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Pharmacy Stage 3 - Endocrine System > Thyroid Hormone - Thyroid Gland > Flashcards

Thyroid Hormone - Thyroid Gland Flashcards

1
Q

what cells are responsible for thyroid hormone production?

A

thyroid follicular cells

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2
Q

why is iodine important in the diet?

A

to help in thyroid hormone synthesis

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3
Q

what amino acid are thyroid hormones formed from?

A

tyrosine

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4
Q

what active forms of the thyroid hormone are also released into the bloodstream?

A

thyroxin (T4) and tri-iodothyronine (T3)

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5
Q

what is the major secreted product from the thyroid gland?

A

T4 - thyroxin

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6
Q

which is more active T4 or T3

A

T3 - iodothyronine

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7
Q

what binds to T3 and T4 in the circulation?

A

thyroxine-binding globulin (TBG)

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8
Q

what is used to convert T4 to T3 at the liver, kidney and other tissues with high blood flow?

A

5’-deiodinase

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9
Q

How does T3 regulate gene transcription?

A
  1. The Thyroid hormone receptor (TR) is in the nucleus, bound to retinoid x receptor (RXR) to form a dimer
  2. This dimer is bound to a thyroid response element (TRE) in the promoter region of hormone-sensitive genes.
  3. In the absence of T3 the dimer binds to a co-repressor protein (CoR) that silences transcription
  4. When T3 enters the nucleus, it displaces CoR and allows co- activator proteins (Co-A) to bind to the complex
  5. this then regulates gene transcription
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10
Q

how are thyroid hormones (T3 and T4 ) released?

A
  1. Hypothalamic nuclei releases Thyrotrophin releasing hormone (TRH)
  2. This stimulates the pituitary gland to release thyroid stimulating hormone (TSH)
  3. This then acts on the thyroid gland to release thyroid hormones (T3 & T4)
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11
Q

What four things does the TSH released from the pituitary gland increase?

A
  1. increases iodine uptake
  2. increases thyroglobulin synthesis
  3. increases endocytosis and proteolysis of thyroglobulin
  4. increases blood flow through the thyroid gland
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12
Q

what causes the negative feedback loop on the pituitary and hypothalamus when T3 and T4 have been released?

A

when the T3 levels are high, a negative feedback loop occurs to slow down/ stop their release

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13
Q

what are the roles of thyroid hormone?

A
  1. role in metabolism

2. role in growth and development

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14
Q

what are the metabolic functions of thyroid hormones?

A
  1. Carbohydrate metabolism - increased glycolysis, increased insulin secretion etc
  2. Fat metabolism - reduced fat stores, increased fatty acid conc in plasma etc
  3. Protein metabolism - increased protein degradation, loss of muscle mass providing fuel for gluconeogenesis
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15
Q

what is the effect of the thyroid hormone on the cardiovascular system?

A
  1. increased cardiac output
  2. increased heart rate
  3. increased ventilation rate
  4. increased sympathetic activity by potentiating catecholamine effects
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16
Q

what occurs when there is a Thyroid deficiency during development?

A

cretinism (congenital hypothyroidism)

17
Q

what is the role of the thyroid hormone in growth?

A

Thyroid hormone is needed to allow bone, cartilage and other connective tissues to respond to growth hormone

18
Q

what is the most common cause of congenital hypothyroidism?

A

iodine deficiency

19
Q

what is the effect of a lack of iodine in your diet and how does this cause enlarged thyroid glands?

A
  1. If iodine in diet is limited, little thyroid hormone can be synthesised
  2. Plasma levels of TSH will therefore rise since no negative feedback
  3. High levels of TSH will produce hypertrophy / hyperplasia of thyroid gland
  4. Increased mass of tissue will produce more thyroid hormone - thyromegaly
  5. In extreme cases – can lead to the development of a goitre
20
Q

where is the main source of iodine in diet?

A
  • sea food / marine salts

- Regions very remote from sea mainly use rock salt in diet

21
Q

what condition can cause high TSH secretion besides a lack of iodine in the diet?

