Thyroid hormones

Question 1

Describe the thyroid gland

Answer 1

there are 2 lobes
histology made of follicles
spherical - follicular cells with colloid in the middle 
isthmus is between lobes 
parafollicular cells surround it

Question 2

describe the production of thyroid hormones

Answer 2

TSHR on basolateral membrane stimulated by TSH
stimulates up take of iodide through the sodium iodide symporter
at the apical membrane the iodide enters the colloid

TSHR make thyroglobulin protein
iodine is added to the AA with tyrosyl residues - iodination catalysed by TPO (thyroid peroxidase)
this forms monoiodotyrosine and diiodotyrosine
these added in coupling reaction – T3 hormone
if 2 diiodotyrosine combined – T4

T3 and T4 enter follicular cells
are chopped up and secreted into the blood

Question 3

name for thyroid hormones

Answer 3

iodothyronines

Question 4

name for T3

Answer 4

tri-iodothyronine

Question 5

namne for T4

Answer 5

tetra-iodothyronine

Question 6

describe the transport in blood

Answer 6

mostly bound to plasma proteins 
- thyroid binding globulin 70-80%
- albumin 10-15%
- prealbumin (aka transthyretin) 
only o.05% T4 and 0.5% T3 unbound bioactive

Question 7

why does deiodination need to occur

Answer 7

T4 main product of thyroid gland - T3 is the active form

need to convert T4 to T3 in peripheral tissue

Question 8

what is reverse T3

Answer 8

product when wrong iodine is removed from T4

inactive

Question 9

describe the mechanism of action of T3

Answer 9

bind to THR - stimulate transcription and protein synthesis

act on non-nuclear ion channels

Question 10

what are the actions of TH

Answer 10

fetal growth and development - tested for hypothyroidism, heel prick test - TH important for development of nervous system, untreated congenital hypothyroidism v= cretinism

basal metabolic rate - protein carb and fat met - lose weight if over active. potentiate actions of catecholamines - tachycardia and lipolysis - over active e= palpitations. effects on CNS (restlessness), GI (diahorrea) and reproductive system. over and underactive affect menstrual - subfertility

Question 11

what is the latent period of the TH

Answer 11

T3 12hours

T4 7-9 hours

Question 12

what are the half lives of the TH

Answer 12

T3 2days

T4 7-9days

Question 13

describe the control of TH production

Answer 13

T4 and T3 inhibit TSH (direct) and TRH (indirect) -ve feedback
if making TH, TSH low. if not TSH high
iodide inhibits hormone release - immediate. wolff-chaikoff effect - used if about to have surgery/have thyroid storm - given KI
oestrogen stimulate
glucocorticoid decrease

Question 14

roles of TSH

Answer 14

stimulate iodine uptake
stimulate the production of thyroxine
stimulate the release of stored thyroxine