The Thyroid Gland Flashcards
vertebral extent of thyroid
C5-T1
the two thyroid hormones
T3 (triiodothyronine) and T4 (thyroxine)
what is contained in the follicles of teh thyroid
colloid
what is colloid
a glycoprotein containing supply of thyroid hormones and thyroglobulin
2 cells types of thyroid
clear cells and follicular cells
what cells surround the colloid
follicular
what do clear cells do
secrete calcitonin
describe the structure of the thyroid gland
follicles filled with colloid with clear cells and capillary interspersed between them, all enclosed in a connective tissue capsule
what do follicular cells do
they secrete thyroglobulin proteins and enzymes which make TH
what amino acid forms with iodide to make TH and where does it happen
tyrosine in the colloid
how does iodide get into colloid
from diet. absorbed into plasma then to follicular cells by coupling to Na+. it is then transported into colloid by pendrin transporter
what is MIT
one iodine and one tyrosine (M for mono)
what is DIT
two iodine and one tyrosine (D for di)
what combination of MIT and DIT makes T3
1 MIT + 1 DIT = T3 (3 iodines)
what combination of MIT and DIT makes T4
2 DIT = T4 (4 iodines)
what enzyme secreted by follicular cells catalyses
thyroid peroxidase
what stimulates TH release
TSH
how is TH released from colloid to plasma
endocytosed into follicular cells –> thyroglobulin cut off –> cross cell membrane into plasma
how is TH bound in colloid
attached to thyroglobulin
TH is lipid/water soluble
lipid soluble
what is TBG
thyroxine binding globulin, binds to TH in plasma
which TH has a longer half life
T4, 6 days compared to 1, its because TBG has a greater affinity for it over T3
what proportion of circulating TH is protein bound
99.8%
which TH binds to more intracellular receptors
T3, despite having shorter life and being far less abundant. cell receptors have a greater affinity for it
what action does T4 undergo due to T3’s greater receptor affintiy
it get deiodinased in plasma and cells
stimuli of TRH release
cold, exercise and pregnancy
action of TRH
TSH release
action of TSH
TH release
what class of hormones that cortisol belongs to inhibits TSH and T4–>T3 conversion
glucocorticoids
inhibitory action of somatostatin (GHIH)
inhibits TSH (makes sense because GHIH inhibits GH and GH need TH permissive effects)
effects of TH on metabolism
increases metabolic rate and creates heat
effect of TH on liver
increases hepatic gluconeogenesis
why is TH not a glucose counter regulatory hormone
although it raises gluconeogenesis it has no effect on insulin so BG stays the same
effect of TH on protein and fat
proteolysis and lipolysis
effect of TH on GH
permissive, without it stunted growth
2 causes of hyperthyroidism
graves disease and a thyroid tumour
what happens in graves disease to produce hyperthyroidism
antibodies mimic TSH at anterior pituitary and continually stimulates TH release. produces negative feedback on TSH but antiboidies keep on stimulating anyway. Thyroid gland gets hyperplastic (big)
given the actions of TH what would you expect in hyperthyroidism presentation
proteolysis/lipolysis –> thin, weak, wasting
increased metabolism –> weight loss, heat intolerance?
also get exaggerated reflexes, tachycardia, high CO and cardiac failure!
why does hyperthyroidism cause heart problems
TH is permissive to epinephrine so b receptors very stimulated
causes of hypothyroidism
hashimoto’s disease, iodine deficiency and idiopathic
what happens in hashimoto’s disease
there is an autoimmune attack on the thyroid gland –> reduced TH production
what would you expect of a hypothroidism presentation
decreased metabolic –> weight gain, cold intolerance
permissive to epinephrine –> reduced CV function
Nervous system –> slow speech, reduced reflexes
more general –> fatigue, brittle nails, thin skin
what is meant by goitre
thyroid enlargement
