Clinical

Question 1

Describe the anatomy of the thyroid gland? Its shape, location and whether it is palpable or visible in life?

Answer 1

It is a butterfly shaped organ which lies across the trachea at the base of the larynx.

It is neither visible nor palpable in health.

Question 2

What two cell types are present in the thyroid gland?

Answer 2

1. C (clear) cells which secrete calcitonin (Ca regulating hormone)
2. Follicular cells which support thyroid hormone synthesis and surround hollow follicles

Question 3

Describe the structure of thyroid follicles?

Answer 3

They are spherical structures whose walls are made of follicular cells.

The centre of the follicle is filled with colloid.

Question 4

What is colloid composed of?

Answer 4

Sticky glycoprotein matrix.

It contains 2-3 months supply of TH.

Question 5

What two things do the follicular cells make?

Answer 5

1. The enzymes which make thyroid hormones

2. Thyroglobulin

Question 6

What happens to the products of the follicular cells?

Answer 6

The enzymes and thyroglobulin are packaged into vesicles and exported from F cells to the colloid.

Question 7

How does iodine get to the colloid and what happens when it does?

Answer 7

Follicular cells actively concentrate iodide from the plasma and transport it into the colloid where it combines with tyrosine residues to form thyroid hormones

Question 8

Where is tyrosine derived?

Answer 8

From the diet.

Question 9

How is iodide obtained by the body?

Answer 9

From the diet.

Question 10

How does iodide enter the follicular cells from the plasma?

Answer 10

Through a NA+/I- transporter (symport).

Question 11

Is the uptake of iodide by follicular cells with/against the concentration gradient?

Answer 11

Against the concentration gradient (via symport transporter).

Question 12

How is iodide transported into the colloid?

Answer 12

Via the pendrin transporter.

Question 13

What happens when iodide becomes iodine?

Answer 13

Loses an electron

Question 14

What is formed when one iodine molecule is added to tyrosine residues on the thyroglobulin molecule?

Answer 14

Monoiodotyrosine (MIT)

Question 15

What is formed when two iodine molecules are added to tyrosine residues on the thyroglobulin molecule?

Answer 15

Diiodotyrosine (DIT)

Question 16

How is triiodothyronine or T3 formed?

Answer 16

Reaction between MIT and DIT.

Question 17

How is tetraiodothyronine or thyroxine (T4) formed?

Answer 17

DIT + DIT

Question 18

What enzyme catalyses thyroid hormone synthesis and where is it located?

Answer 18

Thyroid peroxidase which is located in the apical membrane of the follicular cells.

Question 19

Are T3 and T4 lipid soluble or lipid insoluble?

Answer 19

Lipid soluble (so can pass follicular cell membrane into the plasma)

Question 20

What happens when TSH acts on the colloid?

Answer 20

Portions of the colloid are taken back into the follicular cells by endocytosis.

Within the cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release the thyroid hormones.

Question 21

What is the name of the main plasma protein which thyroid hormones bind to?

Answer 21

Thyroxine binding globulin

Question 22

Thyroxine binding globulin (TBG) has high affinity for T3 or T4?

Answer 22

T4 - so it is released slowly into plasma (longer half-life)

Question 23

Can bound hormones in the plasma exert an inhibitor effect on TSH and TRH?

Answer 23

No, only free hormone can.

Question 24

Which thyroid hormone is there more of (free and bound)

Answer 24

50x more T4 in plasma

Question 25

90% of which thyroid hormone binding to receptors inside cells?

Answer 25

T3 - TH receptor has a much higher affinity for T3 making it 3-5x more physiologically active

Question 26

Where is T4 deiodinated?

Answer 26

50% is deiodinated in plasma while the remaining is deiodinated in the target cells

Question 27

What are the effects of cold, exercise and pregnancy on TRH?

Answer 27

They increase the release of tropic hormones which act of TRH, which increases TSH release and thus increasing T3/T4 release.

Question 28

How does somatostatin inhibit the regulation of thyroid hormone release?

Answer 28

It inhibits TSH

Question 29

How do glucocorticoids inhibit the regulation of thyroid hormone release?

Answer 29

They inhibit TSH and the conversion of T4 to T3

Question 30

How do TH influence metabolic rate and thermogenesis?

Answer 30

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

Question 31

What effect do TH have on gluconeogenesis and blood glucose?

Answer 31

Increased hepatic gluconeogenesis but no effect on BG providing the pancreas is releasing adequate insulin

Question 32

Why are TH critical for growth?

Answer 32

Lack of TH result in retarded growth (it stimulates GH receptor expression)

Question 33

Why is TH essential for brain development in utero?

Answer 33

Maternal iodine deficiency is found to cause congenital hypothyroidism/cretinism. Although physical growth can be corrected after birth, mental growth and development can never be fully recovered.

Question 34

What are the main causes of hyperthyroidism?

Answer 34

Graves Disease (most common) Thyroid adenoma (rare) = hormone-secreting thyroid tumour

Question 35

What is Grave's disease?

Answer 35

Antibodies are produced that mimic TSH and continually activate the thyroid gland - increased release of TH switches off TSH release from the anterior pituitary so plasma [TSH] is very low -- may have enlarged thyroid gland.

Question 36

What are the symptoms of hyperthyroidism

Answer 36

1. Increased metabolic rate and heat production 2. Increased protein catabolism 3. Altered nervous system function 4. Elevated CVS function (TH permissive to epinephrine)

Question 37

What are the main causes of hypothyroidism?

Answer 37

1. Hashimoto's Disease (autoimmune attack on thyroid gland) 2. Deficiency in dietary iodine (only 50mg/year) 3. Idiopathic

Question 38

What are the symptoms of hypothyroidism?

Answer 38

1. Decreased metabolic rate and heat production 2. Disrupted protein synthesis 3. Altered nervous system function 4. Reduced CVS function

Question 39

What is a Goitre and how is it caused?

Answer 39

Enlargement of the thyroid gland and is caused by increased trophic action of TSH on thyroid follicular cells (hypo) or over-activity due to Grave's.

Question 40

Are hypothyroid and hyperthyroid primary or secondary endocrine disorders?

Answer 40

Both are primary (direct effect on endocrine gland producing end hormone)