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USMLE Endocrine > Thyroid > Flashcards

Thyroid Flashcards

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USMLE® Step 1 flashcards
1
Q

What are thyroid follicular cells derived from?

A

Endoderm

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2
Q

What are parafollicular cells derived from (specifically)?

A

Mesoderm - 4th pharyngeal arch

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3
Q

Where would an ectopic thyroid tissue be found?

A

Tongue (lingual thyroid)

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4
Q

What may removal of ectopic thyroid result in?

A

Hypothyroidism

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5
Q

What is the normal remnant of the thyroglossal duct called?

A

Foramen cecum (and pyramidal lobe of thyroid)

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6
Q

What is the pyramidal lobe of the thyroid a remnant of?

A

The thyroglossal duct

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7
Q

Diagnose:

An anterior midline neck mass that moves with swallowing or protrusion of the tongue

A

Thyroglossal duct cyst

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8
Q

What would a cyst in the lateral neck most likely be due to?

A

Pharyngeal cleft cyst due to a persistent cervical sinus

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9
Q

What is the anterior pituitary derived from?

A

Oral ectoderm (Rathke pouch)

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10
Q

What hormone is secreted from the intermediate lobe of the pituitary?

A

Melanotropin (MSH)

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11
Q

What is the blood supply of the thyroid gland?

What are these arteries branches of?

A
  • Superior thyroid artery (from external carotid)

- Inferior thyroid artery (extension of thyrocervical trunk which is a branch of the subclavian artery)

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12
Q

What are thyroid follicles filled with?

A

Colloid (protein material)

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13
Q

What are the single layer of epithelial cells that line the follicle called?

A

Follicular cells

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14
Q

What element is contained in thyroid hormones?

A

Iodine

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15
Q

What are T3 and T4 synthesized from?

A

Tyrosine and iodine

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16
Q

What is thyroglobulin?

A

Large protein produced by thyroid follicular cells which contains numerous tyrosine molecules

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17
Q

When is iodide (I-) oxidised to iodine (I2)?

A

Taken up as iodide (I-) in follicular cells then oxidised to iodine (I2) in the cells

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18
Q

What is the pump that transports iodide into the follicle cell?

A

Na-Iodine symporter (NIS)

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19
Q

What enzyme turns Iodide to I2?

A

Thyroid Peroxidase (TPO) (also catalysis organification)

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20
Q

What are the functions of TPO?

A
  • Oxidises Iodide to I2
  • Organification (adding iodine to tyrosine to make MIT or DIT)
  • Coupling reactions from MIT and DIT to T3 OR DIT + DIT to T4
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21
Q

How many iodines are present on T3 and T4?

A 
T3 = 3 
T4 = 4
22
Q

What is the main hormone produced by the thyroid gland?

A

T4 (~90%)

- Less potent than T3

23
Q

What converts T4 into T3?

A

5’ deiodinase (mostly occurs in peripheral tissues)

24
Q

What does PTU inhibit?

A
  • TPO (decreased T3 and T4 from thyroid)

- 5’ deiodinase (decreased T4 to T3 conversion peripherally)

25
Q

What does Propranolol block along with Beta channels?

A
  • 5’ deiodinase (weakly)
  • Blocks catecholamines
  • Blocks T4 to T3 conversion
  • Excellent drug in thyrotoxicosis
26
Q

What does methimazole inhibit?

A

TPO

27
Q

When are ectopic thyroid glands often discovered?

A

When there is an increased demand for hormones

  • Puberty
  • Pregnancy
28
Q

What is the Wolff-Chaikiff effect?

A

Excess iodine turns off thyroid peroxidase results in decreased production of T3 and T4 - Hypothyroidism
- Form of protective autoregulation

29
Q

What molecules may inhibit the NIS pump?

A
  • Perchlorate (ClO4-)

- Pertechnetate (TcO4-)

30
Q

Tyrosine and Iodine combine in a process called organification
Organification produces what 2 compounds?

A
  • Monoiodotyrosine (MIT)

- Diiodotyrosine (DIT)

31
Q

TPO combines MIT and DIT in what is known as a coupling reaction.
What will be produced as a result?

A

Triiodothyronine (T3)

32
Q

What is the main pathway by which the thyroid self-regulates?

A

Negative feedback by free T3/T4

  • Ant pituitatary becomes less sensitive to TRH
  • Hypothalamus produces less TRH
33
Q

Via what mechanism is T3 and T4 cleaved from thyroglobulin?

A

Proteolysis (by proteases)

- First TG is endocytosed into follicular cell from lumen

34
Q

How can amiodarone (class III antiarrhythmic) cause hypothyroidism?

A

It contains iodine therefore may cause hypothyroidism via the Wolff-Chaikoff Effect

  • It may mimic T4 and inhibit 5’deiodinase
  • TSH rises after start of therapy then normalises
35
Q

What can the isotope I^131 be used for?

A

Emits radiation (B-decay)

  • Small doses - imaging
  • Large doses - Destroys tissue
36
Q

What is most plasma T4 bound to?

A

TBG (Thyroxine-Binding Globulin)

- Some with transthyretin and albumin

37
Q

What is TBG produced by?

A

The liver

38
Q

Is thyroid hormone soluble in water?

A

T4 is poorly soluble in water

39
Q

What are the clinical consequences of less TBG?

A

Less available T3/T4 to tissues

40
Q

What hormone can raise TBG levels?

A

Estrogen (OCP, pregnancy)

- Will raise total T4 levels

41
Q

What is the effect of liver failure of T4 levels?

A

Lowers TBG levels therefore can lower T4 levels

42
Q

What are the functions of thyroid hormone?

A

7 Bs

  • Brain maturation
  • Bone growth (synergism w. GH)
  • B-adrenergic effects
  • Basal metabolic rate (via Na/KATPase) incr. O2 consumption, RR, temp
  • Blood sugar incr
  • Break down lipids (lipolysis)
  • Surfactant synthesis in babies
43
Q

What are causes of cretinism?

A
  • Iodine deficiency
  • Thyroid dysgenesis
  • Inborn errors of hormone synthesis (dyshormonogenesis)
  • TPO most common
44
Q

What is the most common treatable cause of mental retardation?

A

Cretinism (low thyroid hormone)

45
Q

What are some features of cretinism?

A
  • Mental retardation
  • Coarse facial features
  • Short stature
  • Umbilical hernia
  • Enlarged tongue
46
Q

What are the functions of calcitonin?

A

Suppresses resorption of bone and inhibits osteoclasts

  • Inhibits renal reabsorption of calcium, phosphorus
  • Increases calcium in urine
47
Q

What is calcitonin used to treat clinically?

A

Hypercalcemia

48
Q

What does TSH bind to?

A

Receptors on follicular cells

- Activates cAMP/PKA 2nd messenger system

49
Q

How does TSH increase T3/4 release?

A

Increases rate of proteolysis of thyroglobulin which leads to a rapid release if more T3/4
- Also stimulates thyroid cell growth, TG synthesis

50
Q

How does hCG stimulate the thyroid?

A

Same alpha unit as TSH

51
Q

What is calcitonin produced by?

A 
Parafollicular cells (C cells)
- Situated in between the follicles
52
Q

What substances cause decreased TSH release from the ant. pituitary?

A
  • Somatostatin
  • Dopamine
  • Glucocorticoids

USMLE Endocrine (8 decks)

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