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2H. IM > ENDOC- THYROID intro > Flashcards

ENDOC- THYROID intro Flashcards

1
Q

The thyroid (Greek thyreos , shield, plus eidos , form) consists of two lobes connected by an_______________

A
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2
Q

The thyroid gland is located where?

A

It is located anterior to the trachea between the cricoid cartilage and the suprasternal notch.

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3
Q

What is the weight of thyroid?

A

The normal thyroid is 12–20 g in size, highly vascular, and soft in consistency.

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4
Q

What is located posterior to the thyroid gland?

A

Four parathyroid glands, which produce parathyroid hormone (Chap. 353) , are located posterior to each pole of the thyroid.

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5
Q

The _______________ traverse the lateral borders of the thyroid gland and must be identified during thyroid surgery to avoid injury and vocal cord paralysis.

A

recurrent laryngeal nerves

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6
Q

The thyroid gland produces two related hormones, _____________

A

thyroxine (T 4 ) and triiodothyronine (T 3 )

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7
Q

T3 and T4 acting through thyroid hormone receptors α and β, these hormones play a critical role in ____________________________

A
  1. cell differentiation during development
  2. and help maintain thermogenic and metabolic homeostasis in the adult.
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8
Q

Thyroid hormones are derived from____________ a large iodinated glycoprotein

A

Tg,

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9
Q

What happens after secretion into the thyroid follicle, _______________

A

Tg is iodinated on tyrosine residues that are subsequently coupled via an ether linkage.

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10
Q

________________ cell allows proteolysis and the release of newly synthesized T 4 and T 3 .

A

Reuptake of Tg into the thyroid follicular

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11
Q

____________ is a critical first step in thyroid hormone synthesis

A

Iodide uptake

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12
Q

. Ingested iodine is bound to serum proteins, particularly _________. Unbound iodine is excreted in the urine.

A

albumin

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13
Q

The thyroid gland extracts iodine from the circulation in a highly efficient manner. For example, 10–25% of radioactive tracer (e.g., 123 I) is taken up by the normal thyroid gland over 24 hours; this value can rise to **70–90% in Graves’ disease. **

A
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14
Q

Iodide uptake is mediated by _________________-, which is expressed at the basolateral membrane of thyroid follicular cells.

A

NIS

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15
Q

recommended average daily intake of iodine is _____________for adults

A

150–250 μg/d

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16
Q

recommended average daily intake of iodine is______________for children,

A

90–120 μg/d

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17
Q

recommended average daily intake of iodine is______________ ____________- for pregnant and lactating women

A

250 μg/d

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18
Q
A
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19
Q

. Urinary iodine is _______________in iodine-sufficient populations.

A

>10 μg/dL

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20
Q

After iodide enters the thyroid, it is trapped and transported to the____________of thyroid follicular cells, where it is oxidized in an organification reaction that involves TPO and hydrogen peroxide.

The reactive iodine atom is added to selected tyrosyl residues within Tg, a large (660 kDa) dimeric protein that consists of 2769 amino acids.

A

**apical membrane **

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21
Q

The iodotyrosines in Tg are then coupled via an_____________ in a reaction that is also catalyzed by TPO.

Either T 4 or T 3 can be produced by this reaction, depending on the number of iodine atoms present in the iodotyrosines.

A

ether linkage

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22
Q

After coupling, Tg is taken back into the thyroid cell, where it is processed in_____________to release T 4 and T 3

.

A

lysosomes

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23
Q

Uncoupled mono- and diiodotyrosines (MIT, DIT) are deiodinated by the enzyme ____________, thereby recycling any iodide that is not converted into thyroid hormones.

A

dehalogenase

24
Q

What are the steps in the process of thyroid synthesis?

A
  1. Iodide uptake
  2. •Organification
  3. •Coupling
  4. •Storage
  5. •Release
25
Q

– most useful physiologic marker of thyroid hormone action
–31kDa

A

•TSH

26
Q

–_________ common to other glycoprotein hormones (LH, FSH, hCG)

A

α subunit:

27
Q

___________: this subunit is unique to thyroid hormone

A

–β subunit

28
Q

what suppress TSH

A

–Dopamine, glucocorticoids, and somatostatin

29
Q

____________ stimulates pituitary production of TSH,

A

Hypothalamic TRH

30
Q

___________- stimulates thyroid hormone synthesis and secretion.

A

TSH

31
Q

Thyroid hormones, acting predominantly through thyroid hormone receptor ___________, feed back to inhibit TRH and TSH production.

