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Endocrine Physiology > The Thyroid Gland > Flashcards

The Thyroid Gland Flashcards

(60 cards)

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1
Q

Which two hormones does the thyroid gland synthesise?

A

T3 - triiodothyronine

T4 - Thyroxine

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2
Q

What are the two cell types in the thyroid gland?

A

C (clear) cells

Follicular cells

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3
Q

What do C (clear) cells secrete?

A

Calcitonin

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4
Q

What do follicular cells do?

A

Support thyroid hormone synthesis
Surround hollow follicles
Actively concentrate iodide from the plasma and transport it into the colloid

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5
Q

What are the walls of thyroid follicles made from?

A

Follicular cells

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6
Q

What is the centre of the thyroid follicle filled with?

A

Colloid

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7
Q

What is colloid?

A

A sticky glycoprotein matrix

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8
Q

How much supply of thyroid hormone (TH) is in a thyroid follicle?

A

2 - 3 months

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9
Q

What do follicular enzymes make?

A

Enzymes that make thyroid hormone

Thyroglobulin

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10
Q

What is thyroglobulin rich in?

A

Tyrosine residues

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11
Q

Enzymes and thyroglobulin are packaged into vesicles and exported from the follicular cells into where?

A

The colloid

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12
Q

What happens to iodide once it is transported into the colloid?

A

Combines with tyrosine resides to form thyroid hormones

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13
Q

Where are both tyrosine and iodide derived from?

A

The diet

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14
Q

What is thyroid peroxidase?

A

Enzyme present on the colloidal side of the cells

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15
Q

Function of thyroid peroxidase

A

Catalyses the addition of iodide to tyrosine residues is thyroglobulin
In this process iodide loses an electron to become iodine

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16
Q

Addition of one iodine to tyrosine leads to….

A

MIT (monoiodotyrosine)

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17
Q

Addition of a second iodine to MIT leads to……

A

DIT (diiodotyrosine)

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18
Q

MIT + DIT = ?

A

Triiodothyronine or T3

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19
Q

DIT + DIT = ?

A

Thyroxine or T4

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20
Q

How are thyroid hormones released in response to TSH?

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 the thyroid hormones

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21
Q

Solubility of both T3 and T4

A

Lipid soluble

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22
Q

Binding of both T3 and T4

A

They are lipid soluble and so pass across the follicular cell membrane into the plasma, where they bind to plasma proteins, mainly thyroxine-binding globulin

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23
Q

Which of T3 and T4 circulate in the plasma?

A

Both

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24
Q

What molecules is thyroxine made up of?

A

2 tyrosine

4 iodine

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25
What molecules is triiodothyronine made up of?
2 tyrosine | 3 iodine
26
Where is thyroxine peroxidase located?
On the apical membrane of the follicular cells
27
How does iodide enter the follicular cells from the plasma?
Via a Na/I- transporter (symport). | The coupling to Na+ enables the follicular cells to take up iodide against a concentration gradient
28
How is iodide transported into the colloid?
Via the pendrin transporter
29
What is iodide transport into the thyroid gland inhibited by?
Thiocyanates
30
Common origin of thiocyanates
Cigarette smoke
31
How does TH release into the plasma?
Under influence of TSH from pituitary | TSH stimulates the follicular cells to endocytose colloidal thyroglobulin
32
How does T3 and T4 circulate in the plasma?
99.8% bound to plasma protein
33
What does thyroxine binding globulin (TBG) have a particularly high affinity for? What does this result in?
T4 Releasing it slowly into the plasma This accounts for the longer half life
34
What is the half life of T4?
Approx. 6 days
35
What is the half life of T3?
Approx. 1 day
36
Which of T3 and T4 is there more of in the plasma?
50x more total (free + bound) T4 in plasma than T3
37
Which of T3 and T4 is more physiologically active?
T3 3-5x more active than T4
38
90% of TH binding to TH receptors inside cell is either T3 or T4?
T3
39
Which of T3 or T4 does the TH receptor have a much higher affinity for?
T3
40
How is T4 turned into T3?
Deiodinated to T3 by deiodinase enzymes
41
Where is T4 deiodinated into T3?
Half in plasma | Remaining being inside target cells
42
Effects on TRH from hypothalamus
Cold Exercise Pregnancy
43
What hormones have an inhibitory effect on TRH?
Glucocorticoids | Somatostatin
44
What do glucocorticoids inhibit?
TSH | Conversion of T4 to T3
45
What does somatostatin inhibit?
TSH
46
Effects of thyroid hormones
``` Raises metabolic rate Promotes thermogenesis Net increase in proteolysis Increase in hepatic gluconeogenesis Net increase in lipolysis Critical for growth - anabolic - stimulates GH receptor expression ```
47
Causes of hyperthyroidism
Graves disease | Thyroid adenoma
48
Pathology of graves disease
Antibodies produced that bind mimic TSH and continually activate the thyroid gland Increase release of TH switches off TSH release from anterior pituitary so [TSH] in the plasma is very low.
49
How common is graves disease?
Common
50
How common is thyroid adenoma?
Rare
51
Symptoms of hyperthyroidism
``` Weight loss Heat intolerance Muscle weakness Hyperexcitable reflexes Psychological disturbances Increased HR/Contractable force Cardiac failure ```
52
Causes of hypothyroidism
Hashimotos disease Deficiency in dietary iodine Idiopathic
53
What is hashimotos disease?
Autoimmune attack of thyroid gland
54
How much dietary iodine do we need a year?
50mg/year
55
Sources of iodine
Mild Fish Seafood Seaweed
56
Symptoms of hypothyroidism
``` Weight gain Cold intolerance Brittle nails Thin skin Slow speech/reflexes Fatigue Slow HR Weak pulse ```
57
What are thyroid pathologies often accompanied by?
Significant enlargement of thyroid gland = goitre
58
What is goitre?
Significant enlargement of thyroid gland
59
Pathology of goitre in hypothyroidism
Increased trophic action of TSH on thyroid follicular cells
60
Pathology of goitre in graves disease
Over activity as a result of autoimmune disease

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