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Endocrinology > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

What are the units of the thyroid gland?

A

thyroid follicles: epithelial cells in a single layer surrounding a colloid

parafollicular cells: C-cells are a source of calcitonin

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2
Q

What are the functions of the thyroid gland?

A

-secrete thyroid hormone

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3
Q

What regulates the blood flow to the thyroid gland?

A

sympathetic nerves

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4
Q

What are the active forms of thyroid hormone?

A

T3 + T4

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5
Q

What are the inactive forms of thyroid hormone?

A

rT3 (from T4) + T2 (from T3)

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6
Q

What are the steps for the synthesis of T3 and T4?

A
  1. TRAPPING
    active transport of iodine into cell with Na/I symporter (NIS) => Na/K ATPase to lower [Na] intracellular concentration
  2. binding of TSH on TSHR => activates IP3 (golgi = TG synthesis) + cAMP (activates NIS)
  3. PDS throws iodine in colloid
  4. Thyroglobulin synthesis and exocytosis in lumen of follicle
  5. ORGANIFICATION + COUPLING
    iodination of tyrosine residues in TG by TPO
    linking pairs of iodotyrosines in TG to form T3 + T4
  6. INTERNALIZATION
    iodinated TG => proteolysis to release T3 + T4
  7. DEIODINATION
    recycling of I-
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7
Q

What can inhibit iodine intake from thyroid cell?

A
  • perchlorate (ClO4-)
    from diet
  • bromide (Br-)
  • nitrite (NO2-)
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8
Q

What can protect from radioactive iodine in an emergency situation?

A

thyroid shield potassium iodide

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9
Q

What drugs inhibit iodination and how do they work?

A

thiocarbamide inhibitors

they target TPO => low T3 and T4 = no -ve feedback = high TSH = hyperplasia and goiter

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10
Q

How does the iodination of TG occur?

A

Need of hydrogen peroxide
O from blood and H from NADPH (pentose phosphate = high need for glu)
H2O2 formed by DUOX2

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11
Q

What are the types of deiodinases?

A

Type I: T4 -> T3
Type II: T4 -> T3
Type III: T4 -> rT3 (when T3 in excess) and T3 -> T2

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12
Q

How many thyroid hormones are formed within one TG?

A

maximum of 4
from the surface
to keep a steady-state of T3 and T4

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13
Q

Where does type I deiodinase is present?

A

muscle, liver, kidney

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14
Q

Where does type II deiodinase is present?

A

brain + pituitary = feedback on TRH and TSH

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15
Q

What is the syndrome characterized by low T3 and higher rT3?

A

sick euthyroid syndrome

from cancer/anorexia

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16
Q

What are the carrier proteins that allow transport of thyroid hormones?

A
  • TBG (thyroxine-binding protein)
  • transthyretin
  • albumin
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17
Q

What drugs can cause hyperthyroidism?

A

epilepsy treatment = compete with carrier proteins = more free T3 and T4

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18
Q

Which of the thyroid hormone forms has the longer half-life?

A

T4

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19
Q

Which form of thyroid hormone is more active?

A

T3

20
Q

What conditions increase the binding to proteins?

A

pregnancy and oral contraceptives

21
Q

What conditions decrease the binding to proteins?

A

starvation and liver disease (thyroxine protein produced in liver)

22
Q

What rare AA is part of deiodinases?

A

selenocysteine

23
Q

Fill the blanks:

About __% of __ is metabolized by deiodination.

A
  • 80%

- T4

24
Q

What is a positive aspect and a negative one about thyroid hormone storage?

A

storage is high
bad = if release triggered => hyperthyroidism
good = if iodine deficiency (can maintain for 50 days)

25
Q

What are the thyroid hormone functions?

A
  • growth and dev

- maintenance of basal metabolic rate

26
Q

What type of receptor is THR and what does it activate?

A 
Nuclear class II receptor (already bound to RE)
Activates translation of 2 genes: THRA and THRB (2 receptor isoforms by alternative splicing)
27
Q

How does T4 regulate metabolism?

A

it binds to integrins on the surface of cell membranes

and activates translation by MAPK pathway

28
Q

What are the effects of TH in mitochondria?

A

T2 binds to cytochrome C for oxidative phosphorylation
T3 binds to uncoupling proteins (UPC1) to increase heat production
THR transcription in mitochondria

29
Q

What are integrins?

A

proteins that link the cytoskeleton to the extracellular matrix = change shape inside cell

30
Q

What is the calorigenic action of TH?

A

increases O2 consumption and heat production (except uterus, testes, brain, lymph nodes, spleen, and anterior pituitary)
due to FAs mobilization + Na/K ATPase activity

31
Q

What are effects secondary to calorigenesis of TH?

A
  • increased N excretion (muscle wasting)
  • weight loss
  • increased erythropoiesis due to increased demand for O2
  • positive N balance for children
  • hepatic conversion of carotene to vit A (by T4) = carotenemia
32
Q

What are the consequences of a lack of T4?

A

myxedema

skin puffiness and bumps from no degradation of proteins, polysaccharides and hyaluronic acid accumulation

33
Q

what is the effect of TH on the cardiovascular system?

A
  • Myocytes triggered by T3:
    • activates a-myosin cells = increases ATPase activity =
      contraction
    • inhibits b-myosin = decreases ATPase activity
  • heat = vasodilatation => reabsorption of Na and water by kidneys = increased blood volume
  • catecholamines increase HR and BP
34
Q

What are catecholamines?

A

effects similar to T3 and T4: increased metabolic rate, stimulation of nervous and cardiovascular system

35
Q

What drugs can be used to reduce catecholamines activity?

A

beta-blockers

36
Q

What is the effect of TH on nervous system?

A

increased T4 to T3

increased response to catecholamines

37
Q

What is the effects of TH on skeletal muscles?

A

increased protein turnover

38
Q

What is the effects of TH on carbohydrates?

A

increased absorption in GI tract

39
Q

What is the effects of TH on the reproductive system?

A

follicular dev and ovulation
pregnancy
spermatogenesis

40
Q

What is the effects of TH on cholesterol metabolism?

A

decreases C levels by increasing hepatic LDL receptors

41
Q

What is the effects of TH on growth?

A
  • increased bone mass
  • epidermis, hair follicles and nails
  • increased rate + force of skeletal muscle contractions
  • degradation of mucopolysaccharides in subcutaneous tissues
42
Q

What are the types of hypothyroidism?

A
  • primary: thyroid gland failure
  • secondary: pituitary failure
  • tertiary: hypothalamic failure
  • peripheral resistance
43
Q

What is the treatment for hypothyroidism?

A

T4 injections (bc longer 1/2 life)

44
Q

What is Hashimoto’s disease?

A 
autoimmune disease against TPO or TG
low TH with sufficient iodine
=> hypothyroid 
treatment with T4
goiter in children
45
Q

What is Graves’ disease?

A

hyperthyroidism

anti-TSH receptors anti-bodies that mimic TSH

46
Q

What are the different types of THR?

A
  • alpha-1 => widely distributed
  • alpha-2 => inhibitor
  • beta-1 => widely distributed
  • beta-2 => anterior pituitary specific

Endocrinology (9 decks)

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