Week 3- The Thyroid Gland

Question 1

How many lobes does the thyroid have?

Answer 1

Usually 2, some people have 3 (10-15% of people have a third lobe called the pyramid)

Question 2

What is the gap that joins the 2 lobes of the thyroid?

Answer 2

Isthmus

Question 3

What is the thyroid comprised of?

Answer 3

Follicles

Question 4

What glands are embedded in the thyroid?

Answer 4

Parathyroid

Question 5

What nerve associated with the thyroid supplies the vocal chords?

Answer 5

Left recurrent laryngeal nerve

Question 6

What are the 2 glands in the thyroid that are important for calcium metabolism?

Answer 6

Inferior and superior parathyroid glands

Question 7

What part of the cell are the thyroid hormones made?

Answer 7

In the colloid

Question 8

What are the 2 thyroid hormones called?

Answer 8

T3 (triiodothyronine) and T4 (thyroxine)

Question 9

Out of T3 and T4, which is active and which is inactive

Answer 9

T3 is active

T4 is inactive

Question 10

What 2 molecules couple to form T3?

Answer 10

Monoiodotyrosine and diiodotyrosine

Question 11

What 2 molecules couple to form T4?

Answer 11

2 molecules of triiodothyronine

Question 12

What is the main hormone product of the thyroid gland?

Answer 12

T4

Question 13

How is T4 activated?

Answer 13

It is deiodinated to T3 in target tissues

Question 14

What is reverse T3?

Answer 14

A molecule produced when T4 is deiodonated in a different position to produce an inactive molecule

Question 15

What enzyme converts T3 to T4?

Answer 15

Deiodinase enzyme

Question 16

How do thyroid hormones affect cells?

Answer 16

They bind to receptors and change gene expression

Question 17

Why is the mothers thyroid hormone status important in pregnancy?

Answer 17

The foetus gets its thyroid hormone from the mother so essential for growth and development

Question 18

What is cretinism?

Answer 18

Untreated congenital hypothyroidism

Question 19

How is congenital hypothyroidism measured in babies?

Answer 19

Heel prick test

Question 20

What are some of the main actions of the thyroid hormone?

Answer 20

It increases basal metabolic rate, increases cardiac output, affects GI, CNS, reproductive system

Question 21

What is the half life of T3

Answer 21

2 days

Question 22

What is the half life of T4?

Answer 22

7-9 days

Question 23

A lot of which molecule inhibits T3/T4 production?

Answer 23

Iodide

Question 24

Are thyroid disorders more common in women or men, by how much and why?

Answer 24

In women by a 4:1, due to autoimmunity

Question 25

Are over or under active thyroids more common?

Answer 25

Neither they’re the same

Question 26

What happens in hypothyroidism and how does it arise?

Answer 26

Thyroxine levels decline and TSH levels climb, due to autoimmune damage

Question 27

What are the 2 main common thyroid disease?

Answer 27

Hashimoto’s thyroiditis (hypothyroidism) | Graves’ disease (hyperthyroidism)

Question 28

What are common symptoms of hypothyroidism?

Answer 28

``` Fatigue Depression Deep voice Cold intolerance Weight gain Reduced appetite Constipation Bracycardia Eventual myxedema coma ```

Question 29

What medicine is given to treat hypothyroidism?

Answer 29

Levothyroxine | It can also treat hyperthyroidism sometimes

Question 30

What is the common dose of levothyroxine?How is it administered?

Answer 30

100 micrograms | Administered orally

Question 31

What is common thyroid hormone replacement?

Answer 31

Giving a mix of T3 as well as T4

Question 32

What happens in hyperthyroidism?

Answer 32

Thyroxine levels rise | TSH levels drop

Question 33

What are the 3 main causes of hyperthyroidism?

Answer 33

Graves’ disease (whole gland smoothly enlarged and whole gland overactive) Toxic multinodular goitre Solitary toxic nodule

Question 34

What is the mechanism of action in Graves’ disease?

Answer 34

Antibodies bind to and stimulate TSH receptors in the thyroid causing a smooth goitre

Question 35

What happens in Graves’ disease when antibodies bind to muscles behind the eye?

Answer 35

Exophthalmos- the eyes bulge and you can see the whites around the eye

Question 36

What happens in Graves’ disease when antibodies bind to soft growth tissue of shins?

Answer 36

Pretibial myxoedema

Question 37

What are common symptoms of hyperthyroidism?

Answer 37

``` Heat intolerance Weight loss with increased appetite Myopathy Mood swings Diarrhea Tremor of hands Palpitations Sore eyes Goitre ```

Question 38

What 2 cell types are present in thyroid follicles?

Answer 38

Follicular cells and colloid cells

Question 39

Describe the embryology of the thyroid gland

Answer 39

Originates from the base of the tongue (floor of the pharynx) Development of thyroglossal duct Divides into 2 lobes Duct disappears leaving the foramen cæcum Thyroid gland then develops

Question 40

Where does circulating T3 come from?

Answer 40

80% from deiodination of T4 | 20% from direct thyroidal secretion

Question 41

How are thyroid hormones transported in the blood?

Answer 41

They are bound to plasma proteins (99% of the time)

Question 42

What are the 3 main plasma proteins that transport thyroid hormones?

Answer 42

Thyroid binding globulin Albumin Prealbumin/ transthyretin

Question 43

How is thyroid hormone production controlled?

Answer 43

The negative feedback loop prevents excess production as T3/T4 production switches off TRH in hypothalamus (indirectly via switching off TSH) and TSH production in the anterior pituitary (directly)

Question 44

What are potential minor complications of taking levothyroxine?

Answer 44

Weight loss and headache

Question 45

What are potential major complications of taking levothyroxine?

Answer 45

Heart attack and rapid heart rate

Question 46

What is thyroglobulin, where is it produced and what special property does it have?

Answer 46

It is the prohormone produced in the anterior pituitary gland that has a thymine residue that can be iodised. It allows the synthesis of T3 and T4 in the presence of thyroperoxidase

Question 47

Describe the pathway for T3/T4 synthesis in the thyroid gland

Answer 47

TSH is produced by the anterior pituitary gland and binds to TSH receptors on follicular cells Iodide is transported into the follicular cell via sodium iodine receptor and then to the colloid Iodide is oxidized to iodine TSH binding also starts production of thyroglobulin (a proenzyme that has a thymine residue that can be iodised) Thyroperoxidase enzyme is also activated by TSH binding and travels to the colloid where it catalyses iodination reactions Iodination of thyroglobulin allows synthesis of T3 and T4 (that are still bound to thyroglobulin) Protein bound T3 and T4 travel back to follicular cells where peptide bonds are broken and T3/T4 are released into the bloodstream

Question 48

What molecule is essential in the synthesis pathway of T3/T4?

Answer 48

Iodide (but it must be oxidized to iodine in the synthesis pathway)

Question 49

How does TRH from the hypothalamus reach the anterior pituitary?

Answer 49

It is released into the medial leminence and as the vessels are leaky travels via portal circulation

Question 50

What does ‘primary’ in primary hypothyroidism mean?

Answer 50

That it is a problem with the gland

Question 51

What is menorrhagia and what thyroid condition is it a common feature of?

Answer 51

Heavy peroids- a common feature of hypothyroidism

Question 52

How much time after initial levothyroxine prescription is the dosage checked and how is it checked?

Answer 52

Checked after 6/8 weeks If the dose is too high TSH levels will be too low If the dose is too low TSH levels will be too high