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Endocrine > Thyroid Gland and hormones_Parthasarathi > Flashcards

Thyroid Gland and hormones_Parthasarathi Flashcards

1
Q

Thyroid hormones are derivates of what amino acid?

A

Tyrosine

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2
Q

2 features of thyroid gland that distinguish it from all other endocrine glands?

A
  1. Can be palpated
  2. Utilizes iodine
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3
Q

Thyroid produces what 2 main hormones?

A

T3 (Triiodothyronine) & T4 (Thyroxine)

Also produces reverse T3, but has no biologic activity

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4
Q

Thyroid gland mainly produces T3 or T4 ?

Which is considered the active form ?

A

Almost all hormonal output = T4

T3 = Active Form

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5
Q

Since T3 = active form, what must happen to T4 to be used? (Think broadly)

A

Target tissues convert T4 to T3

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6
Q

Thyroid hormones are produced by what cells of thyroid gland?

A

Follicular epithelial cells

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7
Q

Describe orientation of follicular cells.

Which side faces blood, which side faces follicular lumen?

A

Basal membrane faces the blood. Apical membrance faces the follicular lumen

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8
Q

The material in the lumen of follicles is _____, which is composed of ______ attached to ______?

A

colloid = Newly synthesized thyroid hormones + thyroglobulin

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9
Q

There are 3 unusual features of the thyroid hormone synthesis. Name them.

A
  1. Thyroid hormones contain large amounts of iodine, Dietary iodine is required
  2. Synthesis of thyroid hormones is part intracellular and part extraceullar, with the completed hormones stored extracellularly in the follicular lumen until the thyroid gland is stimulated to secrete
  3. T4 = major secretory product, but must be activated to T3
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10
Q

Thyroid hormone synthesis.

Thyroglobulin contains large amounts of what amino acid?

Where is thyroglobulin synthesized?

What occurs following synthesis?

Purpose of this amino acid found in thyroglobulin?

A

Tyrosine

Synthesized on RER and golgi of thyroid follicular cells.

Thryroglobulin put into secretory vesicles and sent across apical membrane into follicular lumen. (Shown in Step 1)

Tyrosine will be iodinated to form the precursors of thyroid hormones

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11
Q

Thyroid hormone synthesis.

What key transporter is crucial to starting synthesis? Where is it found? Active or passive transport?

A

Na+-I- cotransport. Found on basal surface of follicular epithelial cells. ACTIVELY transports I- from blood into follicular epithelial cells (Step 2)

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12
Q

Once I- has been transported into follicular epithelial cell, it heads toward the apical membrane (facing the follicular lumen).

What occurs at the apical membrane? What enzyme is involved?

A

At apical membrane (still within follicular cell), thyroid peroxidase converts I- to I2. (STEP 3)

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13
Q

After thyroid peroxidase converts I- to I2, it is transported across the apical surface into the follicular lumen through what channel?

A

Pendrin channel

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14
Q

What happens to I2 right after it crosses through the pendrin channel into the follicular lumen?

A

I2 combines with tyrosine residues of thyroglobulin (catalyzed by thyroid peroxidase). This combination forms MIT (monoiodotyrosine) and DIT (diiodotyrosine). (STEP 4)

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15
Q

Following formation of MIT and DIT on Thyroglobulin, 2 separate coupling reactions occur b/t MIT and DIT.

2 molecules of DIT combine to form?

1 molecule of DIT & 1 molecule of MIT combine to form?

These 2 reactions collectively result in _____ thyroglobulin?

A

2x DIT = T4

MIT + DIT = T3

These result in iodinated thyroglobulin (Step 5)

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16
Q

After thyroglobulin becomes iodinated, where does it transport to?

A

Thyroglobulin is then endocytosied back into follicular cell (Step 6)

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17
Q

After thyroglobulin (with bound T3, T4, MIT, and DIT) are endocytosed back into follicular cell, what occurs?

A

T3 & T4 are hydrolysized from thyroglobulin via lysosomal enzymes and secreted into circulation. (STEP 7)

18
Q

What happens to MIT and DIT following cleavage from thyroglobulin inside follicular cell?

A

Thyroid deaminase deiodinates MIT and DIT into I- & Tyrosine.

Iodide enters free pull (with Na/I pump iodide)

Tyrosine incorporated into new thyroglobulin synthesis. (STEP 8)

19
Q

Majority of T3 and T4 are bound to what?

