thyroid anatomy and physiology Flashcards
what week of embryogenesis does the thyroid gland start to develop?
4th week
how does the thyroid develop in the embryo?
it begins to develop at the base of the tongue and descends down to infront of the tracheal rings and meets the parathyroid glands
what does the thyroid gland tissue secrete?
- thyroxine (T4)
- Tri iodothyronine (T3)
- Calcitonin
how many parathyroid glands are there?
4
what do parathyroid glands secrete?
PTH (parathyroid hormone)
what supplies the parasympathetic system of the thyroid gland?
vagus nerve
what nerve supplies the sympathetic system of the thyroid gland?
superior, middle and inferior ganglion sympathetic trunk
what arteries supply thyroid gland?
superior and inferior thyroid arteries
what veins drain blood from the thyroid?
superior and middle thyroid vein which drains into the internal jugular
and inferior thyroid drains into the brachiocephalic veins
what is the basic functional subunit of a thyroid gland?
follicle
describe the feedback loop of the thyroid gland
- hypothalamus releases TRH (thyroptropin releasing hormone)
- TRH stimulates thyrotropins in pituitary gland and causes pituitary gland to release TSH (thyroid stimulating hormone)
- TSH stimulates the thyroid gland to produce T4 and T3 hormones
- T3 and T4 are secreted into peripheral tissues
- T3 and T4 in peripheral tissues cause negative feedback loop to be activated and stop both hypothalamus producing TRH and pituitary gland producing TSH
what will occur to TSH levels in an underactive thyroid?
-they will increase as negative feedback isnt happening and it is trying to compensate for lack of T4 and T3
what are the 6 steps of thyroid hormone synthesis?
- Thyroglobulin synthesis
- Uptake and concentration of iodide (I-) by follicle cells
- Oxidation of Iodide (I-) to iodine (I)
- Iodination of thyroglobulin
- Formation of MIT (monoiodotyrosine) and DIT (di iodotyrosine)
- Secretion
what inhibits iodination of thyroglobulin?
-carbimazole + propyltthiouracil
what is T4?
thyroxine
what is T3?
triiodothyronine
what are the majority of thyroid hormones secreted?
T4 (90%)
is T4 or T3 more biologically active?
T3 and it is 4 times more potent
what can convert T4 into T3?
liver and kidney to make it biologically active
how are thyroid hormones transported?
bound to serum proteins e.g. TBG and TBPA
is it bound or unbound thyroid hormone that enters cells?
only unbound thyroid hormones enter cells
if TBG increases what occurs to total and free T4?
total T4- decrease
free T4- stays the same
if TBG decreases what occurs to total and free T4?
total T4= decreases
free T4= stays the same
what effect do thyroid hormones have on basal metabolic rate?
they increase it
what affect do thyroid hormones have on thermogenesis?
they increase it
what affect do thyroid hormones have on carbohydrate, protein and lipid metabolism?
increase them
what can hypothyroidism do to intellectual functions?
-it slows intellectual functions
what does hyperthyroidism do the mood?
- nervousness
- hyperkenesis
- emotional liability
what are the 3 types of de iodinases?
Type I (D1) Type II (D2) Type II (D3)
where is type I (D1) de iodinase usually foubd?
-in the liver and kidneys
where is type II (D2) de iodinase usually found?
heart and skeletal muscle, CNS, fat, thyroid and pituitary
where is type II (D3) de iodinase usually found?
-fetal tissue and placenta and brain
what are some symptoms of hypothyroidism?
- reduced BMR
- slow pulse rate
- fatigue, lethargy, slow response times and mental sluggishness
- cold intolerance
- tendancy to put on weight easily
what are some symptoms of hyperthyroidism?
- increased BMR
- very fast pulse
- increased nervousness and excessively emotional
- insomnia
- sweating and heat intolerance
- tendancy to lose weight easily
