Thyroid Function Flashcards
thyroid axis
stimulus comes –> hypothalamus activated –> release TRH (thyroid releasing hormone) –> anterior pituitary = TSH (thryoid stimulating hormone)
–> thryoid glands = 2 thyroid hormones hormones (T3/T4)
–> target tissues
TSH
thyroid stimulating hormone
helps in regulation or activation of many processes in thyroid synthesis
colloid space
production of the precursor of T3/T4 hormones (thyroglobulin)
thyroglobulin transported into follicular cells and processed there –> released into blood to activate target tissues
parafollicular cells
adjacent to follicular cells
–> secretion of other hormones (calcitonin) = regulates calcium levels in the body
thyroid glands
butterfly shaped
found under vocal box or larynx
two lobes, left and right
–> many cells composed within thyroid gland
–> thyroid follicles :
consists of
* big luminal space = colloid
* surrounded by many cells on periphery = follicular cells
T3/T4 hormone
iodine containing hormones
–> derived from 2 thyro molecules??? = iodinated during maturation
= tyroxine T4 (4 iodine atoms attached)
= triiodothyroine T3 (3 iodine atoms attached)
T4 : prohormone of T3
- less biologically active than T3
T3 :
80% made from T4, 20% from MIT + DIT
increases metabolic rate
rTE :
100% made from T4 and decreases metabolic rate
follicular cells
produce mainly tyroxine/T4 (precursor) into the tciruclation to be taken by targen tissues which can be converted into triiodothryonine/T3 (biologically active hormone) by removing a iodine atom
thyroid hormone synthesis
1 tyrosine molecule is iodinated (added on phenyl ring)
into Monoiodotyrosine (MIT)
–> iodinated into Diiodotyrosine (DIT)
forms 2 combinations
1) DIT + MIT = T3
2) DIT + DIT = T4 (favoured rection)
T4 –> T3 is preffered
T4 can also convert into reverse T3 (rT3)
rT3
reverse T3 is non active but can bind to receptor and cause antagonising effect on downstream signalling pathways (NOT GOOD)
synthesis thyroid hormone mechanism
1) follicular ells makes enzymes and thyroglobulin in the endoplasmic reticulum for colloid space
2) iodine from blood through transporters (active process/ co transporter with Na+) through the follicular cells and into colloid space
3) enzymes add iodine to thyroglobulin
4) thyroid peroxidase iodinates the tyrosine residues on the thyroglobuline [tyrosine –> MIT –> DIT = T3/T4]
5) T3/T4 synthesis initated = T3/T4 complex formed and transported into follicular cells, uptake achieved by the protein megalin
6) in follicular cells, the T3/T4 complex is in lysosome
7) lysosome uses lysosomes enzymes and hydrolysis + low pH (acidic environment) so T3/T4 is removed from backbone of thyroglobulin
8) vesicle degrated and T3/T4 moves into the blood
9) T3/T4 hormones are amine hormones (tyrosine derived) but similar to steriod hormones, bound by carrier proteins to stay longer in the blood circulation
10) only set free and becomes active when needed
hypothyroidoism
thyroid gland secretes too little hormone
**primary hypothyroidoism : **
iodine deficiency in diet
**secondary hypothyroidoism : **
not enough TSH (tumour in anterior pituitary)
**tertiary hypothyroidoism : **
TRH not enough (hypothalamus)
**congenital hypothyroidoism : **
at birth, children born with non-functioning/ undeveloped thyroid glands
Dec oxygen consumpton rate + metabolic rate
Dec protein synthesis –> muscle weakness and muscle tremours
slow speech and though
Dec heartbeat rate
== goiter disease
free T3/T4 homrone function
- affect nearly every organ in the body
- increase BMR and heat production
- increase mitochrondrial activity and glucose oxidation in target cells
- growth and development
- cardiac function, gastric GI tract and memory
hyperthyroidoism
thyroid gland secretes too much hormone
changes to metabolism, nervous systems and heart function
Inc oxygen consumpton rate + metabolic heat production
Inc protein catabolism –> muscle weakness and muscle tremours
Inc heart rate
== graves disease
graves disease
autoimmunity produced
–> thyroid stimulating immunoglobins = activates thyroid glands = product T3/T4
–> no communication with hypothalamus/ anterior pituitary = no physiological stimulus = negative feedback loop broken
thyroid gland tumours
Goiter disease
iodine deficiency in diet
decrease levels of T3/T4 in blood
negative feedback loop = stimulates TSH (anterior pituitary) so TSH increases and causes an enlarged thyroid gland ?? why
