The Thyroid Gland

Question 1

What shape is the thyroid gland?

Answer 1

Butterfly shape

Question 2

What are the thyroid hormones?

Answer 2

T3 - triiodothyronine

T4 - Thyroxine

Question 3

What type of cells are contained in the thyroid gland?

Answer 3

C cells

Follicular cells

Question 4

What are C-cells?

Answer 4

Clear cells - secrete calcitonin

Question 5

What are follicular cells?

Answer 5

Support thyroid hormone synthesis and surround follicles

Question 6

What are follicles?

Answer 6

Spherical structures whose walls are made of follicular cells
Centres filled with colloid

Question 7

What is colloid?

Answer 7

Fluid in the centre of a thyroid follicle

Sticky glycoprotein matrix

Question 8

How much TH is contained within a follicle?

Answer 8

2-3mths worth

Question 9

What is the function of follicular cells?

Answer 9

TH production enzymes
Thyroglobulin production
Concentrate iodide (I-)

Question 10

What is thyroglobulin?

Answer 10

A large protein rich in tyrosine residues

Question 11

How do follicular enzymes and thyroglobulin enter the colloid?

Answer 11

Vesicles

Question 12

How do we get iodide and tyrosine?

Answer 12

Diet

Question 13

How does Iodide enter the cells?

Answer 13

Blood

Na+/I- transporter (symporter)

Question 14

What is the benefit of coupling I- to Na+?

Answer 14

Allows iodide to be taken up against a concentration gradient

Question 15

How is iodide transported into colloid?

Answer 15

Pendrin transporter

Question 16

How is iodide added to tyrosine?

Answer 16

One iodine + tyrosine:
MIT (monoiodotyrosine)
Add a second iodine:
DIT (diiodotyrosine)

Question 17

What is the fate of MIT and DIT?

Answer 17

MIT + DIT = T3

DIT + DIT = T4

Question 18

How is thyroid hormone synthesis catalysed?

Answer 18

Thyroid peroxidase on the apical membrane of follicular cells

Question 19

TSH stimulates what?

Answer 19

Portions of colloid to be taken up into the follicular cells by endocytosis

Question 20

What happens to colloid taken back up into the follicular cells?

Answer 20

Form vesicles with proteolytic enzymes to cut up thyroglobulin into thyroid hormones

Question 21

How is colloid cut into thyroid hormones?

Answer 21

Packaged into vesicles in the follicular cells with enzymes

Question 22

How do T3/4 pass into the blood?

Answer 22

Passive - lipid soluble

Question 23

How do T3/T4 travel in the blood?

Answer 23

Bound to thyroxine-binding globulin

Question 24

What additional proteins are involved in TH movement?

Answer 24

Transporter proteins

Question 25

What aspect of TH control is stimulated by TSH?

Answer 25

Exocytosis of iodinated/conjugated thyroglobulin | Release of TH into the plasma

Question 26

Where does conjugation of thyroglobulin take place?

Answer 26

Colloid

Question 27

TH stimulates follicular cells to do what?

Answer 27

Endocytose colloidal thyroglobulin

Question 28

What is the fate of thyroid hormones in the absence of TSH?

Answer 28

Thyroid hormones are stored in the colloid in the absence of TSH

Question 29

What portion of plasma T3/T4 is bound?

Answer 29

99.8%

Question 30

Thyroxine binding globulin has a particularly high binding affinity for what? What does this mean?

Answer 30

T4 - it is released more slowly, has a longer half life

Question 31

What is the half life of thyroid hormone?

Answer 31

T4 - 6 days | T3 - 1 day

Question 32

Which fraction of the hormone inhibits TSH and TRH?

Answer 32

FREE/unbound TH

Question 33

Most TH circulates in which form?

Answer 33

Protein bound T4 - 100nmoles/L | T3 - 2.3nmoles/L

Question 34

What is the ratio of total (free and bound) TH?

Answer 34

50x more T4 than T3

Question 35

The majority of TH binding to TH receptors inside cells is what?

Answer 35

T3 (90%)

Question 36

Which TH is most physiologically active?

Answer 36

T3 - 3-5x higher than T4

Question 37

What is the fate of T4?

Answer 37

Used by cells | Deiodinated to T3 by deiodinase enzymes

Question 38

What is the role of deiodinase enzymes?

