The Thyroid Gland

Question 1

Where is the thyroid gland located?

Answer 1

Around the front larynx and trachea
Below the thyroid cartilage

Question 2

What is the name of the bridge that joins the 2 lobes of the thyroid gland?

Answer 2

Isthmus

Question 3

Describe the embryological development of the Thyroid gland

Answer 3

An epithelial proliferation in the floor of the pharynx at the base of the tongue

Descends down the Thyroglossal duct and migrates downwards passing in front of the hyoid bone

Question 4

What are thyroid follicles?

Answer 4

When follicular cells are arranged in spheres

Question 5

What do the thyroid follicles contain?

Answer 5

Colloid

Question 6

What is colloid?

Answer 6

It’s made up of deposits of Thyroglobulin

Question 7

Is colloid Intracellular or extracellular?

Answer 7

Extracellular even though it is inside the follicle

Question 8

What is Thyroglobulin?

Answer 8

A protein where thyroid hormone is synthesised

Question 9

What are parafollicular cells and where are they found in thyroid histology?

Answer 9

Cells that produce Calcitonin which is important in calcium metabolism

They are found not producing the spheres/ thyroid follicles

Question 10

What is the only use of iodine in the body?

Answer 10

Synthesis of Thyroid hormones

Question 11

What do thyroid hormones consist of?

Answer 11

Iodinated tyrosine

Question 12

What does T3 (Triiodothyronine) consist of?

Answer 12

2 tyrosines linked together with iodine at 3 positions on the aromatic rings

MONOiodotyrosin + DIiodotyrosine

Question 13

What does T4 (Tetraiodothyronine) or Thyroxine consist of?

Answer 13

2 tyrosines linked together with iodine at 4 positions on the aromatic rings

DIiodotyrosine + DIiodotyrosine

Question 14

Why is Thyroglobulin important in thyroid hormone synthesis?

Answer 14

It acts as a scaffold

It contains the tyrosine residues that will be iodinated
Tyrosine residues on the thyroglobulin protein get iodinate
They also then get coupled to form T3 or T4

Question 15

What happens to the thyroglobulin protein that holds the coupled tyrosine residues, so T3 or T4 when the thyroid hormones need to be released?

Answer 15

It gets degraded (proteolysis)

The colloid that got taken back into the cell fuses with a lysosome which degrades thyroglobulin

Question 16

What is the key enzyme involved in the synthesis of thyroid hormones?

Answer 16

Thyroid Peroxidase

Question 17

What are the 3 key roles of Thyroid peroxidase?

Answer 17

-Oxidation of iodide to iodine (needs H2O2)

-Addition of Iodine to tyrosine residues

-Coupling of MIT or DIT to generate T3 or T4 in the Thyroglobulin protein

Question 18

What form must iodine be converted into before it can be absorbed into the body?

Answer 18

Iodide (I-)

Question 19

What is an Iodine trap?

Answer 19

The sodium iodide symporter found in thyroid epithelial cells which take up iodide from the blood

Question 20

Which enzyme oxidises Iodide to iodine in the thyroid epithelial cell so it can be released into the colloid?

Answer 20

Thyroid peroxidase

Question 21

What happens to the thyroid gland in response to iodine deficiency?

Answer 21

Enlarges
Goitre

Question 22

How are the thyroid hormones stored?

Answer 22

Within the thyroglobulin proteins

Question 23

How does colloid containing Thyroglobulin protein with thyroid hormones stored in it enter back into the thyroid follicular cell?

Answer 23

Pinocytosis

Question 24

Out of T3 and T4, which one is more biologically active?

Answer 24

T3

Question 25

Which is the main thyroid hormone which is secreted?

Answer 25

T4 but T3 is more active

Question 26

What is the name of the protein which T3 and T4 are transported bound to in the blood?

Answer 26

Thyroxine-binding globulin

Question 27

How is thyroid hormone secretion regulated?

Answer 27

Negative feedback

Question 28

Describe the negative feedback Regualtion of thyroid hormone secretion

Answer 28

Too much thyroid hormone stops anterior pituitary releasing TSH Too much thyroid hormone inhibits Hypothalamus producing TRH

Question 29

What type of hormone is Thyroid Stimulating Hormone?

Answer 29

Glycoprotein

Question 30

Why are all glycoprotein hormones similar?

Answer 30

They have the same alpha subunit

Question 31

How does TSH trigger thyroid hormone release? What does it stimulate?

Answer 31

Iodide uptake Iodide oxidation Thyroglobulin synthesis Thyroglobulin iodination Colloid pinocytosis Proteolysis of thyroglobulin

Question 32

What are the general actions of thyroid hormone?

