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Endocrinology > The Thyroid gland > Flashcards

The Thyroid gland Flashcards

1
Q

anatomy of the thyroid

A
  • Lies against and around front larynx and trachea
  • Below thyroid cartilage (Adams apple)
  • 2 lobes joined by an isthmus
  • now tie shaped
  • Isthmus extends from 2nd and 3rd rings of the trachea
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2
Q
A
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3
Q

parathryoid and thyroids are

A

distinct glands

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4
Q

embryological development of the thyroid gland

A
  • First endocrine gland to develop
  • At 3-4 weeks gestation it appears as an epithelial proliferation in floor of pharynx at base of the tongue and takes several weeks to migrate to final position
  • First descends as diverticulum through thyroglossal duct and migrates downwards passing in front of hyoid bone
  • During migration remains connected to tongue by thyroglossal duct which subsequently degenerates
  • Detaches thyroid then continues to its final position over following 2 weeks
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5
Q

follicular cells are arrnaged in spheres called

A

thyroid follicles

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6
Q

thyroid follicles are filled with

A

colloid- a deposit of thyroglobulin

(extracellular even though inside follicle)

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7
Q

The thyroid and parathyroid are distinct glands

A
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8
Q

parafollicular cells (distinct from parathryoid principle cells) produce

A

calcitonin

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9
Q

thryoid hormone synthesis

A
  • T3- triiodothyronine
  • T4- tetraiodothyronine (thyroxine)
  • 2 tyrosine linked together with iodine at three or four position on the aromatic ring
  • Thyroglobulin acts as a scaffold on which thyroid hormones are formed
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10
Q

Thyroglobulin acts as

A

a scaffold on which thyroid hormones are formed

  • occuring on tyrosine residues
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11
Q

dietary idoine is …… before absorption in the small intestine

A

reduced

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12
Q

Thyroid hormone and precursors are the only molecules in the body that contain

A

iodine

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13
Q

thyroid gland contains ……% of iodine in the body

A

90-95%

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14
Q

how is iodide taken up by thyroid epithelial cells

A

Iodide (I-), is taken up from blood by thyroid epithelial cells, which have a sodium iodide symporter (iodine trap)

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15
Q

sources of iodine

A

iodized salt, dairy products, grains and meat

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16
Q

thyroid peroxidase

A

Membrane bound enzyme that regulates 3 separate reactions involving iodine

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17
Q

name the three reactiosn thyroid peroxidase regulates

A
  • Oxidation of iodide to iodine (requires H2O2)
  • Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin
  • Coupling of MIT or DIT to generate thyroid hormones within the thyroglobulin protein
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18
Q

thyroid hormone synthesis

A
  1. Important amino acids imported from the plasma into the cell for thyroglobulin synthesis
  2. Thyroglobulin exported into colloid
  3. Iodide (recued dietary iodine) then imported into the cell from the plasma via the iodide trap (sodium iodide symporter)
  4. Iodide oxidised to iodine by thyroid peroxidase
  5. Iodination of thyroxine residues by thyroid peroxidase
  6. Coupling of tyrosine residues together by thyroid peroxidase
  7. Colloid taken into cell via pinocytosis to form a vesicle full of thyroglobulin
  8. Lysosome fuses with vesicle
  9. These enzymes degrades the thyroglobulin protein releasing thyroid hormone
  10. Deiodinase enzyme will recycle any unused iodine
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19
Q

T3/T4 ratio

A

90% of thyroid hormone secreted is T4- Most T4 is converted to T3 outside thyroid

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20
Q

difference between T3 and T4

A
21
Q

T3 is ……more biologically active than T4

A

4x

22
Q

where is most T4 converted to T3

A

in the liver and kidney

23
Q

80% of ciruclating Thyroid hormone is

A

T3 derived from T4

24
Q

how are T3 and T4 transported in the blood

A

thyroxine-binding globulin

25
Q

regulation of thryoid hormone

A

negative feedback

  • when thyroid hormoen is high it provides negartive feedback to both the AP (stop producing TSH) and the hypothalamus (stop producing TRH)
26
Q

Thyroid hormone effect virtually every cell in the body and have 2 interconnected responses

A
  1. Effects on cellular differentiation and development
  2. Effect on metabolic pathways
27
Q

Tyroid stimulating hormones (TSH)

A
  • Glycoprotein hormone composed of 2 non-covalently bound subunits (alpha and beta)
  • (Alpha subunit also present in FSH and LH)
  • B subunit provides unique biological activity
28
Q

effects of TSH

A

Triggers the release of thyroid hormone

29
Q

TSH stimulates which axtions

A

Iodide uptake

Iodide oxidation

Thyroglobulin synthesis

Thyroglobulin iodination

Colloid pinocytosis into cell

Proteolysis of thyroglobulin

Cell metabolism and growth

30
Q

general action of thryoid hromone

A

1) Increase in basal metabolic rate and heat production
2) Stimulation of metabolic pathways
3) Sympathomimetic effects

31
Q

1) Increase in basal metabolic rate and heat production

A
  • In most tissues (except the brain, spleen and testis), thyroid hormones stimulate the metabolic rate by:
    • Increasing the number and size of mitochondria
    • Stimulating the synthesis of enzymes in the respiratory chain
32
Q

2) Stimulation of metabolic pathways

A

Catabolic pathways are generally stimulated more than anabolic

  • Lipid metabolism: stimulates lipolysis and B-oxidation of fatty acids
  • Carbohydrate metabolism: stimulates insulin-dependent entry of glucose into cells and increase gluconeogenesis and glycogenolysis
33
Q

3) Sympathomimetic effects

A

Increases target cell response to catecholamines by increasing receptor number on target cells

34
Q

Sympathomimetic effects on the cardiovascular system

A
  • increases hearts responsiveness to catecholamines
    • Increased CO (increase heart rate and force of contraction)
    • Peripheral vasodilation to carry extra heat to body surface (B2 adrenergic receptors)
35
Q

Sympathomimetic effects on the nervous system

A

Essential for both development and adult function

Increased myelination of nerves and development of neurones

36
Q

thyroid hormone receptor is a

A

nuclear receptor

37
Q

function of thyroid hormone receptor

A
  • Modulate gene expression
  • Thyroid hormone receptors bind DNA in the absence of hormone leading to transcriptional repression
  • Hormone binding associated with a conformational change in the receptor that causes it to function as a transcriptional activator
38
Q

normal plasma level of thyroid hromone

A
39
Q

goitre

A
  • Enlargement of the thyroid gland
  • Develops when the thyroid gland is overstimulated
  • May accompany either hypo- or hyperthyroidism (but not necessarily present in either)
40
Q

signs of hypothyroidism

A

Low T3

Low T4

Elevated TSH

41
Q

hypothyroidism in infants

A

cretinism

  • dward stature
  • mental deficiency
  • poor bone dev
  • slow pulse
  • muscle weakness
  • GI disturbance
42
Q

hypothyroidism in adults

A

myxedema

  • thick puffy skin
  • muscl weakness
  • slow speech
  • mental deterioration
  • intolerance to cold
43
Q

autoimmune hypothyroidism

A

hashimotos

44
Q

hyperthryoidism signs

A
  • high T3
  • high T4
  • low TSH
45
Q

autoimmune hyperthyroidism

A

graves disease

46
Q

method used to scan the thyroid

A

thyroid scintigraphy

47
Q

thyroid scintigraphy uses

A

Technetium-99m (99mTc) used for isotope scanning of the thyroid with a gamma camera.

48
Q

biological halflife of Technetium-99m

A

1 day (radiation xpsoure low)

Endocrinology (12 decks)

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