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RE > Thyroid Hormone Physiology > Flashcards

Thyroid Hormone Physiology Flashcards

1
Q

What is the largest endocrine gland?

A

Thyroid

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2
Q

What is the main function of the thyroid?

A

Regulate basal metabolic rate

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3
Q

What dietary substance does the thyroid need to function?

A

Iodine

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4
Q

What is dietary iodine (I2) absorbed as?

A

Iodide

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5
Q
A
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6
Q

The thyroid is located below which structure? Which cartilage is it located at either side of?

A

Below the larynx
Either side of thyroid cartilage (cricoid cartilage)

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7
Q

Which two muscles is the thyroid covered by?

A

Strap muscles from the neck and overlapped by sternocleidomastoid

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8
Q

What fascia is the thyroid enclosed by?

A

Pretracheal fascia - attaches thyroid to the larynx

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9
Q

Where does the thyroid gland develop from?

A

Floor of the pharynx, near the root of the tongue.
It descends into he neck as a down growth.

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10
Q

What colloid substance are thyroid follicles rich in?

A

Thyroglobulin

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11
Q

What type of protein is thyroglobulin?

A

Large tyrosine rich dimeric glycoprotein

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12
Q

What hormones does the thyroid gland secrete?

A

T4 - Thyroxine (majority)
T2 - Triiodothyronine

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13
Q

T4 + T3 originate from which hormone?

A

Tyrosine
They have added iodine (T4 = 4, T3 = 3)

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14
Q

Which thyroid hormone has a longer half life?

A

T4

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15
Q

What do thyroid C cells produce and what is it involved in?

A

Calcitonin
Involved in calcium homeostasis

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16
Q

Which hormone is a precursor of T3?

A

T4.
Can be converted to active T3 via removal of one iodine.

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17
Q

T4 is converted to T3 primarily in which extra-thyroidal tissues?

A

Liver and Kidneys

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18
Q

How is iodine removed from T4 to convert this into T3?

A

Via Iodothyronine deiodinase

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19
Q

What are the three types of deiodinases?

A

D1 - liver, kidney, thyroid
D2 - Muscle, brain, pituitary, skin and placenta
D3

20
Q

Which deiodinase creates active T3 and how?

A

D1 + D2
Via outer ring deiodination

21
Q

Which deiodinase creates inactive T3 and how?

A

D3
Via inner ring deiodination

22
Q

How is thyroglobulin synthesised?

A

Thyroglobulin and thyroid peroxidase are synthesised in the RER –> Then transported to Golgi –> Packaged in vacuoles –> Thyroglobulin exocytosis into colloid space through microvilli. Thyroid peroxidase sits on apical membrane.
Iodide enters cell –> transported to apical border via Pendrin where there are microvilli –> Iodide converted to iodine + iodine binds to thyroglobulin, both via thyroid peroxidase.
This creates Iodinated thyroglobulin that is stored in colloid.

23
Q

How is thyroglobulin broken down?

A

Colloid droplets from colloid space go though endocytosis into apical cell membrane.
Lysosome fuses with droplet + enzyme breaks thyroglobulin down into T3 + T4

24
Q

What type of cells are thyroid cells?

A

Cuboidal epithelial cells

25
Q

Which protein assists Iodide across the cell into the colloid space?

A

Pendrin

26
Q

How does iodide enter the basolateral membrane of the thyroid cell?

A

Via Sodium/Iodide symporter

27
Q

Thyroid peroxidase binds tyrosine and iodine to create which molecules in the colloid space?

A

X1 Iodine + Tyrosine = MIT (Monoiodotyrosine)
X2 Iodine + Tyrosine = DIT (Diiodotyrosine)

28
Q

MIT + DIT are coupled together via thyroxine peroxidase and transported back into the apical membrane of the cell via which process?

A

Endocytosis

29
Q

In the Thyroid follicular cell, MIT + DIT, and DIT + DIT are bonded to create which hormones?

A

DIT + DIT = T4
MIT + DIT = T3

30
Q

The Sodium/Iodide Symporter is reliant on the sodium gradient created by which pump?

A

Na+/K+ ATPase

31
Q

The Sodium/Iodide Symporter transports 1 iodide ion against how many sodium ions?

A

2

32
Q

What regulates thyroid hormone secretion?

A

Hypothalamus Pituitary Thyroid Axis

33
Q

How does the HPT Axis work?

A

Hypothalamus releases Thyrotropin Releasing Hormone (TRH) –> Stimulates anterior pituitary to release Thyroid Stimulating Hormone (TSH) –> Production of T3 + T4 in thyroid –> Negative feedback loop to hypothalamus and anterior pituitary to inhibit TRH + TSH secretion.

34
Q

Thyrotropin releasing hormone (TRH) is transported to the pituitary via which system?

A

Hypothalamic - Hypophyseal portal system

35
Q

Which cells in the anterior pituitary gland produces TSH?

A

Thyrotrope cells

36
Q

TSH receptor binding activities which enzymes?

A

AC –> cAMP
PLC

37
Q

How does cold exposure affect thyroid hormone stimulation?

A

Hypothalamus mechanism that controls body temperature gets excited –> increases production and secretion of TRH –> reduces metabolic rate

38
Q

How does fasting affect thyroid hormone stimulation?

A

Fasting reduces leptin levels –> reduces expression/secretion of TRH, TSH + thyroid hormones –> reduces metabolic rate

39
Q

How does emotion affect thyroid hormone stimulation?

A

Emotional reactions stimulate the SS –> decreased TSH secretion.

40
Q

What are T4 + T3 bound to in the blood?

A

Plasma proteins.
E.g. Thyroxine binding globulin (TBG), Thyroxine binding prealbumin or transthyretin (TTR) + Albumin (All liver synthesised)

41
Q

How quickly are T4 + T3 released into tissues from blood?

A

Half of blood T4 released every 6 days
Half of blood T3 released every day as there is lower affinity to plasma proteins.

42
Q

How to T3 + T3 enter cells?

A

via active membrane transport.
MCT8 transporter protein.

43
Q

Which hormone binds to Thyroid hormone receptors?

A

T3 - high affinity binding.

44
Q

Thyroid hormone receptors need to bind to which other receptors to form heterodimers?

A

Retinoid X Receptors (RXR)

45
Q

Once T3 binds to Thyroid Hormone Receptors + Thyroid Hormone Receptors bind to Retinoid X Receptors, what does this allow Thyroid Hormone Receptors to bind to?

A

Thyroid Hormone Response elements/Hormone response elements (HRE).
These are short DNA sequences that regulate gene expression.

46
Q

What cellular mechanisms are affected in response to thyroid hormone activity?

A
47
Q

Which body functions are affected in response to thyroid hormone activity?

A

RE (18 decks)

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