Answer 32

Increase Basal metabolic rate and heat production Stimulates metabolic pathways Sympathomimetic effects

Question 33

How does thyroid hormone stimulate the metabolic rate?

Answer 33

Increase number and size of mitochondria Stimulate synthesis of enzymes in respiratory chain

Question 34

What metabolic pathways are generally stimulated by thyroid hormones?

Answer 34

Catabolic: Lipolysis and B-oxidation of fatty acids Gluconeogenesis, glycogenolysis

Question 35

What is the sympathomimetic effect of thyroid hormone?

Answer 35

Increases number,of Catecholamines receptors (adrenaline) on target cells

Question 36

How does thyroid hormone specifically effect the CVS?

Answer 36

Increases hearts responsiveness to Catecholamines Increased cardiac output = chronotropy (inc Heart rate) and inotropy (stronger contraction) Peripheral vasodilation = more heat to body surface

Question 37

How does thyroid hormone specifically effect the nervous system?

Answer 37

Increased myelination of nerves and development of Neurons

Question 38

How do thyroid hormone receptors act when no thyroid hormone is bound? So how does Thyroid hormone act?

Answer 38

They bind to DNA when the hormone isn’t present preventing transcription/gene expression Regulates gene expression

Question 39

How does binding of the thyroid hormone to its receptor activate transcription?

Answer 39

Causes conformational change to the receptor so it is no longer bound to the DNA allowing it to be transcribed

Question 40

Where is the thyroid hormone receptor actually located?

Answer 40

Inside the nucleus actually on the DNA

Question 41

What is the part of DNA that has the Thyroid Hormone Receptor bound to it called?

Answer 41

Thyroid Response element

Question 42

So what is a hormone response element? (HRE)

Answer 42

A region of DNA which a hormone receptor binds/is bound to

Question 43

What is Goitre?

Answer 43

Enlargement of the thyroid gland

Question 44

When can goitre be present?

Answer 44

Hypothyroidism Hyperthyroidism (when gland is overstimulated)

Question 45

What can cause Hypothyroidism?

Answer 45

Failure of thyroid gland TSH or TRH deficiency Iodine deficiency AUTOIMMUNITY Post surgery Anti thyroid drugs

Question 46

What are the general symtoms of Hypothyroidism?

Answer 46

Obesity Lethargy Slow reflexes Intolerance to cold Brady cardia

Question 47

In Hypothyroidism what hormones are low and what are high?

Answer 47

Low T3 and T4 High TSH

Question 48

What condition can Hypothyroidism cause in adults?

Answer 48

Myxedema

Question 49

What are the signs of Myxedema in Adults due to Hypothyroidism?

Answer 49

Thick puffy skin (waxy swollen skin) Muscle weakness Slow speech Metal deterioration Intolerance to cold

Question 50

What is congenital hypothyroidism caused by in infants?

Answer 50

Iodine deficiency

Question 51

Signs of Congenital hypothyroidism

Answer 51

Mental deficiency GI disturbances Poor bone development Slow pulse muscle weakness

Question 52

What is Hashimotos disease?

Answer 52

An Autoimmune disease which leads to the destruction of thyroid follicles causing hypothyroidism

Question 53

In Hashimotos disease, what hormone levels will be low and which hormone will be high?

Answer 53

Low T3 Low T4 High TSH

Question 54

How is Hashimotos disease treated?

Answer 54

Give oral thyroid hormone

Question 55

What are some causes of Hyperthyroidism?

Answer 55

Autoimmune Graves’ disease Excessive T3 or T4 therapy Drugs amiodarone Ectopic thyroid tissue

Question 56

What are the general symptoms of Hyperthyroidism?

Answer 56

Weight loss Irritability Heat intolerance Tachycardia Increased appetite Tremors

Question 57

What is Exophthalmos?

Answer 57

Bulging eyes

Question 58

Can some hyperthyroidisms cause Exophthalmos (bulging eyes)?

Answer 58

Yes but not all

Question 59

How does Graves Disease cause Hyperthyroidism?

Answer 59

TSI (Thyroid stimulating immunoglobulin) is produced This binds to TSH receptors mimicking the actin of TSH stimulating Thyroid hormone production

Question 60

What hormone levels will be high and low in Graves Disease?

Answer 60

High T3 High T4 Low TSH

Question 61

What does Graves Disease cause?

Answer 61

Increased BMR Sweating Decrease body weight Muscle weakness Heart palpitations Sometimes Exophthalmos

Question 62

What is the name of the isotope used to scan the thyroid with a gamma camera?

Answer 62

Technetium-99m

Question 63

What is the name of an Antithyroid drug and what does it do?

Answer 63

Carbimazole Blocks formation of thyroid hormone so treats Hyperthyroidism