Thyroid gland

Question 1

Where is the thyroid gland found?

Answer 1

Neck - front lower larynx and upper trachea

Question 2

What is the isthmus?

Answer 2

Part of the thyroid gland that connects the two lobes

• Narrower than each lobe which contributes to the bow tie shape
• Extends from the 2nd to 3rd rings of trachea

Question 3

What are the 2 nerves near thyroid gland and what are they involved in?

Answer 3

• Recurrent laryngeal
• External branch of the superior laryngeal

-Speech

Question 4

Describe the embryological development of the thyroid gland

Answer 4

-First endocrine gland to develop

1) Epithelial proliferation in floor of pharynx at the base of tongue
2) Descends as diverticulum through thyroglossal duct and migrates down
3) thyroglossal duct degenerates and detaches, further migrating to final position

Question 5

What are the two major cell types in the thyroid gland?

Answer 5

• Follicular cells

- Parafollicular (C-cells)

Question 6

How are follicular cells arranged?

Answer 6

functional unit : follicle

-spherical lined with epithelial cells surrounding a central lumen containing colloid

Question 7

What does colloid mostly consist of?

Answer 7

Thyroglobulin

Question 8

What is the importance of thyroid peroxidase?

Answer 8

Membrane bound enzyme that regulates 3 separate reactions for thyroid hormone synthesis

Question 9

What are the 3 reactions regulated by thyroid peroxidase?

Answer 9

1) Oxidation = Iodine to iodide
2) Addition of Iodine = to tyrosine residues
3) Coupling = MIT or DIT to generate thyroid hormone

Question 10

What does MIT stand for?

Answer 10

Monoiodotyrosine

-1 iodide on tyrosine residue

Question 11

What does DIT stand for?

Answer 11

Diiodotyrosine

-2 iodide on tyrosine residue

Question 12

What is the role of thyroglubilin during thyroid hormone synthesis?

Answer 12

Acts as a scaffold for tyrosine residues

Question 13

What are some sources of dietary iodine?

Answer 13

• Dairy products
• Grains
• Vegetables
• Meat
• Eggs
• Iodized salt

Question 14

Describe how dietary iodine is taken up by the body

Answer 14

1) Reduced to iodide
2) Absorbed in small intestine
3) Transported to thyroid epithelial cells through blood
4) Taken up by thyroid epithelial cells via sodium-iodide symporter

Question 15

Describe the secretion of thyroid hormone from colloid store

Answer 15

1) Pinocytosis of thryoglobulin with coupled tyrosine residues(hormone)
2) Lysosymes degrade thyroglobulin
3) Exocytosis of thyroid hormones to blood

Question 16

Which hormone is secreted more from thyroid glands?

Answer 16

T4 (longer half life)

Question 17

Where is T4 converted to T3 in the body?

Answer 17

Liver or kidneys

Question 18

How is T3 and T4 transported around the body?

Answer 18

Thyroxine-binding globulin

not all is bound to it, there are some free

Question 19

By what mechanism is thyroid hormone secretion regulated and name the key players to this

Answer 19

Negative feedback

• TSH acts on hypothalamus (-ve)
• Thyroid hormone itself acts on anterior and hypothalamus (-ve)

Question 20

What type of receptors are TSH receptors?

Answer 20

GPCR
(Alpha s = adenyly cyclase)
(Alpha q = Phospholipase C)

Question 21

What does TSH stimulate?

Answer 21

• Iodide uptake
• Iodide oxidation
• Thyroglobulin synthesis
• Colloid pinocytosis
• Proteolysis of thyroglobulin
• Secretion of thyroid hormone

-Has trophic effects (increase size = goitre)

Question 22

What are the general effects of thyroid hormone?

Answer 22

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

Question 23

How does thyroid hormone increase basal metabolic rate and heat production?

Answer 23

By increasing no. and size of mt and also stimulates synthesis of enzymes in respiratory chain

Question 24

How does thyroid hormone stimulate metabolic pathways?

Answer 24

(generally catabolic)

• Lipid metabolism
• Carbohydrate metabolism

Question 25

How does thyroid hormone have sympathomimetic effects?

Answer 25

Increases receptor no. for catecholamine target cells

Question 26

What are specific tissue effects of thyroid hormones?

Answer 26

1) Increase heart response of heart to catecholamines (inc CO) 2) Development and function of nervous system (myelination of nerves)

Question 27

What type of receptors are thyroid hormone receptors and how does it operate?

Answer 27

Nuclear receptors | -usually bound to DNA for transcriptional repression when theres no thyroid hormone

Question 28

Give examples of genes activated by thyroid hormone

Answer 28

- PEPCK - Ca2+ ATPase - Cytochrome oxidase - 6-phosphogluconate dehydrogenase

Question 29

What are normal plasma levels for free T4, T3 and TSH?

Answer 29

``` T4 = 10-25 pM T3 = 3-8 pM TSH = 1-15 pM ```

Question 30

What is goitre?

Answer 30

Enlargement of the thyroid gland when its overstimulated

Question 31

What is hypothyroidism and what are the causes?

Answer 31

Low levels of thyroid hormones in the body (T3 and T4) - Failure of thyroid gland - Anti-thyroid drugs - TSH/TRH deficiency - Inadequate supply of dietary iodine - Autoimmune - Congenital - Post-surgery

Question 32

What are the general symptoms of hypothyroidism?

Answer 32

- Obesity - Lethargy - Intolerance to cold - Bradycardia - Alopecia - Dry skin - Slow reflexes

Question 33

What is cretinism?

Answer 33

Condition from hypothyroidism in infants - dwarfed structure - mental deficiency - poor bone development - slow pulse - muscle weakness

Question 34

What is myxedema?

Answer 34

Condition from hypothyroidism in adults - Thick puffy skin - Muscle weakness - Slow speech - Mental deterioration - Intolerance to cold

Question 35

What would you see to levels of thyroid hormone and TSH of hypothyroidism?

Answer 35

Low T4 and T3 | High TSH

Question 36

What is Hashimoto's disease?

Answer 36

Autoimmune destruction of thyroid follicles leading to hypothyroidism -treated by oral thyroid hormone

Question 37

What is hyperthyroidism and what are the causes?

Answer 37

High levels of T4 and T3 in the body - Toxic multinodular goitre - Solitary toxic adenoma - Drugs - Excessive T4 therapy - Ectopic thyroid tissue - Autoimmune

Question 38

What are the general symptoms of hyperthyroidism?

Answer 38

- Increased BMR - Irritability - Heat intolerance - Tachycardia - Sweating - Bulging eyes - Increased appetite - Fatigue

Question 39

What would you see to levels of thyroid hormone and TSH of hyperthyroidism?

Answer 39

High T4 and T3 | Low TSH

Question 40

What is Grave's disease?

Answer 40

Autoimmune disease resulting in hyperthyroidism cause by production of thyroid stimulating immunoglobulin

Question 41

What is thyroid scintigraphy and what chemical do they usually use?

Answer 41

A procedure to visualise the thyroid gland | -Technetium-99m

Question 42

What are antithyroid drugs and give an example

Answer 42

Treatment for hyperthyroidism that blocks thyroid hormone formation -Carbimazole (pro-drug that is converted to methimazole in the body) prevents thyroid peroxidase from coupling and iodinating tyrosine residues

Question 43

What causes TRH release?

Answer 43

- Stress - Decreased temperature (cold) - Low thyroid hormones level in the blood

Question 44

How are thyroid disorders assessed?

Answer 44

- Blood count (Thyroid hormones and TSH levels) - Neck examination (palpate) - Ultrasound - Radio-isotope scan - Plain X-ray - CT scan