A

β2 (TRβ2)

32
Q

_____________secreted by the thyrotrope cells of the anterior pituitary, plays a pivotal role in control of the thyroid axis and serves as the **most useful physiologic marker of thyroid hormone action. **

It is a 31-kDa hormone composed of α and β subunits;

the α subunit is common to the other glycoprotein hormones [luteinizing hormone, follicle-stimulating hormone, human chorionic gonadotropin (hCG)], whereas the TSH β subunit is unique to TSH.

A

TSH,

33
Q

________________-suppress TSH but are not of major physiologic importance except when these agents are administered in pharmacologic doses.

A

Dopamine, glucocorticoids, and somatostatin

34
Q

TSH regulates thyroid gland function through the ______________, a seventransmembrane G protein–coupled receptor (GPCR). Most of these activating mutations occur in the transmembrane domain of the receptor. They mimic the conformational changes induced by TSH binding or the interactions of thyroid-stimulating immunoglobulins (TSI) in Graves’ disease.

A

TSH-R

35
Q

Thyroid follicles are formed by thyroid epithelial cells surrounding proteinaceous colloid, which contains_______________.

A

thyroglobulin

36
Q

______________- which are polarized, synthesize thyroglobulin and carry out thyroid hormone biosynthesis (see text for details).

A

Follicular cells,

37
Q

T4 is secreted from the thyroid gland in about___________excess over T3 (Table 341-2)

.

A

** twentyfold **

38
Q

Thyroid hormones bind to what proteins?

A
  • thyroxine-binding globulin (TBG),
  • transthyretin (TTR, formerly known as thyroxine-binding prealbumin, or TBPA), and
  • albumin.
39
Q

What is the purpose of the plasma-binding proteins_________________

A
  • increase the pool of circulating hormone,
  • delay hormone clearance,
  • and may modulate hormone delivery to selected tissue sites.
40
Q

When the effects of the various binding proteins are combined, approximately 99.98% of T 4 and 99.7% of T 3 are protein-bound.’

A
41
Q

________________ may be thought of as a precursor for the more potent T 3.

A

T 4

42
Q

T4 is converted to T 3 by the_____________ (see slide 3)

A

deiodinase enzymes.

43
Q

T 3 is bound with 10–15 times greater affinity than T4 , which explains its____________-. Though T 4 is produced in excess of T 3 , receptors are occupied mainly by T 3 , reflecting T 4 → T 3 conversion by peripheral tissues, greater T 3 bioavailability in the plasma, and receptors’ greater affinity for T 3 .

A

increased hormonal potency

44
Q

Plasma proteins:

_____________________ - carries about 80% of the bound hormones

A

Thyroxine-binding globulin

45
Q

• - __________________-carries about 10% of T 4 but little T3

A

Transthyretin

46
Q

______________ - binds up to 10% of T 4 and 30% of T3

A

•Albumin

47
Q

The deiodinases

A
  • Type I
  • Type II
  • Type III
48
Q

Type I deiodinase, which is located primarily in___________, ___________ and ___________ has a relatively low affinity for T 4.

A
  • ** thyroid,**
  • liver,
  • and kidneys,
49
Q

Type II deiodinase has a higher affinity for T 4 and is found primarily in the __________, ____________-, ____________ and _________

A
  • pituitary gland,
  • brain,
  • brown fat,
  • and thyroid gland.
50
Q

Expression of type II deiodinase allows it to________________-, a property that may be important in the context of levothyroxine (T 4 ) replacement.

Type II deiodinase is also regulated by thyroid hormone; hypothyroidism induces the enzyme, resulting in enhanced T 4 → T 3 conversion in tissues such as brain and pituitary

A

regulate T 3 concentrations locally,

51
Q

. T 4 → T 3 conversion is impaired by________________

A
  • fasting,
  • systemic illness
  • or acute trauma,
  • oral contrast agents,
  • and a variety of medications (e.g., propylthiouracil, propranolol, amiodarone, glucocorticoids).
52
Q

__________________- inactivates T 4 and T 3 and is the most important source of reverse T 3 (rT 3 ).

A

Type III deiodinase

53
Q

_____________- that express type III deiodinase are a rare cause of hypothyroidism in infants.

A

Massive hemangiomas

54
Q

Measurement of thyroid hormones

  • TSH levels change dynamically in response to alterations of T4 and T3
  • finding of an abnormal TSH level must be followed by **measurements of circulating thyroid hormone levels **
A
55
Q

Because TSH levels change dynamically in response to alterations of T 4 and T 3 , a logical approach to thyroid testing is to first determine whether TSH is ____________, _________ and _______

A

suppressed, normal, or elevated.

56
Q

The finding of an abnormal TSH level must be followed by _____________to confirm the diagnosis of hyperthyroidism (suppressed TSH) or hypothyroidism (elevated TSH).

A

measurements of circulating thyroid hormone levels

57
Q
A

2H. IM (13 decks)

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