7 Thyroid function

Question 1

which molcule is thyroid hormone based around?

Answer 1

amino acid tyrosine

Question 2

describe the parts of the thyroid gland and which molecules relate to these parts

Answer 2

follicular cells= functional unit where thyroid hormones are made

protein rich colloid : lumen for storage of TH

parafollicular C cells: secrete calcitonin

Question 3

terms for lots of tyrosine

Answer 3

thyrosine

Question 4

chemical structure of synthesis of TH

Answer 4

tyrosine + iodine -> MIT (3 position) or DIT (3 &5 position)

DIT+ DIT-> T4

MIT+ DIT-> T3

DIT+ MIT-> reverse T3

Question 5

rT3 characteristics:

Answer 5

biologically inactive

antagonist against T3 receptors

Question 6

synthesis of iodine:

1. how is iodine taken up?
2. how iodide moved into colloid?
3. how is iodide oxidised?

Answer 6

1. active uptake of iodine - trapping
2. movement of iodide across apical membrane , via pendrin
3. oxidation of iodide via thyroid peroxidase TPO: integral membrane protein in apical membrane

Question 7

synthesis of thyroid hormones:

what is organification?

what is the product of this?

what happens after organifiction?

Answer 7

1. Organification: iodination of tyrosine molcules of thyroglobulin (Tg) -> MIT and DIT
2. coupling of iodotyrosines-> T3, T4 in colloid

Question 8

what are thyroid homrones synthesied on? how is this molecule transported

Answer 8

thyroglobulin = a large glycoprotein with many tyrosine molecules readily available (130-140/ thyroglobulin molecule)

• it is transported into colloid by exocytosis

Question 9

which reactions do thyroid peroxidase catalyse?

where do this reactions occur?

Answer 9

1. oxidation of iodide
2. organification (iodination of tyrosine on Tg)
3. coupling of iodotyrosines

occur at apical/ luminal membrane

Question 10

where is iodinated Tg stored? how long can these stores supply the body?

Answer 10

in the colloid (follicular lumen)

stored to supply body for several weeks

Question 11

thyroid hormone release

Answer 11

1. Thyroglobulin molecule is taken up into follicular cell by endocytosis of some of colloid
2. fusion with lysosomes results in proteolysis and release of T3/ T4 into plasma

Question 12

which hormone from ant pituitary stimulates synthesis of TH

what happens if this hormone is in excess?

Answer 12

TSH thyroid stimulating hormone of anterior pituitary (stimulated by TRH from hypothalamus)

As a trophic hormone, TSH maintains integrity of thyroid gland

• in excess, increases size and numbers of thyroid follicular cells

Question 13

other factors that regulate thyroid hormone secretion

Answer 13

cold (neonate) - increases TRH

stress (eg trauma)- decreases TRH

Question 14

thyroid hormones in the circulation

Answer 14

T4 and T3 are bound to plasma proteins:

thyroxine- binding globulin

transthyreitin

albumin

Question 15

t1/2 of T4, T3, rT3

Answer 15

T4= 7 days

T3= 1 day

rT3= 4 hours

free T3 and T4 collerates to thyroid function

Question 16

what are the thyroid hormones

Answer 16

thyroxine = T4

tri iodothyronine= T3

calcitonin

Question 17

T3 vs T4 functional differences

Answer 17

T4 functions largely as prohormone

T3 principal active hormone

T3 is 4-5x more potent than T4

Question 18

actions of thyroid hormones

Answer 18

1. increase metabolism in all cells

• increase metabolism of CHO, proteins, fats-> increase O2 consumption, heat production -> increase BMR
• T3 acts on mitochondria in cells

2. stimulate growth and development: normal development and CNS
3. synergistic actions of SNS/ catecholamines (thyroid hormones upregulate expression of beta adrenoreceptors)

Question 19

thyroid hormone mechanism of action

Answer 19

Question 20

what can alter peripheral conversion of thyroid hormones and how?

Answer 20

via altered deiodinase activity

disease (kidney, liver)

acute stress (surgery, illness, injury)

some drugs

diet (low protein, starvation)

Question 21

what would be the effects of increased rT3 and lowered T3

Answer 21

lower metabolic rate