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2 MEH > 19 Thyroid Gland > Flashcards

19 Thyroid Gland Flashcards

. • Describe how the activity of the thyroid gland is controlled. • Describe the effects of thyroid hormones on cells and the body as a whole. • Describe the consequences of over- and under- secretion of thyroid hormones.

1
Q

Where is the thyroid gland located?

A

In front of larynx and trachea.

Below thyroid cartilage (Adam’s apple).

Isthmus crosses 2nd and 3rd rings of trachea (parathyroid glands sit behind)

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2
Q

Describe (in general terms the structure of the thyroid gland.

A

2 lobes, ISTHMUS joins lobes, bow tie shape

(parathyroid glands sit behind)

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3
Q

What is the first endocrine gland to develop embryologically?

A

Thyroid gland

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4
Q

How does the thyroid gland develop embryologically?

A

= EPITHELIAL PROLIFERATION at floor of pharynx at floor of tongue–> migrates to final position–> migrates down- passes in front of hyoid bone–> thyroglossal duct whichconnects tongue to gland- degenerates.

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5
Q

What is a follicular cell?

A

Cells-make up thyroid gland. =arranged in spheres (thyroid follicles)

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6
Q

What is colloid?

A

= deposit of thyroglobulin- fills follicles (=’extracellular)

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7
Q

How can you differentiate between parathyroid and thyroid gland cells histologically?

A

Parathyroid

=consists of ‘chief cells’-produce PTH

Thyroid

=Follicular cells produce Thyroid hormone

=Parafollicular cells produce Calcitonin

=Colloid stores Thyroglobulin

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8
Q

What are tetraiodothryonine (thyroxine) and triiodothyronine respectively?

A

T3 and T4 hormones

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9
Q

What are T3 and T4 formed from?

A

Two tyrosines + iodine

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10
Q

What’s the name of the protein that acts as a scaffold on which thyroid hormones are formed?

A

Thryoglobulin

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11
Q

What are the 3 reactions required for the generation of thyroid hormones from thyroglobulin proteins?

A

1) Oxidation- iodine-iodine
2) Addition of Iodine -to tyrosine acceptor residues on protein thyroglobulin
3) Coupling- tyrosines to generate thyroid hormones

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12
Q

Which enzyme regulates the 3 reactions required for the generation of thyroid hormones from thyroglobulin proteins?

A

Thyroid Peroxidase

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13
Q

Where can 90% of the iodine in the body be found?

A

Thyroid gland

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14
Q

In what form is iodine absorbed in the small intestine?

A

Iodide (reduced iodine)

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15
Q

How is iodide taken up from the blood by thyroid epithelial cells?

A

Sodium-iodide symporter (‘iodine trap’)

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16
Q

Give examples of common dietary sources of iodine.

A

Dairy products, grains, meat, vegetables

17
Q

What is the relationship between T3 and T4?

A
  • 90% secreted thyroid hormone= T4
  • Biological activity of T3= 4x higher than T4
18
Q

Where is most T4 converted to T3?

A

Liver and kidneys

19
Q

How are T3 and T4 transported in the blood?

A

Bound to protein: THYROXINE BINDING GLOBULIN

20
Q

In general terms, what effects do thyroid hormones have?

A
  1. Effect METABOLISM
  2. Effect GROWTH AND DEVELOPMENT
21
Q

How does the negative feedback work in thyroid hormone regulation?

A

1- TSH negative feedback on hypothalamus

2- Thyroid hormone negative feedback on hypothalamus and anterior pituitary

22
Q

What is the structure of TSH?

A

Glycoprotein hormone

23
Q

How does TSH trigger Thyroid hormone release?

A

TSH stimulates: iodide uptake and oxidation, thyroglobulin synthesis, thyroglobulin iodination, colloid pinocytosis into cell, proteolysis- thyroglobulin, cell metabolism and growth

24
Q

What are the 2 second messenger pathways induced by TSH?

A

Adenyl cyclase & Phopholipase C

25
Q

What actions does thyroid hormone exhibit?

A
  1. INCREASE BASAL METABOLIC RATE AND HEAT PRODUCTION- increase number and size of mitochondria, stimulate synthesis of enzymes in respiratory chain
  2. STIMULATE METABOLIC PATHWAYS-lipid metabolism (lipolysis), carbohydrate metabolism- increase gluconeogenesis and glycogenolysis
  3. SYMPATHOMIMETIC EFFECTS- increase receptor number on CATECHOLAMINE target cells
26
Q

What happens if the heart’s responsiveness to catecholamines is increased?

A

CO increases, increase peripheral vasodilation- carry heat to body surface

27
Q

What effect does Thyroid hormone have on the nervous system?

A

Increase myelination of nerves and neutron development

28
Q

How do Thyroid Hormone receptors function? (=in nucleus)

A

=Hormone-activated transcription factors- modulate gene expression. Bind DNA- in absence of hormone- transcriptional repression

29
Q

What happens once Thyroid hormone has entered the cell?

A

Thyroid hormone= lipid soluble- enters cell through thyroid hormone transporters. Receptor=pre-bound to DNA sequences. Thyroid hormone binds- relieves repression- new protein expressed

30
Q

Give an example of a thyroid hormone activated gene.

A

PEPCK, Sodium- potassium ATPase

31
Q

Apart from TSH- what other variations are thyroid hormone levels sensitive to?

A

Albumin changes, dilution effects

2 MEH (18 decks)

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