
Q

What happens to iodide when it enters the colloid?

A

It is oxidised to form iodine (loses electron) which will be added to tyrosine to form precursor molecule which will bind to form T3 or T4

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13
Q

What is the name of the enzyme that catalyses he addition of iodine to tyrosine?

A

Thyroid peroxidase (aka thyroperoxidase)

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14
Q

After the addition of iodine to tyrosine, what two molecules are formed?

A
  • Addition of one iodine to tyrosine –> MIT (monoiodotyrosine).
  • Adding a second iodine –> DIT (diiodotyrosine)
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15
Q

What are the two molecules which bind to form T3 or T4?

A
  • MIT (monoiodotyrosine)

* DIT (diiodotyrosine)

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16
Q

How is triiodothyronine (T3) formed?

A

MIT + DIT

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17
Q

How is tetraiodothyronin or thyroxine (T4) formed?

A

DIT + DIT

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18
Q

How many tyrosine and iodine molecules does T3 contain?

A

2 tyrosine and 3 iodine

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19
Q

How many tyrosine and iodine molecules does T4 contain?

A

2 tyrosine and 4 iodine

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20
Q

Where is thyroid peroxidase found?

A

Apical membrane of follicular cells

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21
Q

What happens to TH once it has been formed?

A

Stored in the colloid as thyroglobulin for ~2mnths until TSH activated are released

22
Q

What is the effect of TSH on TH?

A

Portions of the colloid are taken back up into the follicular cell by endocytosis.
Within the cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release thyroid hormones.

23
Q

How does TH travel from the follicular cells into the plasma?

A
  • Lipid soluble and so pass across the follicular cell membrane
  • MCT transporter also involved as mutation in the protein cause major disruption to TH balance
24
Q

What happens to TH when it enters the plasma?

A

99.8% bind to plasma proteins, mainly thyroxine-binding globulin.

25
Q

What releases TSH?

A

Anterior pituitary

26
Q

Why does T4 have a longer have a longer half life than T3?

A

As Thyroxine Binding Globulin (TBG) has particularly high affinity for T4 releasing it only slowly into the plasma.

27
Q

What form of the TH in the plasma can affect TSH and TRH?

A

Only free hormone exerts an inhibitor effect on TSH and TRH.

28
Q

How much T4 and T3 exists in plasma?

A

T4 - 100nmoles/l

T3 - 2.3nmoles/l

29
Q

Why is T3 more physiologically active than T4?

A

As the TH receptor on cells has a much higher affinity for T3 than T4 (x3-5 more active)

30
Q

What does T4 do if TH receptors have a higher affinity for T3?

A

T4 is deiodinated to T3 by deiodinase enzymes. Around half the T4 is deiodinated in plasma, the remaining fraction being deiodinated inside target cells.

The level of deiodinase activity can be altered at different times in different tissues to suit demand.

31
Q

How does deionination control TH?

A

Increase in deionination -> increase in T4 being converted into T3 -> Increase in TH activity

32
Q

What is responsible for a continuous secretion and stable [plasma] TH?

A

Thyrotropin-releasing hormone (TRH) from hypothalamus

33
Q

What three factors can increase the release of TRH from the hypothalamus?

A

Cold, exercise and pregnancy

34
Q

What two hormones can inhibit the release of TSH?

A

Glucocorticoids and somatostatin

35
Q

What are the effects of a rise in [TH]plasma?

A
  • Neg. feedback on hypothalamus to release TRH

* Neg. feedback on the anterior pituitary to release TSH

36
Q

What are the effects of glucocorticoids?

A

Inhibits TSH and conversion of T4 to T3

37
Q

What is the function of TH?

A

Binds to nuclear receptors in target cells, where they change transcription and translation to alter protein synthesis.

38
Q

What is the effect of TH?

A

Raises metabolic rate and promotes thermogenesis, typically through promoting cycles of simultaneous catabolism and anabolism.

39
Q

What are the actions of TH?

A

Increases hepatic gluconeogenesis, although no effect on BG providing pancreas is releasing adequate insulin (therefore not a counter regulatory hormone)

  • Net increase in proteolysis
  • Net increase in lipolysis
  • Critical for growth (lack of TH results in retarded growth) - stimulates GH receptor expression
40
Q

What is TH essential for in term of development?

A

Brain deveopment in utero

41
Q

What does maternal iodine deficiency cause?

A

Congenital hypothyroidism / cretenism

42
Q

Name two conditions which cause hyperthyroidism

A
  • Graves disease (common)

* Thyroid adenoma (rare) - hormone-secreting thyroid tumour

43
Q

How does graves disease cause hyperthyroidism?

A

Antibodies produced that bind mimic TSH and continually activate the thyroid gland.

Increased release of TH switches off TSH release from anterior pituitary so [TSH]plasma very low.

Thyroid gland may be 2-3x normal size due to hyperplasia. Hyperactivity of cells also apparent.

44
Q

What are the 4 symptoms of hyperthyroidism?

A
  1. Increased metabolic rate and heat production
    • -> weight loss/ heat intolerance
  2. Increased protein catabolism
    - -> muscle weakness/weight loss
  3. Altered nervous system function
    - -> hyperexcitable reflexes and psychological disturbances
  4. Elevated CVS function. TH is permissive to epinephrine, Beta receptors
    • -> increased HR/contractile force, high output, cardiac failure
45
Q

Name three causes of hypothyroidism

A
  • Hashimoto’s Disease - autoimmune attack of thyroid gland
  • Deficiency in dietary iodine
  • Iodine - may be linked to thyroiditis
46
Q

What are sources of dietary iodine?

A

Milk, Fish, seafood and seaweed .

47
Q

What are the four symptoms of hypothyroidism?

A
  1. Decreased metabolic rate and heat production
    • -> weight gain/cold intolerance
  2. Disrupted protein synthesis
    • -> brittle nails/thin skin
  3. Altered nervous system function
    • -> slow speech/reflexes, fatigue
  4. Reduced CVS function
    • -> slow heart rate/weaker pulse
48
Q

What is goitre?

A

Significant enlargement of the thyroid gland associated with hypo and hyperthryoidism

49
Q

What causes goitre to occur?

A

By increased trophic action of TSH on thyroid follicular cells (hypothyroidism) or over-activity as a result of autoimmune disease (Graves Disease)

Results in hypertrophy of thyroid gland

50
Q

What type of disorders are hypo and hyperthyroidism?

A

First degree primary disorders of endocrine hormone release

Endocrine System (29 decks)

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