The Thyroid gland

Question 1

anatomy of the thyroid

Answer 1

• Lies against and around front larynx and trachea
• Below thyroid cartilage (Adams apple)
• 2 lobes joined by an isthmus
• now tie shaped
• Isthmus extends from 2nd and 3rd rings of the trachea

Question 3

parathryoid and thyroids are

Answer 3

distinct glands

Question 4

embryological development of the thyroid gland

Answer 4

• First endocrine gland to develop
• At 3-4 weeks gestation it appears as an epithelial proliferation in floor of pharynx at base of the tongue and takes several weeks to migrate to final position
• First descends as diverticulum through thyroglossal duct and migrates downwards passing in front of hyoid bone
• During migration remains connected to tongue by thyroglossal duct which subsequently degenerates
• Detaches thyroid then continues to its final position over following 2 weeks

Question 5

follicular cells are arrnaged in spheres called

Answer 5

thyroid follicles

Question 6

thyroid follicles are filled with

Answer 6

colloid- a deposit of thyroglobulin

(extracellular even though inside follicle)

Question 7

The thyroid and parathyroid are distinct glands

Answer 7

Question 8

parafollicular cells (distinct from parathryoid principle cells) produce

Answer 8

calcitonin

Question 9

thryoid hormone synthesis

Answer 9

• T3- triiodothyronine
• T4- tetraiodothyronine (thyroxine)
• 2 tyrosine linked together with iodine at three or four position on the aromatic ring
• Thyroglobulin acts as a scaffold on which thyroid hormones are formed

Question 10

Thyroglobulin acts as

Answer 10

a scaffold on which thyroid hormones are formed

• occuring on tyrosine residues

Question 11

dietary idoine is …… before absorption in the small intestine

Answer 11

reduced

Question 12

Thyroid hormone and precursors are the only molecules in the body that contain

Answer 12

iodine

Question 13

thyroid gland contains ……% of iodine in the body

Answer 13

90-95%

Question 14

how is iodide taken up by thyroid epithelial cells

Answer 14

Iodide (I-), is taken up from blood by thyroid epithelial cells, which have a sodium iodide symporter (iodine trap)

Question 15

sources of iodine

Answer 15

iodized salt, dairy products, grains and meat

Question 16

thyroid peroxidase

Answer 16

Membrane bound enzyme that regulates 3 separate reactions involving iodine

Question 17

name the three reactiosn thyroid peroxidase regulates

Answer 17

• Oxidation of iodide to iodine (requires H2O2)
• Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin
• Coupling of MIT or DIT to generate thyroid hormones within the thyroglobulin protein

Question 18

thyroid hormone synthesis

Answer 18

1. Important amino acids imported from the plasma into the cell for thyroglobulin synthesis
2. Thyroglobulin exported into colloid
3. Iodide (recued dietary iodine) then imported into the cell from the plasma via the iodide trap (sodium iodide symporter)
4. Iodide oxidised to iodine by thyroid peroxidase
5. Iodination of thyroxine residues by thyroid peroxidase
6. Coupling of tyrosine residues together by thyroid peroxidase
7. Colloid taken into cell via pinocytosis to form a vesicle full of thyroglobulin
8. Lysosome fuses with vesicle
9. These enzymes degrades the thyroglobulin protein releasing thyroid hormone
10. Deiodinase enzyme will recycle any unused iodine

Question 19

T3/T4 ratio

Answer 19

90% of thyroid hormone secreted is T4- Most T4 is converted to T3 outside thyroid

Question 20

difference between T3 and T4

Question 21

T3 is ……more biologically active than T4

Answer 21

4x

Question 22

where is most T4 converted to T3

Answer 22

in the liver and kidney

Question 23

80% of ciruclating Thyroid hormone is

Answer 23

T3 derived from T4

Question 24

how are T3 and T4 transported in the blood

Answer 24

thyroxine-binding globulin

Question 25

regulation of thryoid hormone

Answer 25

negative feedback - when thyroid hormoen is high it provides negartive feedback to both the AP (stop producing TSH) and the hypothalamus (stop producing TRH)

