Thyroid

Question 1

Where is the thyroid gland located?

Answer 1

In the neck

Question 2

What is the colour of the thyroid gland and why?

Answer 2

Red, due to extensive blood supply and circulation

Question 3

What is the function of the follicle cell?

Answer 3

Secretes thyroglobulin into the colloid

Question 4

What is the function of the colloid?

Answer 4

An extracellular space where thyroglobulin with attached iodine atoms are stored

Question 5

What is the thyroid follicle?

Answer 5

The combination of follicle cells and colloid, serving as the functional unit for thyroid hormone production

Question 6

What makes up the thyroid hormone?

Answer 6

Tyrosine + Iodine

Question 7

Where is tyrosine obtained from?

Answer 7

Synthesised by the body, cannot be obtained from diet

Question 8

Where is iodine obtained from?

Answer 8

Must be obtained from dietary intake

Question 9

What is the process of Thyroglobulin formation?

Answer 9

Follicle cells produce Tg with tyrosine incorporated.
Exported into the colloid via exocytosis

Question 10

How is iodine absorbed?

Answer 10

Iodide from the blood is captured by the thyroid and transferred into the colloid by a Na+ K+ pump where Na+ and I- is coupled

Question 11

What is the mode at which iodide is passed to colloids and why?

Answer 11

Active transport against its concentration to ensure full uptake

Question 12

What transformation does iodide undergo and by which enzyme?

Answer 12

Iodide is oxidised into active iodide by membrane bound thyroperoxidase (TPO)

Question 13

How is iodide attached to Tg?

Answer 13

Iodide enters the colloid via a channel and is attached to tyrosine within the thyroglobulin molecule by TPO

iodide + tyrosine in Tg = mono-iodityrosine
2 iodide + tyrosine = di-iodotyrosine

Question 14

How does T3/T4 form?

Answer 14

Iodo-tyrosine molecuels are coupled!
MIT + DIT = T3, DIT + DIT = T4

MIT + MIT does not occur!

Question 15

What is the mechanism of TH release?

Answer 15

With appropriate stimulation, follicular cells phagocytose a piece of colloid to internalise the Tg-hormone complex

Engulfed vesicle is attacked by lysosomes, splitting the iodinated products from Tg (i.e. MIT, DIT, T3, T4)

T3&4 diffuse through outer membranes of the follicular cells and into the blood

Question 16

How is TH stored in the plasma?

Answer 16

Bound to plasma proteins

Question 17

How is TH stored in the colloid?

Answer 17

Bound to thyroglobulin

Question 18

Which TH molecule is more active?

Answer 18

T3 is active, derived from T4

Question 19

Where does T4 to T3 conversion occur?

Answer 19

Mainly in the liver, kidney and pituitary

Question 20

What is the feedback loop that TH has?

Answer 20

Negative feedback loop

Question 21

What is the function of the hypothalamus?

Answer 21

Secretes thyrotropin releasing hormone (TRH)

Question 22

What is the function of the anterior pituitary gland?

Answer 22

Secretes thyroid stimulating hormone (TSH)

Question 23

What is the effect of TH on metabolic rate and heat/sweat production?

Answer 23

High TH = High BMR due to:
- More O2 consumption & energy use under resting conditions (more mitochondria etc. for oxidative phosphorylation)
- Higher heat production due to increased metabolic effects

Question 24

What is the sympathomimetic effect of TH?

Answer 24

TH simulates sympathetic actions, such as increased heart rate and higher cell responsiveness to catecholamines

Question 25

What are causes of hypothyroidism?

Answer 25

1. Failure of the thyroid gland 2. Secondary deficiency of TRH, TSH or both 3. Inadequate iodine supply

Question 26

What are common symptoms?

Answer 26

Weight gain (due to low BMR), extreme fatigue, intolerance to cold

Question 27

What is cretinism?

Answer 27

Infant hypothyroidism, which can lead to dwarfism and mental retardation

Question 28

What is myxedemia?

Answer 28

Adult hypothyroidism, causing low body temperature, lethargy, muscular weaknesses and slow reflex.

Question 29

What are causes of hyperthyroidism?

Answer 29

1. Hypersecreting thyroid tumour 2. Production of TSI (most common!!) 3. Secondary excess of TRH, TSH or both

Question 30

What is TSI and how does it lead to hyperthyroidism?

Answer 30

TSI is an antibody which binds to TSH receptors, thereby mimicking TSH activity, leading to increased thyroid growth and secretion. However, TSI is not subjected to the negative feedback inhibition by TH, so TH secretion continues!

Question 31

What is goitre?

Answer 31

Enlarged thyroid gland due to overstimulation

Question 32

In which cases of hyper and hypothyroidism is goitre present?

Answer 32

Hypothyroidism: Failure of thyroid gland, lack of dietary I- (because stimulation is still there!) Hyperthyroidism: Presence of TSI, excess TSH/TRH secretion

Question 33

What is the mechanism of action of anti-thyroid drugs as a mode of treatment for hyperthyroidism?

Answer 33

Anti-thyroid drugs interfere with TH synthesis (e.g. block I- uptake or inhibits TPO which helps conversion)

Question 34

How does administration of radioactive iodine as a treatment for hyperthyroidism work?

Answer 34

Radioactive iodine is picked up by thyroid cells, enters the colloid and destroys the forming TH