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endocrine > the thyroid gland > Flashcards

the thyroid gland Flashcards

1
Q

two active forms of thyroid hormone

A

T3 (triiodothyronine)

T4 (thyroxine)

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2
Q

cell types within thyroid gland

A

C (clear) cells and

follicular cells

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3
Q

clear cells

A

secrete calcitonin (Ca2+ regulating hormone)

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4
Q

follicular cells

A

support thyroid hormone sythesis and surround hollow follicles

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5
Q

thyroid follicles

A

spherical structures whose walls are made of follicular cells,
centre of follicle filled with colloid = sticky glycoprotein matrix,
contains 2-3mo supply TH

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6
Q

what do follicular cells manufacture

A

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

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7
Q

thyroglobulin

A

large protein rich in tyrosine residues

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8
Q

what do follicular cells do with iodide

A

actively concentrate iodine from the plasma and transport it into the colloid

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9
Q

what happens to iodide in the colloid

A

it combines with the tyrosine residues to form the thyroid hormones

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10
Q

where are tyrosine and iodide derived from

A

the diet

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11
Q

how does iodide enter follilular cells from plasma

A

via a Na+/I- transporter (symport)

the coupling of Na+ enables the follicular cells to take up iodide against a concentration gradient

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12
Q

how is iodide transported from follicular cell to colloid

A

via the pendrin transported

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13
Q

thyroid peroxidase (aka thyroperoxidase)

A

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

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14
Q

addition of one iodine to tyrosine

A

MIT (monoiodotyrosine)

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15
Q

adding two iodine to tyrosine

A

DIT (diiodotryosine)

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16
Q

what happens to MIT and DIT

A

undergo conjugation reactions

17
Q

MIT + DIT

A

triiodothyronine (T3)

18
Q

DIT + DIT

A

tetraiodothyronine - thyroxine (T4)

19
Q

what does TSH stimulate

A

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

20
Q

which plasma protein do T3 and T4 mainly bind to

A

thyroxine-binding globulin

21
Q

circulating thyroid hormone

A

> 99.8% T3 and T4 circulates bound to plasma protein

only free hormone exerts an inhibitor effect on TSH and TRH

22
Q

T3 vs T4

A

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

23
Q

T4 deiodination

A

T4 is deioinated to T3 by deiodinase enzymes

around half of T4 deiodination occurs in plasma, the rest inside target cells

24
Q

TSH secretion

A

continuous secretion and stable [plasma] driven by TRH from hypothalamus

25
Q

TSH inhibition

A

glucocorticoids and somatostatin (GHIH) are inhibitory

26
Q

how does TH work

A

bind to nuclear receptors inside target cells where they change transcription and translation to alter protein synthesis

27
Q

actions of TH

A
  • 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
28
Q

graves disease

A

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

29
Q

hyperthyroidism symptoms

A
  • weight loss + heat intolerance
  • muscle weakness
  • hyperreflexia
  • psychological disturbances
  • inc HR
30
Q

Hashimoto’s disease

A

hypothyroidism

autoimmune atttack of thyroid gland

31
Q

hypothyroidism symptoms

A 
weight gain 
cold intolerance
brittle nails
slow speech 
hyporreflexia 
fatigue 
slow HR
32
Q

goitre

A

enlargement of thyroid gland

can occur in hyper and hypo-thyroidism

33
Q

cause of goitre formation

A

increased trophic action of TSH on thyroid follicular cells (hypo) or over-activity as result of autoimmine (Graves_)

–> results in hypertrophy of thyroid gland

endocrine (19 decks)

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