21. The Thryoid Gland

Question 1

Where is the thyroid gland?

Answer 1

Lies against and around front larynx and trachea

Below thyroid cartilage (Adams apple)

Question 2

What are the 2 lobes of the thyroid joined by?

Answer 2

Isthmus

Question 3

How does the thyroid gland develop?

Answer 3

First endocrine gland to develop
Appears as an epithelial proliferation in floor of pharynx at base of tongue
First descends as divertculum through thryoglossal duct

Question 4

What is the histology of thyroid tissue?

Answer 4

Follicular cells arranged in spheres called thyroid follicles
Follicles filled with colloid, a deposit of thyroglobulin
Colloid is extracellular even though it is inside the follicle

Question 5

What is the general structure of thyroid hormones?

Answer 5

2 tyrosine linked together with iodine at 3 or 4 positions on the aromatic rings
MIT + DIT = T3
DIT + DIT = T4 (thyroxine)

Question 6

What is the role of thyroglobulin?

Answer 6

Acts as a scaffold on which thyroid hormones are formed

Question 7

What is thyroid peroxidase?

Answer 7

Membrane bound enzyme that regulates 3 separate reactions involving iodide

Question 8

What are the reactions that thyroid peroxidase is involved in?

Answer 8

1. Oxidation of iodide to iodine
2. Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin
3. Coupling of MIT or DIT to generate thyroid hormones within the thyroglobulin protein

Question 9

What has to happen to dietary iodine in the body?

Answer 9

Iodine reduced to iodide before absorption principally in the small intestine

Question 10

What are the only molecules in the body that contain iodine?

Answer 10

Thyroid hormones and precursors

Question 11

What are the dietary sources of iodine?

Answer 11

Iodised salt
Grains
Dairy products

Question 12

Where is most T4 converted to T3?

Answer 12

Liver and kidneys

Question 13

How are T3 and T4 transported in the blood?

Answer 13

Bound to the protein thyroxine-binding globulin

Question 14

How is thyroid hormone regulated?

Answer 14

Via negative feedback

Question 15

Describe the structure of the thyroid stimulating hormone

Answer 15

Glycoprotein hormone composed to 2 non-covalently bound subunits (alpha and beta)
The alpha subunit is also present in FSH and LH
Beta subunit provides unique biological activity

Question 16

What does TSH stimulate?

Answer 16

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

Question 17

What happens when a TSH receptor couples to G alpha s subunit?

Answer 17

Adenylyl cyclase activated
Activates second messenger cAMP
Then activates protein kinase A
Leads to stimulation of thyroid hormone synthesis and release

Question 18

What happens when a TSH receptor couples to a G alpha q subunit?

Answer 18

Activates phospholipase C
Activates second messengers DAG and IP3
Causes Ca2+ release via IP3 receptor and PKC activation
Leads to stimulation of thyroid hormone synthesis and release

Question 19

What are the 3 general action of the thyroid hormone?

Answer 19

Increase in basal metabolic rate and heat production
Stimulation of metabolic pathways
Sympathomimetics effects

Question 20

How doe thyroid hormones stimulate the metabolic rate in most tissues?

Answer 20

Increasing number and size of mitochondria

Stimulating the synthesis of enzymes in the respiratory chain

Question 21

Which metabolic pathways are stimulated by thyroid hormones?

Answer 21

Lipid metabolism: stimulates lipolysis and betaoxidation of fatty acids
Carbohydrate metabolism: stimulate insulin dependent entry of glucose into cells and increase gluconeogenesis and glycogenolysis

Question 22

What are the sympathetic effects of the thyroid hormone?

Answer 22

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

Question 23

What are the effects of the thyroid hormone on the cardiovascular system?

Answer 23

Increases hearts responsiveness to catecholamines
Increased cardiac output
Increased peripheral vasodilation to carry extra heat to body surface

Question 24

What are the effects of the thyroid hormone on the nervous system?

Answer 24

Essential for both development and adult function

Increased myelination of nerves and development of neurons

Question 25

What type of receptor is the thyroid hormone receptor?

Answer 25

Nuclear receptor

Question 26

What is goitre?

Answer 26

Enlargement of the thyroid gland
May accompany either hypo or hyperthyroidism
Develops when thyroid gland is overstimulated

Question 27

What are the causes of hypothyroidism?

Answer 27

Failure of thyroid gland
TSH or TRH deficiency
Inadequate dietary supply of iodine
Radioactive iodine
Autoimmunity
Post surgery
Congenital
Anti-thyroid drugs

Question 28

What are the general symptoms of hypothyroidism?

Answer 28

Obesity
Lethargy
Intolerance to cold
Bradycardia
Dr skin
Alopecia
Hoarse voice
Constipation
Slow reflexes

Question 29

What is hashimotos disease?

Answer 29

Autoimmune disease resulting in destruction of thyroid follicles
Leads to hypothyroidism

Question 30

What is the treatment of hasimotos disease?

Answer 30

Oral thyroid hormone

T4 used since longer half-life

Question 31

What are the causes of hyperthyroidism?

Answer 31

Autoimmune Graves’ disease
Toxic multinodular goitre
Solitary toxic adenoma
Excessive T4 or T3 therapy
Drugs - amiodarone
Ectopic thyroid tissue

Question 32

What are the general symptoms of hyperthyroidism?

Answer 32

Weight loss
Irritability
Heat intolerance, sweating
Tachycardia
Fatigue, weakness
Increased bowel movements

Question 33

What is Graves’ disease?

Answer 33

Autoimmune disease resulting in hyperthyroidism
Caused by production of thyroid stimulating immunoglobulin
TSI continuously stimulates thyroid hormone secretion outside normal negative feedback control

Question 34

What are the symptoms of Graves’ disease?

Answer 34

Increase in BMR
Excessive sweating
Decease in body weight
Muscle weakness
Heart palpitations 
Bulging eyes

Question 35

What are the anti thyroid drugs?

Answer 35

Used to treat overactive thyroid
Block formation of thyroid hormone
Carbimazole - pro-drug converted to methimazole in body
Prevents thyroid peroxidase from coupling ad iodinating tyrosine on thyroglobulin