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regub 5: thyroid hormone 1 (iodine (norm level = 20-50 mg (60% in thyroid)…
regub 5: thyroid hormone 1
embryology
1st endocrine gland to develop
starts @ day 24
arises from median surface of floor of 1st pharyngeal arch as a proliferation of endodermal epithelial cells
develops to form hollow thyroid diverticulum
solidfies + becomes bilobed (connected by an isthmus)
descends to trachea
still connected to tongue via thyroglossal duct
50% present as midline cystic masses below hyoid bone
complete by week 8
ectopic thyroid gland
associated with congenital hypothyroidism
may occur anywhere along descent path
complete descent failure
most common @ base of tongue (lingual thyroid)
incomplete descent
high in neck
below hyoid bone
associated endocrine glands/cells
parathyroid glands
2 pairs on post surface of thyroid
secrete parathyroid hormone (increases blood Ca2+)
parafollicular / C cells
single cells in epithelial lining of follicle
secrete calcitonin (decreases blood Ca2+ levels)
or in groups in connective tissue of follicle
thyroid gland structure
divided into clusters of follicles by connective tissue
one follicle consists of...
thin layer of thyrocytes (epithelium)
make TH + thyroglobulin (prohormone + strorage form, taken into cell by pinocytosis)
colloid
proteinaceous material containing thyroglobulin
thyroglobulin iodinated here by thyroid peroxidase into mono/di iodotyrosine
thyroid hormones
structure = 2 tyrosines (provided by thyroglobulin) linked together with iodine @ positions 3 + 5
lipophilic
T3 = triiodothyronine (90% more activity)
T4 = tetraiodothyronine (10%)
stimulated by TSH (then -ve feedback)
iodine
essential trace element
norm level = 20-50 mg (60% in thyroid)
depends on amount in food, which depends on soil
taken up by NA-I symporter (iodine trap) on outer membrane of thyrocytes
symporter activity increased by TSH
dietary I2 absorbed in GIT
converted to iodide (I-)
excreted by kidneys
I- -> I+ via peroxidase in lumen of follicle
control of TH synthesis
TRH
from hypothal
causes TSH release
blocked by TH
TSH (aka thyrotropin)
from ant pit (by thyrotropes)
glycoprotein with alpha+beta subunits
beta suppressed by T3
higher levels @ night
increases size + vascularity of gland
iodine availability
integrity of gland