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USMLE Endocrine > Thyroid > Flashcards

Thyroid Flashcards

(52 cards)

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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?

Study These Flashcards
A 
T3 = 3 
T4 = 4
22
Q

What is the main hormone produced by the thyroid gland?

Study These Flashcards
A

T4 (~90%)

- Less potent than T3

23
Q

What converts T4 into T3?

Study These Flashcards
A

5’ deiodinase (mostly occurs in peripheral tissues)

24
Q

What does PTU inhibit?

Study These Flashcards
A
  • TPO (decreased T3 and T4 from thyroid)

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

25
What does Propranolol block along with Beta channels?
- 5' deiodinase (weakly) - Blocks catecholamines - Blocks T4 to T3 conversion - Excellent drug in thyrotoxicosis
26
What does methimazole inhibit?
TPO
27
When are ectopic thyroid glands often discovered?
When there is an increased demand for hormones - Puberty - Pregnancy
28
What is the Wolff-Chaikiff effect?
Excess iodine turns off thyroid peroxidase results in decreased production of T3 and T4 - Hypothyroidism - Form of protective autoregulation
29
What molecules may inhibit the NIS pump?
- Perchlorate (ClO4-) | - Pertechnetate (TcO4-)
30
Tyrosine and Iodine combine in a process called organification Organification produces what 2 compounds?
- Monoiodotyrosine (MIT) | - Diiodotyrosine (DIT)
31
TPO combines MIT and DIT in what is known as a coupling reaction. What will be produced as a result?
Triiodothyronine (T3)
32
What is the main pathway by which the thyroid self-regulates?
Negative feedback by free T3/T4 - Ant pituitatary becomes less sensitive to TRH - Hypothalamus produces less TRH
33
Via what mechanism is T3 and T4 cleaved from thyroglobulin?
Proteolysis (by proteases) | - First TG is endocytosed into follicular cell from lumen
34
How can amiodarone (class III antiarrhythmic) cause hypothyroidism?
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
What can the isotope I^131 be used for?
Emits radiation (B-decay) - Small doses - imaging - Large doses - Destroys tissue
36
What is most plasma T4 bound to?
TBG (Thyroxine-Binding Globulin) | - Some with transthyretin and albumin
37
What is TBG produced by?
The liver
38
Is thyroid hormone soluble in water?
T4 is poorly soluble in water
39
What are the clinical consequences of less TBG?
Less available T3/T4 to tissues
40
What hormone can raise TBG levels?
Estrogen (OCP, pregnancy) | - Will raise total T4 levels
41
What is the effect of liver failure of T4 levels?
Lowers TBG levels therefore can lower T4 levels
42
What are the functions of thyroid hormone?
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
What are causes of cretinism?
- Iodine deficiency - Thyroid dysgenesis - Inborn errors of hormone synthesis (dyshormonogenesis) - TPO most common
44
What is the most common treatable cause of mental retardation?
Cretinism (low thyroid hormone)
45
What are some features of cretinism?
- Mental retardation - Coarse facial features - Short stature - Umbilical hernia - Enlarged tongue
46
What are the functions of calcitonin?
Suppresses resorption of bone and inhibits osteoclasts - Inhibits renal reabsorption of calcium, phosphorus - Increases calcium in urine
47
What is calcitonin used to treat clinically?
Hypercalcemia
48
What does TSH bind to?
Receptors on follicular cells | - Activates cAMP/PKA 2nd messenger system
49
How does TSH increase T3/4 release?
Increases rate of proteolysis of thyroglobulin which leads to a rapid release if more T3/4 - Also stimulates thyroid cell growth, TG synthesis
50
How does hCG stimulate the thyroid?
Same alpha unit as TSH
51
What is calcitonin produced by?
``` Parafollicular cells (C cells) - Situated in between the follicles ```
52
What substances cause decreased TSH release from the ant. pituitary?
- Somatostatin - Dopamine - Glucocorticoids

USMLE Endocrine (8 decks)

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