the thyroid gland Flashcards
two active forms of thyroid hormone
T3 (triiodothyronine)
T4 (thyroxine)
cell types within thyroid gland
C (clear) cells and
follicular cells
clear cells
secrete calcitonin (Ca2+ regulating hormone)
follicular cells
support thyroid hormone sythesis and surround hollow follicles
thyroid follicles
spherical structures whose walls are made of follicular cells,
centre of follicle filled with colloid = sticky glycoprotein matrix,
contains 2-3mo supply TH
what do follicular cells manufacture
enzymes that make thyroid hormones as well as thyroglobulin ,
the enzymes and thyroglobulin are packaged into vesicles and exported from the folliluclar cells into the colloid
thyroglobulin
large protein rich in tyrosine residues
what do follicular cells do with iodide
actively concentrate iodine from the plasma and transport it into the colloid
what happens to iodide in the colloid
it combines with the tyrosine residues to form the thyroid hormones
where are tyrosine and iodide derived from
the diet
how does iodide enter follilular cells from plasma
via a Na+/I- transporter (symport)
the coupling of Na+ enables the follicular cells to take up iodide against a concentration gradient
how is iodide transported from follicular cell to colloid
via the pendrin transported
thyroid peroxidase (aka thyroperoxidase)
enzymes exocytosed into the colloid, along with thyroglobulin, catalysses the oxidation of iodide to iodine and the addition of iodine to tyrosine residues on the thyroglobulin molecule
addition of one iodine to tyrosine
MIT (monoiodotyrosine)
adding two iodine to tyrosine
DIT (diiodotryosine)
what happens to MIT and DIT
undergo conjugation reactions
MIT + DIT
triiodothyronine (T3)
DIT + DIT
tetraiodothyronine - thyroxine (T4)
what does TSH stimulate
portions of the colloid are taken back up into follicular cells by endocytosis
within cells they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release thyroid hormones
T3 and T4 are lipid soluble so pass across follicular cell membranes into plasma and bind to plasma proteins
which plasma protein do T3 and T4 mainly bind to
thyroxine-binding globulin
circulating thyroid hormone
> 99.8% T3 and T4 circulates bound to plasma protein
only free hormone exerts an inhibitor effect on TSH and TRH
T3 vs T4
most TH circulates in form of protein bound T4
however, TH receptor has much higher affinity for T3, making T3 more physiologically active than T4
T4 deiodination
T4 is deioinated to T3 by deiodinase enzymes
around half of T4 deiodination occurs in plasma, the rest inside target cells
TSH secretion
continuous secretion and stable [plasma] driven by TRH from hypothalamus
TSH inhibition
glucocorticoids and somatostatin (GHIH) are inhibitory
how does TH work
bind to nuclear receptors inside target cells where they change transcription and translation to alter protein synthesis
actions of TH
- raise metabolic rate and promote thermogenesis
- increase hepatic gluconeogenesis (no effect BG)
- net increase proteolysis
- net increase lipolysis
- critical for growth - stimulate GH receptor expression
- essential for brain development in utero
graves disease
hyperthyroidism
antibodies produced that mimic TSH and continually activate thyroid gland
inc release of TH switches off TSH release from anterior pituitary so TSH is low
hyperthyroidism symptoms
- weight loss + heat intolerance
- muscle weakness
- hyperreflexia
- psychological disturbances
- inc HR
Hashimoto’s disease
hypothyroidism
autoimmune atttack of thyroid gland
hypothyroidism symptoms
weight gain cold intolerance brittle nails slow speech hyporreflexia fatigue slow HR
goitre
enlargement of thyroid gland
can occur in hyper and hypo-thyroidism
cause of goitre formation
increased trophic action of TSH on thyroid follicular cells (hypo) or over-activity as result of autoimmine (Graves_)
–> results in hypertrophy of thyroid gland