Question 26

Thyroid hormone effect virtually every cell in the body and have 2 interconnected responses

Answer 26

1. Effects on cellular differentiation and development 2. Effect on metabolic pathways

Question 27

Tyroid stimulating hormones (TSH)

Answer 27

* Glycoprotein hormone composed of 2 non-covalently bound subunits (alpha and beta) * (Alpha subunit also present in FSH and LH) * B subunit provides unique biological activity

Question 28

effects of TSH

Answer 28

Triggers the release of thyroid hormone

Question 29

TSH stimulates which axtions

Answer 29

Iodide uptake Iodide oxidation Thyroglobulin synthesis Thyroglobulin iodination Colloid pinocytosis into cell Proteolysis of thyroglobulin Cell metabolism and growth

Question 30

general action of thryoid hromone

Answer 30

1) Increase in basal metabolic rate and heat production 2) Stimulation of metabolic pathways 3) Sympathomimetic effects

Question 31

1) Increase in basal metabolic rate and heat production

Answer 31

* In most tissues (except the brain, spleen and testis), thyroid hormones stimulate the metabolic rate by: * Increasing the **number and size of mitochondria** * Stimulating the synthesis of **enzymes in the respiratory chain**

Question 32

2) Stimulation of metabolic pathways

Answer 32

Catabolic pathways are generally stimulated more than anabolic * Lipid metabolism: stimulates lipolysis and B-oxidation of fatty acids * Carbohydrate metabolism: stimulates insulin-dependent entry of glucose into cells and increase gluconeogenesis and glycogenolysis

Question 33

3) Sympathomimetic effects

Answer 33

Increases target cell response to catecholamines by increasing receptor number on target cells

Question 34

Sympathomimetic effects on the cardiovascular system

Answer 34

* increases hearts responsiveness to catecholamines * Increased CO (increase heart rate and force of contraction) * Peripheral vasodilation to carry extra heat to body surface (B2 adrenergic receptors)

Question 35

Sympathomimetic effects on the nervous system

Answer 35

Essential for both development and adult function Increased myelination of nerves and development of neurones

Question 36

thyroid hormone receptor is a

Answer 36

nuclear receptor

Question 37

function of thyroid hormone receptor

Answer 37

* Modulate gene expression * Thyroid hormone receptors bind DNA in the absence of hormone leading to transcriptional repression * Hormone binding associated with a conformational change in the receptor that causes it to function as a transcriptional activator

Question 38

normal plasma level of thyroid hromone

Answer 38

Question 39

goitre

Answer 39

* Enlargement of the thyroid gland * Develops when the thyroid gland is overstimulated * May accompany either hypo- or hyperthyroidism (but not necessarily present in either)

Question 40

signs of hypothyroidism

Answer 40

Low T3 Low T4 Elevated TSH

Question 41

hypothyroidism in infants

Answer 41

cretinism - dward stature - mental deficiency - poor bone dev - slow pulse - muscle weakness - GI disturbance

Question 42

hypothyroidism in adults

Answer 42

myxedema - thick puffy skin - muscl weakness - slow speech - mental deterioration - intolerance to cold

Question 43

autoimmune hypothyroidism

Answer 43

hashimotos

Question 44

hyperthryoidism signs

Answer 44

- high T3 - high T4 - low TSH

Question 45

autoimmune hyperthyroidism

Answer 45

graves disease

Question 46

method used to scan the thyroid

Answer 46

thyroid scintigraphy

Question 47

thyroid scintigraphy uses

Answer 47

Technetium-99m (99mTc) used for isotope scanning of the thyroid with a gamma camera.

Question 48

biological halflife of Technetium-99m

Answer 48

1 day (radiation xpsoure low)