thyroid physiology Flashcards
in embryology where does the thyroid develop
foramen caecum
when does the thyroid reach its final position and where is it (embryology)
week 7 - C5-T1
what are the stages of thyroid development
1) endothermal thickening 2) thyroglossal duct formation 3) bifurcation at lower end of thyroglossal duct 4) thyroid develops 5) isthmus forms
what nerve supplies the thyroid
valgus (CNX)
what vasculature does the thyroid have
sup/ inf thyroid arters + 3 pairs of veins
what cells is the thyroid made of
follicles surrounded by flat and cuboidal follicle cells + occasional C/ para-follicular cells (larger)
what do thyroid follicles do
convert thyroglobulin –> T3/4
what do the centre of thyroid follicles look like
pink centre and contain thyroglobulin
what do C cells contain and what do they do
calcitonin which inhibits osteoclasts and lowers calcium (opposite to PTH)
how is the thyroid controlled by negative feedback
hypothalamus secretes TRH –> ant pit secretes TSH –> thyroid secretes T3 and T4 which reduce TRH and TSH production
how does thyroid stimulating hormone (TSH) activate thyroid cells
binds to receptor cell on thyroid, Ga protein increases cAMP which increases T3/ 4
how to functional follicle cells make thyroglobulin (5)
1) iodide uptake, 2) oxidation to iodine, 3) iodination of thyroglobulin 4) formation of MIT and DIT the precursors of T3/4 5) T3/4 stored until secretion
how are T3 + 4 made from MIT and DIT
MIT (mono) + DIT (di) –> T3.
DIT + DIT –> T4
what is the most abundant thyroid hormone and what is the major active thyroid hormone
90% thyroxine (T4), triiodothyronine (T3) more potent and majorly active
where is the majority of T4 converted to T3
liver and kidney
thyroid hormones have to travel bound to plasma proteins what are the main ones (3)
thyroxine binding globulin (TBG) 70%, thyroxine binding pre-albumin, albumin
what increases TBG and lowers free T4 (5)
pregnancy, oestrogen, Hep A + biliary cirrhosis, acute intermittent porphoryia, herion
what lowers TBG and does not effect free T4
androgens, cushing’s, acromegaly
how do thyroid hormones enter cells
unbound
what do thyroid hormones control generally (4)
METABOLISM, growth + development, reproduction, mood
what effect do thyroid hormones increase in cells
mitochondria number/ size, ATP hydrolysis, , increase met rate and glucose uptake, gylcogenolysis (decrease glycogenesis), liposysis (decreases lipogenesis) resp rate, HR, contractility (adrenaline + NA)
what effect do thyroid hormones have on growth and development
GHRH needs thyroid hormones - hypothyroidism causes slow intellectual function, hyperthyroidism causes hyper emotions
what enzyme degrades thyroid hormones
de-iodinase enzyme
where do you find D1, D2 + D3 enzymes for degredation
D1 = liver/ kidney, D2 = heart, skeletal, CNS, fat, thyroid, pit, D3 = fetal tissue, placenta, brain
what is a goitre
enlargement of thyroid gland
what is a diffuse goitre
smooth swelling of whole gland common in females and young people, normally associated with under/overactive thyroid
what is a multi-nodular goitre
solid multiple nodules, older patients, can predispose cancer, can rupture/ block airway
what thyroid function is seen with multi-nodular goitre normally
TSH normal/ low, high T3/4
what cells are thyroid stimulating hormones (TSH) released from in response to thyroid releasing hormone (TRH)
thyrotroph cells in ant pit
