thyroid anatomy and physiology

Question 1

what week of embryogenesis does the thyroid gland start to develop?

Answer 1

4th week

Question 2

how does the thyroid develop in the embryo?

Answer 2

it begins to develop at the base of the tongue and descends down to infront of the tracheal rings and meets the parathyroid glands

Question 3

what does the thyroid gland tissue secrete?

Answer 3

• thyroxine (T4)
• Tri iodothyronine (T3)
• Calcitonin

Question 4

how many parathyroid glands are there?

Answer 4

4

Question 5

what do parathyroid glands secrete?

Answer 5

PTH (parathyroid hormone)

Question 6

what supplies the parasympathetic system of the thyroid gland?

Answer 6

vagus nerve

Question 7

what nerve supplies the sympathetic system of the thyroid gland?

Answer 7

superior, middle and inferior ganglion sympathetic trunk

Question 8

what arteries supply thyroid gland?

Answer 8

superior and inferior thyroid arteries

Question 9

what veins drain blood from the thyroid?

Answer 9

superior and middle thyroid vein which drains into the internal jugular

and inferior thyroid drains into the brachiocephalic veins

Question 10

what is the basic functional subunit of a thyroid gland?

Answer 10

follicle

Question 11

describe the feedback loop of the thyroid gland

Answer 11

• hypothalamus releases TRH (thyroptropin releasing hormone)
• TRH stimulates thyrotropins in pituitary gland and causes pituitary gland to release TSH (thyroid stimulating hormone)
• TSH stimulates the thyroid gland to produce T4 and T3 hormones
• T3 and T4 are secreted into peripheral tissues
• T3 and T4 in peripheral tissues cause negative feedback loop to be activated and stop both hypothalamus producing TRH and pituitary gland producing TSH

Question 12

what will occur to TSH levels in an underactive thyroid?

Answer 12

-they will increase as negative feedback isnt happening and it is trying to compensate for lack of T4 and T3

Question 13

what are the 6 steps of thyroid hormone synthesis?

Answer 13

1. Thyroglobulin synthesis
2. Uptake and concentration of iodide (I-) by follicle cells
3. Oxidation of Iodide (I-) to iodine (I)
4. Iodination of thyroglobulin
5. Formation of MIT (monoiodotyrosine) and DIT (di iodotyrosine)
6. Secretion

Question 14

what inhibits iodination of thyroglobulin?

Answer 14

-carbimazole + propyltthiouracil

Question 15

what is T4?

Answer 15

thyroxine

Question 16

what is T3?

Answer 16

triiodothyronine

Question 17

what are the majority of thyroid hormones secreted?

Answer 17

T4 (90%)

Question 18

is T4 or T3 more biologically active?

Answer 18

T3 and it is 4 times more potent

Question 19

what can convert T4 into T3?

Answer 19

liver and kidney to make it biologically active

Question 20

how are thyroid hormones transported?

Answer 20

bound to serum proteins e.g. TBG and TBPA

Question 21

is it bound or unbound thyroid hormone that enters cells?

Answer 21

only unbound thyroid hormones enter cells

Question 22

if TBG increases what occurs to total and free T4?

Answer 22

total T4- decrease

free T4- stays the same

Question 23

if TBG decreases what occurs to total and free T4?

Answer 23

total T4= decreases

free T4= stays the same

Question 24

what effect do thyroid hormones have on basal metabolic rate?

Answer 24

they increase it

Question 25

what affect do thyroid hormones have on thermogenesis?

Answer 25

they increase it

Question 26

what affect do thyroid hormones have on carbohydrate, protein and lipid metabolism?

Answer 26

increase them

Question 27

what can hypothyroidism do to intellectual functions?

Answer 27

-it slows intellectual functions

Question 28

what does hyperthyroidism do the mood?

Answer 28

- nervousness - hyperkenesis - emotional liability

Question 29

what are the 3 types of de iodinases?

Answer 29

``` Type I (D1) Type II (D2) Type II (D3) ```

Question 30

where is type I (D1) de iodinase usually foubd?

Answer 30

-in the liver and kidneys

Question 31

where is type II (D2) de iodinase usually found?

Answer 31

heart and skeletal muscle, CNS, fat, thyroid and pituitary

Question 32

where is type II (D3) de iodinase usually found?

Answer 32

-fetal tissue and placenta and brain

Question 33

what are some symptoms of hypothyroidism?

Answer 33

- reduced BMR - slow pulse rate - fatigue, lethargy, slow response times and mental sluggishness - cold intolerance - tendancy to put on weight easily

Question 34

what are some symptoms of hyperthyroidism?

Answer 34

- increased BMR - very fast pulse - increased nervousness and excessively emotional - insomnia - sweating and heat intolerance - tendancy to lose weight easily