Physiology of the Thyroid Gland

Question 1

Describe what colloid is in terms of tructure of the thyroid gland.

Answer 1

colloid = tyrosine containing thyroglobulin filled spheres enclosed by follicular cells

Question 2

What do parafollicular C cells secrete and why is this important?

Answer 2

Secrete cacitonin - involved in cacium metabolism

Question 3

Briefly describe how T3 and T4 are synthesised, stored and released.

Answer 3

Iodine moves into the follicular cell from the blood
This attaches to tyrosine residues on thyroglobulin to form T3 and T4. These can be secreted by pinocytosis or stored in colloid thyroglobuin until required

Question 4

What exactly is T3 made up of?

Answer 4

MIT and DIT

Question 5

If only one iodine joins the tyrosine residue what is created?

Answer 5

Monoiodotyrosine unit (MIT)

Question 6

If 2 iodines join the tyrosine residue, what is the resulting molecule called?

Answer 6

Di-iodotyrosine unit (DIT)

Question 7

If two DIT units join together, what is made?

Answer 7

T4

Question 8

What is the method of action of Carbimazole and Propylthiouracil?

Answer 8

Prevent iodine attaching to tyrsine residues to stop the manufacturing of T3 and T4 - used to treat hyperthyroidism

Question 9

Which thyroid hormone is secreted the most?

Answer 9

T4 = thyroxine - ~90% of thyroid hormones

Question 10

Which thyroid hormone is most potent?

Answer 10

T3 (4x more potent than T4)

Question 11

Can T4 be converted to T3?

Answer 11

yes in the liver and kidney

Question 12

Which thyroid hormone is the major biologically active thyroid hormone?

Answer 12

T3

Question 13

What is the main regulator of thyroid hormone release?

Answer 13

Thyroid stimulating hormone (TSH)

Question 14

Are T3 and T4 hydrophilic or hydrophobic?

Answer 14

Hydrophobic and lipophilic so bind to plasma proteins

Question 15

What is the main plasma protein that T3 and T4 bind to?

Answer 15

Thyroxine binding globulin (TBG)

Question 16

What other plasma proteins can T3 and T4 bind to?

Answer 16

Thyroxine bindding prealbumin (~20%)

Albumin (~5%)

Question 17

The bound/unbound thyroid hormone is biologically active adn therefore available to the tissue?

Answer 17

Unbound

Question 18

What allows the release of T3 and T4 from the colloid?

Answer 18

Thyroid stimulating hormone (TSH)

Question 19

Which thyroid hormone has more rapid onset and offset kinetics?

Answer 19

T3

Question 20

Does the metabolic state correlate more closely with the free hormones or total hormone concentration?

Answer 20

Free

Question 21

Do thyroid hormones increase or decrease basal metabolic rate?

Answer 21

Increase

Question 22

Do thyroid hormones increase or decrease thermogenesis?

Answer 22

Increase

Question 23

What is the effect of thyroid hormones on carbohydrate metabolism?

Answer 23

Increase blood glucose

Increase insulin-dependent glucose uptake into cells

Question 24

What is the effect of thyroid hormones on lipid metabolism?

Answer 24

Mobilise fats from adipose tissue

Incerase fatty acid oxidation in tissues

Question 25

What is the effect of thyroid hormones on protein metabolism?

Answer 25

increase protein synthesis

Question 26

How do thyroid hormones indirectly effect growth?

Answer 26

growth hormone releasing hormone and growth hormone itself required thyroid hormones

Question 27

In the development of foetal and neonatal brain, what exactly requires thyroid hormones?

Answer 27

Myelinogenesis and axonal growth require thyroid hormones

Question 28

What is the effect of thryroid hormones on permissive sympathomimetic action?

Answer 28

Increase responsiveness to adrenaline and NA by increasing number of receptors

Question 29

What class of drugs can be given to treat symptoms in initial stages for hyperthyroidism?

Answer 29

Beta-blockers (e.g. Propanolol)

Question 30

How is the production of thyroid hormones regulated?

Answer 30

- Thyrotrophin releasing hormone (TRH), stimulates thyroid stimulating hormone (TSH) release from anterior pituitary - TSH = major stimulant to release of T3 and T4 from thyroid gland - T3 and T4 exert negative feedback control of release of TRH and TSH

Question 31

What are the other 3 minor regulators of thyroid hormone release?

Answer 31

Low temperature Stress Circadian rhythm

Question 32

What does the thyroid secrete?

Answer 32

Throxine (T4) Tri-iodothyronine (T3) Calcitonin

Question 33

What do the parathyroid glands secrete and what does this act on?

Answer 33

``` parathyriod hormone (PTH) Calcium metabolism ```

Question 34

The Hypothalamus secretes ... that acts on the anterior pituitary to secrete...? What does this allow the release of

Answer 34

TRH TSH T3&T4

Question 35

What are the parathyroid cells composed of?

Answer 35

Chief cells supported by oxyphil cells

Question 36

Describe the cell histology of a normal parathyroid gland.

Answer 36

Round cells with moderate cytoplasm and bland round central nuclei