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Nutritional Physiology > Thyroid Hormone Metabolism & Hormonal Regulation of Growth > Flashcards

Thyroid Hormone Metabolism & Hormonal Regulation of Growth Flashcards

1
Q

What are the features of the thyroid gland?

A
  • located at base of neck
  • has a butterfly shape
  • 2 lobes connected by a central isthmus
  • high blood flow -> extensive capillary network
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2
Q

What cells are found in the thyroid gland?

A 
Follicular cells
C Cells (involved in calcium homeostasis)
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3
Q

What is found in the lumens of follicular cells?

A

Precursor to thyroid hormones

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4
Q

What are the 2 thyroid hormones?

A
  1. Thyroxine - T4 - 90%

2. Triiodothyronine - T3 - 10% (more potent)

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5
Q

How does T4 become T3?

A

De-iodination of outer I

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6
Q

What is needed for thyroid hormone synthesis?

A
  1. Iodine

2. Thyroglobulin precursor

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7
Q

Iodine characteristics

A
  • scarce dietary element (DRI 1mg/day)
  • low levels are absorbed
  • powerful iodine lump in follicular cells (20-50x higher in follicular cells)
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8
Q

Thyroglobulin precursor characteristics

A
  • over 5000 AAs
  • major component for T3 and T4
  • synthesised in follicular cells
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9
Q

What is the process of thyroid hormone synthesis? (6)

A
  1. Iodine trapping
  2. Synthesis of thyroglobulin
  3. Oxidation of iodine
  4. Iodination of tyrosine
  5. Coupling of tyrosine residues
  6. Pinocytosis and digestion of colloid
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10
Q
  1. Iodine trapping
A

Pump is and I-/Na+ symport - electrical neutrality

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11
Q
  1. Synthesis of thyroglobulin
A
  • occurs in endoplasmic reticulum
  • modified in Golgi body
  • vesicles of thyroglobulin are moved into lumen via exocytosis
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12
Q
  1. Oxidation of iodine
A
  • iodide moved into lumen by Cl-/I- antiport (pendrin)

- oxidised to iodide by hydrogen peroxidase

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13
Q
  1. Iodination of tyrosine
A
  • 1 I- = monoiodotyrosine (MIT)

- 2 I- = diiodotyrosine (DIT)

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14
Q
  1. Coupling of tyrosine residues
A
  • DIT + DIT= T4

- MIT + DIT = T3

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15
Q
  1. Pinocytosis and digestion of colloid
A
  • lysosomes cleave off T3 and T4 from backbone
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16
Q

Which transporter transports thyroid hormones?

A

Monocarboxylate transporter

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17
Q

How are thyroid hormones transported in the blood?

A
  • thyroxine binding globulin: 70%
  • transthyretin or thyroxine binding prealbumin - 10-15%
  • albumin: 15-20%
  • unbound T4 (fT4): 0.03%
  • unbound T3: 0.3%
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18
Q

How are thyroid hormones transported into cells?

A

Monocarboxylate transporter

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19
Q

What do thyroid hormones do in the cell?

A
  • all T4 is converted to T3: requires Se and Zn co factors

- T3 nucleus and binds to a hormone-responsive element: protein synthesis and various metabolic effects

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20
Q

What are the 4 effects of TSH?

A
  1. Increases hormone synthesis
  2. Increases hormone secretion into bloodstream
  3. Hyperplasia
  4. Increases blood flow to thyroid
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21
Q

GH is the most abundant hormone in the ………

A

Adenhypophysis

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22
Q

GH release is………..

A

Diurnal and pulsatile

23
Q

What are the effects of growth hormone?

A
  • increased protein synthesis
  • increased amino acid transport
  • increased lipolysis and release of FFAs
  • increased glucose production, serum glucose, decreases glucose uptake by adipose tissue
  • increased fibroblast differentiation
  • increased IGF production in liver and fibroblasts
24
Q

What does the hypothalamus release?

