Thyroid and parathyroid glands Flashcards

1
Q

what is the first gland to appear in the embyro? when?

A

thyroid

4th week

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2
Q

what germ layer makes up the primitive thyroid gland?

A

endoderm that lines the primitive pharynx

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3
Q

when does the thyroid gland begin to function prenatally?

A

end of 3rd month

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4
Q

what is the role of the thyroglossal duct?

A

connects developing thyroid gland to the tongue

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5
Q

the thyroid capsule is developed from what tissue?

A

deep cervical fascia

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6
Q

the thyroid gland synthesizes what hormones?

A

T3
T4
calcitonin

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7
Q

what is the basic structural and functional unit of the thyroid gland?

A

follicle

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8
Q

the thyroid follicle is surrounded by what layers?

A

basal lamina
reticular fibers
fenestrated capillary plexus

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9
Q

what is contained within the lumen of the thyroid follicle?

A

colloid

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10
Q

the follicular epithelium is made up of what cell types? what are their activity levels?

A

follicular cells
principal cells

simple squamous - low activity
simple cuboidal - normal activity
simple columnar - high activity

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11
Q

which cell type synthesizes and secrete calcitonin?

A

parafollicular cells

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12
Q

thyroglobulin is contained in what organelle?

A

vesicles

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13
Q

by which two mechanisms does calcitonin lower calcium levels?

A
  • inhibits bone breakdown by osteoclasts

- promotes calcium deposition (osteoid calcification)

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14
Q

colloid consists of what glycoprotein?

A

thyroglobulin

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15
Q

synthesis of thyroid hormones is controlled by what mechanisms?

A

TSH

iodine levels

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16
Q

what cells synthesize thyroglobulin?

A

follicular cells

17
Q

how is iodide converted to iodine in the thyroid gland?

A

iodide in bloodstream is pumped into follicular cells by sodium / iodide symporters and is oxidized on the apical (microvillar) cell membrane facing the colloid into its active form - iodine

18
Q

how are MIT and DIT made?

A

iodine iodinates tyrosine residues of each thyroglobulin molecule to form MIT and DIT

19
Q

what cells secrete TSH?

A

basophils (thyrotropes)

20
Q

what organelle is the site of thyroglobulin synthesis?

21
Q

what organelles are the sites of thyroglobulin glycosylation?

A

RER and GA

22
Q

where does oxidation of iodide take place?

A

in the active sites of thyroid peroxidase (membrane bound enzyme) on the follicular APICAL cell membrane - facing the colloid

23
Q

what is needed for oxidation of iodide?

A

hydrogen peroxide

24
Q

where does oxidation of iodide occur?

25
what are the two pathways by which thyroglobulin can be processed following endocytosis? which is the main pathway?
lysosomal (main) | transepithelial
26
lysosomal pathway
- vesicles containing colloid unite with endosomes in cytoplasm - endosomes contain proteases which split the iodinated tyrosine residues from the thyroglobulin - iodinated tyrosine residues are released into the cytoplasm as MIT, DIT, T3, and T4 - iodotyrosine dehalogenase splits iodine from MIT and DIT - T3 and T4 are liberated from follicular cell basolateral membrane into surrounding CT to be picked up by capillaries -
27
T4 is only produced by which cell?
follicular cell
28
T3 is mainly produced how?
conversion from T4 by kidney, liver, and heart
29
transepithelial pathway
- thyroglobulin binds to megalin, a transmembrane protein receptor on apical follicular cell membrane facing colloid - thyroglobulin evades / bypasses lysosomal pathway and instead endocytic vesicles are transported to basolateral membrane of follicular cell - small amounts of T3 and T4 bound to thyroglobulin are released from basal follicular cell membrane into the blood and lymphatic capillaries
30
inside the cell T3 and T4 bind to what receptor type?
nuclear thyroid hormone receptor proteins
31
the parathyroid glands arise from which pharyngeal pouches?
inferior parathyroid glands - 3rd | superior parathyroid glands - 4th
32
what are the cell types of the parathyroid glands?
chief cells oxyphil cells intermediate cells
33
which parathyroid cell has secretory granules containing PTH?
chief cells
34
what is the main role of PTH?
regulation of calcium and phosphate levels in blood
35
PTH has influence on what organs?
bone kidney GI (vitamin D)
36
how does PTH regulate bone physiology?
- PTH attaches to osteoblast receptors | - causes osteoblasts to release osteoclast-stimulating factor - frees calcium from bone
37
how does PTH regulate kidney physiology?
- PTH prevents calcium loss in urine | - PTH promotes phosphate loss in urine
38
how does PTH regulate GI physiology?
- regulated formation of vitamin D in kidney - controls rate of calcium absorption from GI - vitamin D facilitates calcium absorption from GI
39
what would be the effect of complete removal of parathyroid glands?
- drop in calcium level | - tetanic contraction of muscles (laryngeal / respiratory muscles leads to death)