A

TBG (Thyroxine binding globulin). Small amounts bound to albumin & T4 binding prealbumin

20
Q

Hepatic failure causes what to occur to TBG (thyroxine binding globulin)?

Relate this to thyroid hormone production

A

Decreases TBG, results in transient increase in level of free thyroid hormones. This causes negative feedback inhibition of thyroid hormone synthesis?

21
Q

What can cause TBG to increase? Mechanism?

A

Pregnancy, high levels of estrogen inhibits hepatic breakdown of TBG and increases its levels. Decreases free thyroid hormone. Feedback mediated Increase in thyroid hormone synthesis

22
Q

What has a longer 1/2 life, bound T3 or bound T4?

A

Bound T3 = 24 hours

Bound T4 = 8 days

23
Q

This is the structure of T4. If the iodine (shown in red circle) is cleaved, what will this become? Via what enzyme?

A

T3, via 5’3’ deiodinase

24
Q

What happens if the inner ring (shown in red circle) is cleaved off T4? via what enzyme?

A

Becomes reverse T3, via 5/3-Deiodinase

25
Q

Type 1 5’/3’ Deiodinase produces T3 that acts on what organs/glands? What occurs?

A

Liver, kidney, thyroid. Triggers release of active T3

26
Q

Type 2 5’/3’ Deiodinase produces T3 that acts on what organs/glands?

A

Pituitary gland, CNS, Placenta. Act more as feedback, limiting the extent of TSH produced

27
Q

Once T3 is produced inside target cells, it enters ____ and binds to a ____ _____? This binding causes what to occur?

A

nucleus, binds with nuclear receptor

Binding triggers DNA transcription.

28
Q

As a result of T3 having to undergo transcription when it reaches its target cell, what can you say regarding the speed with which thyroid hormones act?

A

Slow (hours)

29
Q

Hyperthyroidism effects

Basal metabolic rate?

Carbohydrate metabolism?

Protein Metabolism (muscle wasting)

Lipid Metabolism?

Thermogenesis?

A
30
Q

Hypothyroidism effects

Basal metabolic rate?

Carbohydrate metabolism?

Protein Metabolism (muscle wasting)

Lipid Metabolism?

Thermogenesis?

A
31
Q

What occurs to glucose levels in hyper or hypothyroidism?

A

Glucose will not change, even in the face of increasing/decreasing carbohydrate metabolism

32
Q

What happens to serum cholesterol in hyperthyroidism?

A

Serum cholesterol decreases (lipid metabolism increases)

33
Q

Regulation of Thyroid hormone secretion occurs primarily via what mechanism?

A

Negative feedback

34
Q

What is considered to be the major regulator of Thyroid Function?

What is another name for this regulator?

A

TSH aka Thyrotropin

35
Q

TRH is secreted from ______? It acts on _______ to release ______? This release then acts on ______ to produce _____?

A

TRH secreted by hypothalamus, acts on anterior pituitary to release TSH. TSH acts on thyroid gland to release T3 and T4

36
Q

TSH effect on Na/I symporter?

A

Increases activity

37
Q

TSH has 3 effects on thryoglobulin. What are they?

A
  1. Stimulates the iodination of thyroglobulin in follicular lumen
  2. Stimulates endocytosis of iodinated thyroglobulin into follicular cells
  3. Stimualtes proteolysis of iodinated thyroglobulin in lysoendosomes
38
Q

TSH leads to generation of T3 and T4 via what mechanism?

A

Stimulates conjugation of iodinated tyrosines to generate T3 and T4

39
Q

TSH causes what to increase in thyroid cells (vague question)?

A

Hypertrophy (increase size) and hyperplasia (increase number) of cells

40
Q

Mechanism that occurs with Hashimoto Thyroiditis? Clinical presentation?

A

Anti-thyroid antibodies that block and destroy thyroid function.

Clinical presentation: painless goiter, peripheral edema, headache & joint ache, anovulation

41
Q

Mechanism that occurs in Grave’s Disease? Clinical presentation?

A

Caused by immunoglobulin antibodies (Thyroid stimulating Immunoglobulin). These antibodies bind to TSH receptors on thyroid and continually drive thyroid hormone production.

Clinical presentation: hyperexcitability, weight loss, sweating, diarrhea, extreme fatigue, tremor in hands, Exopthalamos (eyeball protrusion, dry epithelial layers of the eye, ulceration of the cornea)

Endocrine (6 decks)

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