A

GHRH (+ve feedback) and somatostatin (-ve feedback)

25
Q

What factors affect the hypothalamus? (+ve feedback)

A
  • sleep
  • stress
  • exercise
  • ghrelin
  • amino acids
26
Q

Many of the growth effects of GH are stimulated indirectly through……..

A

The increased production of IGF

27
Q

Where is IGF produced?

A
  1. Liver (source of circulating IGF)

2. Skeletal muscle, heart and bone (paracrine and autocrine effects)

28
Q

IGFs are…………

A

Multifunctional hormones that regulate cellular proliferation, differentiation and metabolism

29
Q

Why are IGFs called IGFs?

A

Resemble insulin in structure and function

30
Q

Forms of IGF

A

IGF-1: major in adults

IGF-2: foetus

31
Q

What does IGF do?

A
  • promotes growth do cartilage, bone and soft tissues
  • glucose and AA uptake
  • protein and DNA synthesis
  • mitogens: increase proliferation
32
Q

During puberty:

A
  • GH and IGF levels increase simultaneuously
33
Q

……….. levels reflect the rates at which a child grows

A

IGF

34
Q

What is the role of insulin in IGF production?

A
  • stimulates production of IGF
  • GH cannot increase levels of IGF without insulin
  • ensures growth only occurs when sufficient nutrition is available
35
Q

Amino acids increase ……. levels

A

GH

36
Q

GH receptors:

A
  • found in all known target tissues
  • activated by dimerisation
  • have tyrosine kinase 2 activity, eg Janas associated kinase (JAK2)
37
Q

What does JAK2 do?

A

Activates transcription factors known as STATs, which increase protein synthesis

38
Q

IGF receptors:

A
  • similar to insulin receptors

- have tyrosine kinase activity

39
Q

Negative feedback of increased glucose and FFAs

A
  • hypothalamus

- secretes somatostatin

40
Q

What is a tumour of the pituitary called?

A

Adenoma

41
Q

What are the symptoms of acromegaly?

A
  • cartilage and bone enlarges: impinges in nerves (nerves in face: Bell’s palsy, reduced vision)
  • joint and bone pain
  • soft tissues enlarge: cardiac hypertrophy
42
Q

What causes undersecretion of GH?

A
  • tumours of hypothalamus so somatostatin is over secreted
  • infections
  • irradiation
  • GH receptor defects
43
Q

What is Larson syndrome?

A
  • genetic defect in GH receptor
  • high GH levels
  • IGF 1 levels low or undetectable
44
Q

What are the general effects of thyroid hormones?

A
  • increase nutrient breakdown

- increase ATP production

45
Q

What are the effects of TH on BMR?

A
  • increase BMR 60-100%
  • increase VO2
  • increase no. and size of mitochrondia
  • increase active transport of ions
46
Q

What is Hashimoto’s thyroiditis?

A

Autoimmune destruction of thyroid tissue

47
Q

What is post-ablative hypothyroidism?

A

Treatment for hyperthyroidism overcompensates

48
Q

What is a common cause of hypothyroidism?

A

Iodine deficiency

49
Q

What is hypothyroidism characterised by?

A
  • low T3 and T4

- high TRH and TSH

50
Q

What is hyperthyroidism characterised by?

A
  • high T3 and T4

- low TSH and TRH

51
Q

What happens in Graves’ disease?

A
  • thyroid stimulating IgG antibodies produced
  • same effect as TSH
  • treatment in medication that decreases TH production
52
Q

What happens when hypothyroidism occurs in children? (4)

A
  1. Severe cretinism
  2. Short stature
  3. Bone age retardation
  4. Malformation of facial bones
53
Q

What cause the outbreak of thyrotoxicosis in the US?

A
  • consumption of ground beef prepared from neck trimmings
  • contained TH
  • TH is orally active

Nutritional Physiology (13 decks)

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