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Physiology- Endocrine > Thyroid Hormones > Flashcards

Thyroid Hormones Flashcards

1
Q

What are the two major thyroid hormones? What is a third thyroid hormone?

A

(1) T4- Thyroxine
(2) T3- 3.5.3 Triiodothyronine
(3) Calcitonin

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2
Q

What is calorigenesis?

A

Increase in heat production, basal metabolic rate (BMR)

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3
Q

What does T3 promote?

A

(1) Utilization of energy substrates

(2) O2 consumption

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4
Q

Where is T3 and T4 made?

A

Thyroid follicle; colloid core

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5
Q

What type of input stimulates thyroid gland?

A

Sympathetic nervous input

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6
Q

T/F Under normal conditions, the thyroid gland does expand and contract.

A

TRUE; the follicular epithelia is cuboid in resting state but when “stimulated” to make thyroid hormone, will become elongated

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7
Q

Thyroid Hormone Synthesis

A

Tyrosine +I2= MIT and DIT

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8
Q

T4 synthesis

A

DIT+DIT

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9
Q

T3,rT3 synthesis

A

MIT+DIT

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10
Q

Which hormones are calorigenic

A

T3 and T4, NOT rT3

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11
Q

Where do we get our sources of iodide? What is needed for “normal” function?

A

Bread, milk, iodized salt; 1mg per week (we get ~3mg per week)

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12
Q

Of the iodide ingested, what percentage goes to which tissues?

A

30% thyroid; 50-70% kidneys (excretion in urine); remainder to salivary glands, stomach

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13
Q

How much of the iodide that we ingest goes to the thyroid?

A

around 1/3

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14
Q

How does the thyroid gland actively accumulate iodide?

A

via a Sodium/Iodide co-transporter (NIS)

- the NIS allows iodide to be trapped within the thyroid gland

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15
Q

What is “iodide trapping” and when does it occur?

A

Sodium will go back out of the cell while iodide will stay in the cell regardless of the gradient; It occurs against concentration and electrochemical gradients

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16
Q

Basal levels of hormone synthesis is maintained by what? which NT?

A

Sympathetic Nervous System; Norepinephrine goes into the gland

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17
Q

What are the effects of TSH on Thyroid Hormone Synthesis?

A

(1) Increases iodide uptake (NIS)
(2) Activates Thyroid Peroxidase
(3) Increases Thyroglobulin Synthesis
(4) Increases Secretion of T3/T4, NOT storage

18
Q

What are the steps of Thyroid hormone synthesis and storage?

A
  • Thyroglobulin is made from RER and stored in colloid-
    (1) uptake of iodide from NIS
    (2) Iodide reaction with TPO to convert to iodine
    (3) Iodine and TPO stimulate iodination of thyroglobulin/tyrosine molecule
    (4) TPO stimulates coupling into T3/T4 that is stored
19
Q

What are the steps to the secretion of Thyroid hormones?

A

(1) microvilli engulf portion of colloid (endocytosis)
(2) Meets with lysosome that engulfs it
(3) the TG molecules broken down by proteolytic enxymes to produce free T3/T4
(4) The T3 and T4 are released into the interstitial fluid that goes into capillaries
(5) MIT and DIT leftover are metabolized by intrathyroidal deiodinase (Iodide is recycled)

20
Q

Relative levels of T3, T4 and rT3 in a normal, complete lack, and deficient body

A

(NORMAL) T4 90%, T3 10%, rT3 <0.1%
(LACK) No T4 or T3 production/secretion
(DEFICIENT) Total T4 and T3 production/secretion decreases; the proportion of T3 synthesized/secreted increases (e.g. 50% of each)

21
Q

What is TSH? Does T3/T4 regulate TSH and TRH secretion?

A

Thyroid Stimulating Hormone; YES

22
Q

T/F Thyroid hormones are usually not bound to proteins.

A

FALSE

23
Q

What are the thyroid binding proteins? (4)

A

(1) 70% thyroxine binding globulin (TBO)
(2) 10-15% transthyretin (TTR)
(3) 15-20% bound to albumin
(4) 3% lipoproteins

24
Q

What is the major function of the binding protein?

A

To take the T3/T4 around to the target cell

25
Q

What are the functions of Thyroid binding proteins? (4)

A

(1) Transport to target cells
(2) Prevent excretion in urine
(3) prevent metabolism by liver
(4) maintain circulating storage pool

26
Q

What are the half lives of T4 and T3?

A

T4- 7 days; T3- 1 day

27
Q

What are the ways T3 and T4 enter target cells?

A

(1) Transporters- NCTP, OATP, LATs
(2) Diffusion
(Has to have deep binding away from the TBG to yield free T3 and T4 to enter the cell)

28
Q

What is the biologicall reactive hormone? Why do you keep more of the other in circulation?

A

T3; T4 is a prohormone with a LONGER half life so you have a larger pool because it can metabolize in the target cell

29
Q

How does the T4 become T3 in the target cell? What happens to the iodide?

A

Through Type 1 and Type 3 Deiodinase; recycled or excreted (T1 and T2 are excreted in urine)

30
Q

Why is rT3 important? (This happens with deiodination of T4 in target cells as well)

A

Under starvation conditions an adaptation occurs which allows for less production of the calorigenic hormone T3, essentially conserving food energy stores (rT3 is produced)

31
Q

What is T3’s action at the target cells? (4)

A

(1) Respiratory Enzymes
(2) Gluconeogenic Enzymes
(3) Na/K pump
(4) B-adrenergic receptors

32
Q

T/F Some of the T4 that enters the target cell converts to T3 and some stays as T4.

A

FALSE; ALL T4 is converted to T3 that binds onto THR (thyroid hormone receptor) where transcription is initiated

33
Q

What are the 3 Physiological actions of T3- Noncalorigenic

A

(1) Tropic/Growth Effects- bone growth, tooth eruption, stimulates GH and IGF-1 production
(2) Nervous System Effects- normal development, neuronal differentiation; normal emotions
(3) Reproductive Effects- normal ovarian cycle, spermatogenesis

34
Q

What can happen if T3 is absent or low during fetal and early neonatal development?

A

Sever mental retardation can occur!

35
Q

What are the 2 Physiological Actions of T3- Calorigenic

A

(1) Calorigenesis- includes cardiovascular, respiratory, and metabolic actions in support of the increase in BMR
(2) Stimulates appetite

36
Q

What is associated with hyperthyroidism?

A

(1) Increased appetite
(2) Weight loss
(3) Heat intolerance
(4) Increased sweating

37
Q

What is associated with hypothyroidism?

A

(1) Reduced appetite
(2) Weight gain
(3) Cold intolerance
(4) Decreased sweating

38
Q

What happens with Hyperthyroidism (Grave’s disease)?

A

An autoantibody (thyroid-stimulating immunoglobulins) stimulate the thyroid and mimic TSH to secrete excess levels of T3/T4 (why you have a goiter) which leads to increased negative feedback to decrease plasma TSH but the autoantibody still stimulates

39
Q

What happens with Hypothyroidism?

A

An autoantibody that inhibits thyroid peroxidase which leads to low plasma T3 and T4 which has a decreased negative feedback for more plasma TSH to make more thyroglobulins (why there’s a goiter) but since it inhibits TPO there is no production of more T3 and T4

40
Q

What happens with Iodide deficiency?

A

Similar process to Hypothyroidism instead iodide uptake is reduced which is needed in the iodination process

Physiology- Endocrine (7 decks)

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