Answer 38

Convert T4 to T3

Question 39

Where does Deiodination take place?

Answer 39

50% in plasma | 50% in cells

Question 40

What environments stimulate TRH release

Answer 40

PREGNANCY Cold Exercise

Question 41

Where is TRH released from?

Answer 41

The hypothalamus

Question 42

Which hormones are inhibitory to TH?

Answer 42

Somatostatin | Glucocorticoids

Question 43

Thyroid hormones cause an increased excretion of what?

Answer 43

Glucocorticoids

Question 44

How do somatostatins effect TH?

Answer 44

Inhibit TSH

Question 45

How do glucocorticoids effect TH?

Answer 45

Inhibit TSH and conversion of T4 to T3

Question 46

What is the function of thyroid hormone?

Answer 46

``` Increased metabolic rate Promotion of thermogenesis Increase hepatic gluconeogenesis Net Increase proteolysis and lipolysis Growth Brain development in utero ```

Question 47

What are futile cycles?

Answer 47

Counteracting cycles of hormones simultaneously causing catabolism and anabolism

Question 48

What is the effect of TH increasing hepatic gluconeogenesis?

Answer 48

No effect on plasma [Glucose] so long as pancreas is supplying adequate insulin

Question 49

What is the effect of TH on proteins?

Answer 49

Simultaneous protein synthesis and proteolysis but NET EFFECT IS PROTEOLYSIS

Question 50

What is cretinism?

Answer 50

Congenital hypothyroidism caused by maternal iodine deficiency

Question 51

Why is TH essential for growth?

Answer 51

TH stimulates GH receptor expression

Question 52

What are the causes of hyperthyroidism?

Answer 52

Graves' disease | Thyroid adenoma

Question 53

What is a thyroid adenoma?

Answer 53

A hormone secreting thyroid tumour

Question 54

What is Graves' disease?

Answer 54

Antibodies produced that mimic TSH and cause the thyroid to be constantly on Plasma[TSH] is very low Often goitre Cell hyperactivity

Question 55

What are the symptoms of hyperthyroidism?

Answer 55

``` Weight loss/heat intolerance Muscle weakness Excitable reflexes Psychological disturbances Increased HR Cardiac failure ```

Question 56

What is the cause of weight loss in hyperthyroidism?

Answer 56

Increased protein catabolism with increased metabolic rate

Question 57

What is the cause of hyper-excitable reflexes in hyperthyroidism?

Answer 57

Altered nervous system function

Question 58

What is the cause of increased CO/HR in hyperthyroidism?

Answer 58

TH is permissive to epinephrine (beta receptors) - increased cardiovascular function

Question 59

What are the causes of hypothyroidism?

Answer 59

IDIOPATHIC Hashimoto's disease Iodine deficiency

Question 60

Idiopathic hypothyroidism may be linked to what?

Answer 60

Thyroiditis

Question 61

What are the dietary sources of iodine?

Answer 61

Milk Seafood Seaweed

Question 62

What is Hashimoto's disease?

Answer 62

Autoimmune attack on thyroid

Question 63

What are the symptoms of hypothyroidism?

Answer 63

``` Weight gain/cold intolerance Brittle nails/hair/skin Slow speech/reflexes Fatigue Slow HR, weak pulse ```

Question 64

How does hypothyroidism cause weight gain/cold intolerance?

Answer 64

Decreased metabolic rate and heat production

Question 65

How does hypothyroidism cause brittle nails/thin skin?

Answer 65

Disrupted protein synthesis

Question 66

How does hypothyroidism cause slow speech/reflexes, fatigue?

Answer 66

Altered nervous system function

Question 67

How does hypothyroidism cause slow HR/Weak pulse?

Answer 67

Reduced cardiovascular function

Question 68

What is a goitre?

Answer 68

Hypertrophy of the thyroid gland

Question 69

What causes a goitre?

Answer 69

Hyper/hypothyroidism

Question 70

How does hypothyroidism cause a goitre?

Answer 70

Increased trophic action of TSH on the thyroid follicular cells

Question 71

How does hyperthyroidism cause a goitre?

Answer 71

Overactivity due to autoimmune disease (Graves' disease)

Question 72

Low levels of TH stimulates what?

Answer 72

Activity in the anterior pituitary

Question 73

What is a primary disorder of TH release?

Answer 73

Diseases of the thyroid directly