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Mpharm 1 > PH1124 - thyroid hormones > Flashcards

PH1124 - thyroid hormones Flashcards

1
Q

how and when does the thyroid gland develop? (3)

A
  • appears very early in life

- develops about four weeks after conception as an epithelial invagination of the tongue

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2
Q

what is the anatomy of the thyroid gland? (4)

A
  • composed of 2 large lobes either side of the trachea below the larynx and joined by the isthmus
  • there is a rich blood supply from thyroid arteries
  • innervated by the autonomic nervous system
  • pyramidal lobe is often present
  • butterfly shape
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3
Q

what is the cellular structure of

the thyroid gland? (3)

A
  • its cells are arranged in follicles
  • there are millions of follicles in a thyroid gland
  • follicles are spherical bodies composed of epithelial cells surrounding a colloidal storage protein (thyroglobulin)
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4
Q

what does the height of epithelial cells depend on in the thyroid gland? (2)

A
  • inactive; epithelial cells are flattened with a large colloidal mass
  • active; cells are columnar in shape with a small colloidal lumen
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5
Q

what are the parafollicular C cells?

A
  • large epithelial cells that lie between the follicles and produce calcitonin
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6
Q

what are the main hormones produced by the thyroid gland? (2)

A
  • T3

- T4

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7
Q

how are thyroid hormones made?

A
  • dietary iodide is taken up by an iodide pump from the blood
  • at the apical surface I- is oxidised by hydrogen peroxide from thyroid peroxidase to I2
  • I2 is immediately incorporated into tyrosyl residue covalently bound to thyroglobulin TG at apical border to form MIT and DIT
  • MIT and DIT are coupled by thyroid peroxidase to form T3 and T4
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8
Q

how are thyroid hormones secreted?

A
  • TSH causes endocytosis and proteolysis by lysosomes of TG-hormone complex
  • this is where thyroids hormones are stored in the colloid
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9
Q

what simulates the uptake of iodide by the thyroid? (3)

A
  • TSH (thyroid stimulating hormone)
  • iodine deficiency
  • TSH receptor antibodies
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10
Q

what are the anions that inhibit the iodine pump? (4)

A
  • I-
  • perchlorate (ClO4-)
  • thiocyanate (SCN-)
  • pertechnetate (TcO4-)
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11
Q

how are the thyroid hormones transported? (2)

A
  • transported in the plasma bound to proteins where T3 is less avidly bound than T4
  • only free hormone not bound is biologically active
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12
Q

what are the proteins that thyroid hormones bind to in plasma? (3)

A
  • thyroid binding globulin
  • transthyretin
  • albumin
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13
Q

which thyroid hormone is more potent?

A
  • T3 is approx 10 times more potent than T4
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14
Q

how are thyroid hormones metabolised? (2)

A
  • T4 is de-iodinated to T3 in the periphery (liver and kidney)
  • T3 and T4 are conjugated to glucuronide or sulphate in the liver and excreted in the bile
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15
Q

how is iodine excreted?

A
  • excreted in the urine or re-circulated to the thyroid
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16
Q

what is TRH?

A
  • thyrotropin releasing hormone
17
Q

how is the release of thyroid hormones regulated?

A
  • a stimulus causes the hypothalalmus to release TRH which causes the anterior pituitary to release TSH
  • this causes the thyroid glad to produce T3 and T4
  • this directly stops the productions of TSH from the anterior pituitary therefore its a negative feed back loop
18
Q

how do somatostatin and dopamine effect the production of T3 and T4?

A
  • they cause the release of TSH from the anterior pituitary to be inhibited
  • less T3 and T4 is secreted from the thyroid gland
19
Q

what is the role of TRH? (2)

A
  • stimulates the release of thyrotrophin or thyroid stimulating hormone (TSH) from the anterior pituitary
  • releases prolactin
20
Q

what is TSH?

A
  • thyroid stimulating hormone
21
Q

what does TSH do to the thyroid?

A
  • acts on specific receptors on basal membrane

- increases synthesis of iodine pumps, thyroglobin and thyroid hormones

22
Q

what is the release of TSH inhibited by? (6)

A
  • thyroid hormones
  • cortisol
  • growth hormone
  • oestrogens
  • dopamine
  • somatostatin
23
Q

what is the mechanism of action for thyroid hormones at the nuclear level? (5)

A
  • T3 and T4 enter the cell by passive diffusion or active uptake
  • T4 is then de-iodinated to T3
  • T3 is actively transported into the nucleus and binds to cytosolic thyroid hormone binding protein (CTBP)
  • this ensures T3 binds to thyroid receptors in the nucleus already bound to regulatory thyroid hormone response elements TREs
  • thyroid hormone receptor auxiliary protein (TRAP) is then necessary to stabilise TR binding to DNA to either transcribe or inhibits genes
24
Q

what is the mechanism of action for thyroid hormones at the cellular level? (3)

A
  • T3 binds to membrane-associated receptors to activate Na+/K+ ATPase pump
  • this causes an increased uptake of glucose and amino acids
  • T3 also directly activates mitochondria to increase energy production
25
Q

how do thyroid hormones control basal metabolic rate?

A
  • via calorienesis
26
Q

what is calorigenesis?

A
  • increase O2 consumption in all tissues (except brain, spleen, testes and anterior pituitary) causing increased heat production
  • iimportant for thermoregulation
  • does this by synthesizing new ATPase pumps in cell membrane but also directly activates mitochondria
27
Q

what does calorigenesis block?

A
  • blockers of Na+/K+ ATPase pumps such as cardiac glycosides (dogoxin) inhibit actions of thyroid hormones
28
Q

what are the physiological actions of the thyroid hormones?

A
  • calorigenesis
  • carbohydrate, protein and fat metabolism
  • maturation of central nervous system
  • skeletal growth and maturation

Mpharm 1 (59 decks)

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