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REB > 4. Thyroid Hormone > Flashcards

4. Thyroid Hormone Flashcards

1
Q

When does the thyroid gland begin to develop?

A

Day 24

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2
Q

What does the thyroid gland originate from?

A

Proliferation of endodermal epithelial cells on median surface of developing pharyngeal floor

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3
Q

What does the thyroid precursor develop from?

A

Thyroglossal duct

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4
Q

What happens in relation to the thyroid gland by weeks 7-10?

A

Thyroglossal duct obliterates

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5
Q

When is the thyroid gland fully developed?

A

10 weeks

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6
Q

What is the term used to describe disorders of thyroid gland formation?

A

Thyroid dysgenesis

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7
Q

What is an ectopic thyroid gland?

A

Thyroid gland doesn’t reach final position at front of neck

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8
Q

What is the aetiology of a thyroglossal duct cyst?

A

Duct doesn’t atrophy

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9
Q

Where can the parathyroid glands be found?

A

Posterior surface of the thyroid

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10
Q

What is the function of parathyroid hormone?

A

Increases blood calcium levels

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11
Q

What are parafollicular or c-cells?

A

Single cells in epithelial lining of follicle or in groups in connective tissue between follicles
Secrete calcitonin

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12
Q

What is the function of calcitonin?

A

Lowers blood calcium levels

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13
Q

Describe the structure of the thyroid gland

A

Divided into clusters of follicles by connective tissue

Follicles contain a thin layer of epithelial cells and colloid

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14
Q

What does the synthesis of thyroid hormones require?

A

Thyroglobin

Iodine

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15
Q

What type of molecule is thyroglobin?

A

Large glycoprotein

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16
Q

Where is thyroglobulin made?

A

Epithelial cells surrounding follicle

17
Q

What stimulates the production of thyroglobulin?

A

TSH

18
Q

Describe thyroid hormone synthesis

A
  1. Thyroglobulin secreted into follicle lumen
  2. TSH causes cells to actively transport I- into cytosol
    I- is converted to active iodine
  3. Iodine enters follicle lumen
    Attaches to tyrosine in colloid
    Forms DIT and MIT
  4. Iodinated tyrosines link together to form T3 and T4
  5. Thyroglobulin colloid is endocytosed
    Combined with lysosome
  6. Lysosomal enzymes cleave T3 and T4 from thyroglobulin colloid
    Hormones diffuse from follicle cell into the bloodstream
19
Q

What is thyroperoxidase?

A

Enzyme on colloid facing membrane of epithelial ells

20
Q

What reactions does thyroperoxidase catalyse?

A

Ioidination of tyrosines on thyroglobulin

Coupling reactions

21
Q

How are T3 and T4 transported in the blood?

A

Bind to throxine-binding globulins or albumin

22
Q

What are the benefits of having T3 and T4 bound to transport proteins?

A

Stops them having unregulated effects
Protects from enzyme attack and renal excretion
Longer half life than other hormones

23
Q

How much iodine is contained in the average adult body?

A

20-50mg

24
Q

What percentage of the iodine in the body is contained in the thyroid?

A

60%

25
Q

What types of areas have low iodine in the soil?

A

Glaciated, mountainous, heavy rainfall

26
Q

What foods are sources of iodine?

A

Seafood, iodized salt, milk

27
Q

How does dietary iodine get into the bloodstream?

A

Absorbed in the GI tract
Converted to iodide in the gut
Circulates bound to serum proteins

28
Q

What transports iodide into follicular cells?

A

Na+/I- co-transporter

29
Q

What inhibits the production of TRH?

A

Intracellular T3

Circulating T4

30
Q

What regulates TSH synthesis?

A

Increased by TRH
Decreased by T3
Slightly higher during the night

31
Q

What are the functions of TSH?

A
  1. Increases size and vascularity of the thyroid gland
  2. Increases iodide transport into follicular cells
  3. Increases production and iodination of thyroglobulin
  4. Increases endocytosis of colloid from lumen
32
Q

What plasma proteins can thyroid hormones bind to?

A

Thyroxine binding globulin
Thyroxine binding pre-albumin
Albumin

33
Q

Which plasma protein has high affinity and low capacity for thyroid hormones?

A

TBG

34
Q

What is the type of receptor that thyroid hormones bind to?

A

Retinoid X Receptor

REB (29 decks)

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