The thyroid gland

Question 1

Describe the appearance of the thyroid gland

Answer 1

butterfly - heavy (15-20g) - lies across trachea at base of larynx
not palpable or visible in health

Question 2

What are the 2 physiologically active hormones which the thyroid gland secretes?

Answer 2

T3 and T4(thyroxine)

Question 3

What are the 2 types of cells found in the thyroid gland?

Answer 3

follilcular cells

clear cells

Question 4

What do clear cells secrete?

Answer 4

calcitonin

Question 5

What do follicular cells do?

Answer 5

support thyroid hormone synthesis

surround hollow follicles

Question 6

What is the middle of the follicle filled with?

Answer 6

colloid

Question 7

What does the colloid contain?

Answer 7

sticky glycoprotein matrix containing 2-3 month supply of TH

Question 8

What do follicular cells make?

Answer 8

enzymes

thyroglobulin

Question 9

What do follicular cells do in terms of thyroid hormone (and iodide)

Answer 9

concentrate iodide from the plasma and transport it into the colloid where it combines with tyrosine residues to form thyroid hormone

Question 10

Describe the thyroglobulin molecule?

Answer 10

Large protein residues rich in tyrosine residues

Question 11

Where do you get tyrosine and iodide from?

Answer 11

the diet

Question 12

What differentiates T3 and T4?

Answer 12

T3 has 3 iodide ions from the 2 tyrosine molecules

Question 13

What enzyme catalyses the reaction to form thyroid hormone and where is it found?

Answer 13

thyroid peroxidase

apical membrane of follicular cells

Question 14

How does iodide enter the follicular cells from the plasma?

Answer 14

Na+/I- transporter –> symport

Question 15

What is the advantage of coupling iodide to sodium for transport?

Answer 15

Can uptake against the concentration gradient

Question 16

How is iodide transported from the follicular cells to the colloid?

Answer 16

pendrin transporter

Question 17

What inhibits iodide transport into follicular cells and where is this found in daily life?

Answer 17

thiocyanates

cigarette smoke

Question 18

What does thyroperoxidase catalyse?

Answer 18

iodide to tyrosine residues in thyroglobulin

Question 19

After T3 and T4 are made what happens to release them into the blood?

Answer 19

TSH stimulates endocytosis of parts of the colloid into follicular cells and form vesicles with proteolytic enzymes to cut thyroglobulin to release thyroid hormones

Question 20

What plasma proteins do thyroid hormones bind to?

Answer 20

thyroxine binding hormone

Question 21

Does TBG have a particularly high affinity for T3 or T4?

Answer 21

T4

Question 22

What does TBG do in terms of T4 half life?

Answer 22

make it longer by releasing it slowly into the plasma

Question 23

What does the thyroid hormone need to be to exert an inhibitory effect on TSH and TRH?

Answer 23

free (physiologically active and not bound to plasma proteins)

Question 24

Does most TH circulate as T3 or T4?

Answer 24

T4

Question 25

Do most of TH binding to intracellular receptors T3 or T4?

Answer 25

T3

Question 26

Does TH receptor have higher affinity for T3 or T4?

Answer 26

T3

Question 27

Is T3 or T4 more physiologically active?

Answer 27

T3

Question 28

How is T4 converted to T3?

Answer 28

deiodinated by deiodinase enzymes

Question 29

Where is T4 deiodinated?

Answer 29

half in plasma | half inside target cells

Question 30

Can levels of deiodinase of T4 be changed?

Answer 30

yes in response to suit demand

Question 31

Positive inputs to TRH?

Answer 31

cold, exercise and pregnancy

Question 32

What does glucocorticoids inhibit in terms of thyroid hormones?

Answer 32

TSH and conversion of T4-->T3

Question 33

What does GHIH inhibit in terms of thyroid hormones?

Answer 33

TSH

Question 34

Thyroid hormone functions

Answer 34

increase lipolysis increase proteolysis raise metabolic rate and promote thermogenesis increase hepatic gluconeogenesis Growth - stimulate GH receptor expression and anabolic foetal brain development

Question 35

2 causes of hyperthyroidism

Answer 35

Graves disease - autoimmune | adenoma of thyroid

Question 36

Graves disease

Answer 36

Autoimmune stimulation of thyroid hormone due to antibodies mimic TSH and increase negative feedback and turn off TSH from anterior pituitary so plasma TSH is low large thyroid gland --> hyperplasia hyperactivity

Question 37

Thyroid adenoma

Answer 37

rare | hormone secreting thyroid tumour

Question 38

Link hyperthyroidism symptoms to what has happened

Answer 38

increased metabolic rate and heat production - weight loss and heat intolerance protein catabolism - loss of muscle and weight altered nervous system function - hyperexcitable reflexes and psychological Elevated CV function as TH is permissive to epinephrine so increased HR, palpitations

Question 39

3 causes of hypothyroidism

Answer 39

hashimotos disease iatrogenic iodine deficiency

Question 40

Hashimotos disease

Answer 40

autoimmune destruction of thyroid gland

Question 41

Symptoms of hypothyroidism

Answer 41

weight gain, tiredness, slow heart rate, brittle nails and skin, slow speech and reflexes

Question 42

What is Goitre?

Answer 42

enlarged thyroid gland - hypo and hyperthyroidism

Question 43

Where do primary, secondary and tertiary thyroid disorders occur?

Answer 43

primary - thyroid gland secondary - anterior pituitary tertiary - hypothalamus