The Thyroid Gland Flashcards
what 2 hormones does the thyroid gland synthesise?
- T3 (triiodothyronine)
- T4 (thyroxine)
what are the 2 cell types that the thyroid gland contains?
- C (clear cells)
- Follicular cells
what is the function of C cells?
secrete calcitonin (calcium regulating hormone)
what is the function of follicular cells?
support thyroid hormone synthesis and surround hollow follicles
what is the centre of the thyroid follicle composed of?
a sticky glycoprotein matric called colloid
what are 3 functions of follicular cells ?
- produce enzymes that make thyroid hormones
- thyroglobulin
- concentrates iodine from plasma & transports it into the colloid
what is thyroglobulin?
protein rich in tyrosine residues
what happens to the enzymes and thyroglobulin one they are made in the follicles?
they are packaged into vesicles & exported from the cell into the colloid
where are tyrsoine and iodine derived from?
diet
describe the steps involved in thryoid hormone synthesis (3)
- Iodide enters the follicular cells from the plasma via Na+/I- transporter. The coupling to sodium enables the follicular cells to take up iodide against a concentration gradient.
- iodide then transported into colloid via pendrin transported –Iodide transport into thyroid gland is inhibited by thiocyanates, formed from detoxification of cyanide- origin is cigarette smoke
-
thyroid peroxidase enzyme adds iodine to tyrosine residues in thyroglobulin to make T3 & T4
- in the process, iodide loses an electron and becomes IODINE
Addition of one iodine to tyrosine –> MIT (monoiodotyrosine)
Adding a second iodine –> DIT (diiodotyrosine)
MIT & DIT then undergo reactions. What are the products of these reactions?
- MIT + DIT –> triiodothyronine or T3
OR
- DIT + DIT –> tetraiodothyronine or Thyroxine T4.
what happens to the thyroid follice in repsonse to TSH?
- parts of colloid are taken back up into the follicular cell by endcoytosis. Within the cells, they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to free hormones T3&T4
- both T3 & T4 are lipid soluble so pass across follicular cell membrane into plasma, where >99.8% of them bind to Thyroxine binding globulin (TBG)
- free T3 & T4 enter circulation
to what does TBG have a particular affinity for?
T4; therefore releases it slowly into plasma hence why it has a logner half-life
only what kind of hormone exerts an inibitor effect on TSH & TRH?
free hormone ie not bound to protein
what does most TH circulate in the form of?
protein bound T4- 50x more than T3
although there is more circulating T4 in plasma, 90% of TH binding to TH receptors inside cells is to T3. So, how does this happen?
T4 is deiodinated to T3 by deiodinase enzymes:
- around half is deiodinated in plasma
- remaining fraction inside target cells
what are 4 effects of thyroid hormone function?
- raises metabolic rate & promotes thermogenesis
- increases hepatic gluconeogenesis
- net increase in proteloysis in (also build up, but break down is greater)
- net increase in lipolysis
why is TH critical for growth?
- anabolic at normal levels, but catabolic at abnormal levels
- facilitates action of GH by increasing GH recpetor expression
- required for foetal brain development (deficinecy = congenital hypothyroidism)
What is a goitre?
a signficant enlargement of the thyroid gland, seen in thyroid pathologies such as hypo & hyperthyroidism
What may goitre formation be caused by?
- increased trophic action of TSH on thyroid follicular cells (hypothyroidism)
OR
- overactivity as a result of autoimmune disease (hyperthyroidism)
outline the flowchart which shows how goitre occurs due to low iodine in diet –> hypothyrodism
if the defect is in…
- the thyroid
- the pituitary
- hypothalamus
what are they called as?
- primary hypo/hyperthyroidism
- secondary hypo/hyper…
- tertiary hypo/hyper…
