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Med 3: Thyroid Disease > Thyroid Hormones > Flashcards

Thyroid Hormones Flashcards

1
Q

Which hormones do we mean when we say “thyroid hormones”?

A

T4 (thyroxine/tetraiodothyronine), T3 (tri-iodothyronine), calcitonin

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2
Q

What does the ‘T’ in T4 and T3 mean? How about the number?

A 
T= Compound based around tyrosine
Number= number of iodines attached
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3
Q

How and where are thyroid hormones produced?

A

. Tyrosine iodinated on thyroglobulin (large glycoprotein with many tyrosine molecules), catalysed by thyroid peroxidase (TPO)
. In cuboidal follicular cells (in thyroid gland)

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4
Q

What type of cells are found in the thyroid gland? Describe the structure of the thyroid gland.

A

. Cuboidal follicular cells (with microvilli) surround colloid (apical membrane of follicular cells facing in to colloid, basal membrane facing out)
. These are surrounded by capillaries and parafollicular C-cells

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5
Q

How is T3 made?

A

. Tyrosine + iodine –> MIT (tyrosine iodinated at 3’ of ring)
. MIT + DIT –> T3 (triiodothyronine)

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6
Q

How is T4 made?

A

. Tyrosine + iodine –> DIT (tyrosine iodinated at 3’ and 5’ of ring)
. DIT + DIT –> T4 (tetraiodothyronine/thyroxine)

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7
Q

What are MIT and DIT?

A

Mono-iodotyrosine and di-iodotyrosine

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8
Q

How is rT3 (reverse T3) made?

A

. Tyrosine + iodine –> MIT or DIT

. DIT + MIT –> rT3

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9
Q

How is the production of T3 and rT3 different?

A

MIT + DIT –> T3

DIT + MIT –> rT3

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10
Q

What does rT3 do?

A

Acts as an antagonist against T3

Binds to T3 receptor but doesn’t activate it, just prevents T3 from binding

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11
Q

Which side of the follicular cell faces the colloid?

A

Apical/luminal membrane

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12
Q

How is iodine obtained by the body to make MIT and DIT?

A

Most iodine comes from diet as iodide, then enters bloodstream and transported across follicular cells and oxidised to iodine to be added to tyrosine

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13
Q

How is iodide uptaken from the blood and transported into the colloid?

A

. Sodium-iodide symporter transports iodide from blood across basal membrane into follicular cell
. Travels through cell then released out other side of cell through apical membrane via pendrin (transporter)
. Thyroid peroxidase (TPO, protein in cell membrane)oxidises iodide to iodine
. Iodine added to tyrosine to make thyroid hormones

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14
Q

What is thyroid peroxidase, what does it do, and where is it found?

A

. Membrane protein
. Catalyses oxidation of iodide to iodine, iodination of tyrosine (on thyroglobulin), coupling of iodotyrosines (MIT/DIT)
. Found in apical/luminal membrane of follicular cells

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15
Q

How is thyroglobulin transported from the follicular cells to the colloid?

A

Exocytosis

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16
Q

What is the colloid?

A

The follicule lumen

17
Q

Where is iodinated thyroglobulin stored? How is it uptaken back into the follicular cell?

A

Stored in the colloid

Uptaken into follicular cell by endocytosis when needed

18
Q

Describe how thyroid hormones are released into the bloodstream from being stored in the colloid

A

. Thyroid hormones bound to thyroglobulin (Tg) in colloid
. Apical membrane pinches some colloid and takes some iodinated Tg into follicular cell in endosome by endocytosis
. In cytoplasm, lysosomal proteases degrade endosomes to release T3 and T4
. T3 and T4 released into bloodstream

19
Q

Where is thyrotropin-releasing hormone (TRH) secreted from?

A

Hypothalamus

20
Q

Where is thyroid-stimulating hormone (TSH/thyrotropin) secreted from?

A

Anterior pituitary gland

21
Q

Starting from the hypothalamus, describe the pathway by which secretion of thyroid hormones T3 and T4 are regulated

A

Hypothalamus releases thyrotropin-releasing hormone (TRH)
Stimulates anterior pituitary to release thyroid-stimulating hormone (TSH)
TSH stimulates synthesis and release of T3/T4 from thyroid gland

22
Q

What does TSH do?

A

. Stimulates synthesis and release of T3/T4 from thyroid gland
. Maintains integrity of thyroid gland (trophic hormone)

23
Q

What does excess TSH result in?

A

Increase in size and number of thyroid follicular cells in thyroid gland, resulting in excess production of T3/T4

24
Q

Describe how cold exposure to new-borns can cause an increase in thyroid hormones. Why is this beneficial?

A

. Neonate warm inside mother then when born exposed to cold of outside world
. Increased TRH –> increased TSH –> increase T3/T4
. Increased metabolism generates heat energy

25
Q

Describe how stress (e.g. trauma) can cause a decrease in thyroid hormones. Why is this beneficial?

A

. Stress = decrease TRH, increase somatostatin
. Causes decreased release of TSH –> decreased T3/T4
. Decreased metabolism to reduce heat loss, so can harness energy to deal with stressor

26
Q

How are thyroid hormones transported in the blood? Are they active in the blood?

A

By carrier plasma proteins because they’re lipophilic

No, protein-bound hormones are biologically inert

27
Q

Which measurement correlates more closely to thyroid function: free T4 or total T4?

A

Free T4

28
Q

Which group of enzymes is responsible for converting T4 to T3/rT3?

A

Deiodinase enzymes

29
Q

Describe the relative amounts of T3 and T4 produced by the thyroid gland

A

Produces mostly T4, but in target tissues most T4 is converted to mostly T3 and some rT3 (which is more biologically active)

30
Q

Which is more potent, T3 or T4? What is the effect of this?

A

. T3 more potent than T4, so most activity in target cells is mediated by T3
. T3 is the main active hormone, T4 is mainly prohormone (converted to mostly T3 and some rT3)

31
Q

What do thyroid hormones do?

A

. Increase metabolism
. Stimulate growth and development
. Synergistic actions with sympathetic NS/catecholamines (by upregulating expression of B-adrenoceptors)

32
Q

How is a cellular response generated when thyroid hormones bind to intracellular receptors?

A

Intracellular receptors act as hormone-regulated

transcription factors –> Alter production of proteins + biological effects

33
Q

What effect would an decrease in T3 and a increase in rT3 have?

A

Lower metabolic rate
(rT3 inhibits effect of T3 by blocking T3 receptors, and low T3 plus antagonistic effect of increased rT3= slower metabolic rate)

34
Q

What would happen to the production of T3 and rT3 if the body wanted to conserve energy or in the case of disease etc.?

A

More rT3 produced than T3 (lowers metabolic rate because of rT3’s antagonist effect on T3)

35
Q

Which kind of system regulates thyroid hormone secretion?

A

Negative feedback system between hypothalamic TRH, anterior pituitary TSH, and thyroid T3/T4

36
Q

What does calcitonin do? How does it do this? Which cells produce calcitonin?

A

. Decreases blood calcium levels (opposes parathyroid hormone, PTH)
. Inhibits action of osteoclasts (usually break down bone) and decreases renal reabsorption of calcium
. C-cells (parafollicular cells)

37
Q

How many parathyroid glands do people normally have? What do they do and how do they do this?

A

4

Secrete PTH to increase blood calcium and phosphate levels

Med 3: Thyroid Disease (2 decks)

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