A

A pituitary tumour

22
Q

what is hyperthyroidism and what is the most common cause of hyperthyroidism?

A

it is an overactive thyroid gland in which excess thyroid hormone is released and it is most commonly caused by Graves disease.

23
Q

what happens in graves disease to cause hyperthyroidism?

A

B-lymphocytes secrete glycoproteins which activate the TSH receptors on the thyroid gland stimulating thyroid hormone release

24
Q

what pharmacological treatment is given for hyperthyroidism?

A

Thioamides (e.g. methimazole)

25
Q

how does the thioamine methimazole work?

A

inhibits T3/T4 synthesis by inhibiting thyroid peroxidase

26
Q

what is hyperthyroidism and what is the most common cause of hyperthyroidism?

A

it is an underactive thyroid gland in which little to no thyroid hormone is released and it is most commonly caused by cause Hashimoto thyroiditis – autoimmune destruction of thyroid follicular cells. A lack of thyroid hormone being released results in high TSH levels.

27
Q

in which will TSH levels be raised: hypothyroidism or hyperthyroidism?

A

hypothyroidism

28
Q

what is used to treat hypothyroidism?

A

Levothyroxine - T4

L-tri-iodothyronine (T3) for faster action

29
Q

where are parathyroid glands found and what do they do?

A
  • found behind the thyroid: Two on each side of body

- Secrete parathyroid hormone

30
Q

what does parathyroid hormone do?

A

plays a crucial role in the control of circulating Ca2+ levels

31
Q

what happens to the parathyroid gland when calcium levels changes?

A
  1. Cells of parathyroid gland express a G protein-coupled receptor for Ca2+
  2. If [Ca2+ ] is high – PTH secretion inhibited
  3. If [Ca2+] is low – PTH is secreted
  4. This means that circulating [PTH] is dependent upon circulating [Ca2+]
32
Q

how is Vitamin D activated by parathyroid hormone (PTH) and Calcium?

A
  1. Vitamin D is metabolically inactive and must undergo 2 hydroxylation reactions to become active:
    - First occurs in the liver
    - Can undergo one of two reactions in kidney catalysed by aromatases:
    - Hydroxylation at 1 position (1alpha- aromatase) leads to activation
    - Hydroxylation at 24 position (24alpha- aromatase) leads to inactive metabolite
  2. Low [Ca2+] and PTH both stimulate 1a- aromatase therefore PTH (and low [Ca2+]) stimulate activation of vitamin D
33
Q

how does vitamin D increase circulating calcium concentration?

A
  1. Stimulates the absorption of Ca2+ by the small intestine – increases amount of Ca2+ obtained from diet
  2. Stimulates reabsorption of Ca2+ from the glomerular filtrate – this reduces the amount of Ca2+ lost in the urine
  3. Stimulates Ca2+ release from bone
34
Q

how does PTH control calcium levels?

A
  • when low levels of Calcium present, PTH is released
  • This stimulates 1,25-(OH)2-cholecalciferol (vitamin D)
  • vitamin D then causes Ca2+ levels to increase in the blood
35
Q

what is the direct effect of PTH on bone?

A
  1. PTH receptors are found on osteoblasts (bone formation) but not on osteoclasts (bone degradation)
  2. Initially PTH activates osteoblasts and evokes transient deposition of bone
  3. Long term stimulation of osteoblasts leads to release of cytokines from osteoblasts
  4. These cytokines stimulate the numbers and activity of bone degrading osteoclasts
  5. Long term effect of PTH stimulation is therefore to promote the release of Ca2+ from bone
  6. Restores circulating Ca2+ levels
36
Q

what hormone is released by the parafollicular cells found in the thyroid gland?

A

calcitonin

37
Q

how does calcitonin control Ca2+ levels?

A
  1. Calcitonin is secreted in response to elevated [Ca2+]
  2. Calcitonin decreases activity of osteoclasts so tends to shift balance in favour of deposition of Ca2+ in bone hence reducing plasma calcium levels
  3. Calcitonin has weak effects on kidney which favour a loss of Ca2+ from body via urine

Pharmacy Stage 3 - Endocrine System (9 decks)

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