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FHB Exam 4: Endocrinology and Reproduction > Endocrine V: Thyroid > Flashcards

Endocrine V: Thyroid Flashcards

1
Q

Which cellular component makes up 30% of the thyroid mass?

A

colloid, which is the extracellular storage site of T3/T4 and thyroglobulin

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2
Q

What is the blood supply of the thyroid?

A
  • arteries: superior (ext. carotid) and inferior (thyrocervical trunk) thyroid arteries
  • veins: venous plexus on surface giving rise to superior, middle, inferior thyroid veins (drain into int. jugular)
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3
Q

What is the innervation of the thyroid?

A

middle and inferior cervical ganglia (SNS)

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4
Q

What happens with dietary iodide that is ingested in excess?

A

it is secreted into the urine as iodine

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5
Q

Which two precursors do thyroid hormones (iodothyronines) require?

A

thyroglobulin (TG) and iodide

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6
Q

What is the average dietary intake of iodide? What is the amount that will result in thyroid hormone deficiency?

A

average dietary intake is 400 micrograms/day; less than 20 micrograms will result in hormone deficiency

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7
Q

How is the Wolf-Chaikoff effect useful clinically?

A

in hyperthyroid patients, very high iodide doses are used to rapidly shut down thyroid hormone production

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8
Q

What is the most preventable cause of mental retardation?

A

thyroid hormone deficiency (due to iodide deficiency)

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9
Q

How are thyroid hormones primarily transported?

A

more than 99% of thyroid hormone is bound to transport proteins in circulation; the majority is bound to TBG

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10
Q

What is unique about TBG?

A

it can reversibly release T4 to target tissues

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11
Q

Do thyroid hormones affect cholesterol in the body? If so, how?

A

Yes, they affect cholesterol clearance rates by increasing hepatic LDL receptors.

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12
Q

Which is the most active form of thyroid hormone?

A

T3, triiodothyronine; it is a pleiotropic hormone, similar to cortisol, and regulates BMR

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13
Q

Is T3 the precursor to T4, or is T4 the precursor to T3?

A

T4 is the precursor to T3 (T4 is deiodinated to T3 in thyrotropes and brain by type II deiodinase)

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14
Q

Where is thyrotropin releasing hormone (TRH) synthesized and secreted?

A

PVN of hypothalamus

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15
Q

Where do T3 and T4 exert their negative feedback effects?

A

At the anterior pituitary (on thyrotrophs) and at hypothalamus (on PVN neurons)

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16
Q

What are the tonic inhibitors of TSH?

A

somatostatin and dopamine

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17
Q

True or false: all steps of thyroid synthesis are stimulated by TSH.

A

true!

18
Q

What are the steps in thyroid hormone synthesis?

A
  1. Iodide trapping
  2. Transport
  3. Iodination
  4. Conjugation
  5. Endocytosis
  6. Proteolysis
  7. Secretion
19
Q

Which residues of thyroglobulin are iodinated?

A

tyrosyl residues

20
Q

Which enzyme serves as as thyroid hormone sensor?

A

type II deiodinase in thyrotropes and brain

21
Q

What does the thyroid peroxidase enzyme do?

A

It oxidizes iodide to form iodine, which is needed for iodination of tyrosyl residues on TG.

22
Q

What does carbimazole (methimazole) do?

A

inhibits TPO and is used as a treatment for hyperthyroidism

23
Q

How is reverse T3 distinguished from the active form of T3?

A

presence of 2 iodinated residues on the OUTER ring, as opposed to 1 on the outer and 1 on the inner

24
Q

What is the HL of T3 vs. T4 and why?

A

T3=1 day, T4=7 days; this is because T4 is more tightly bound to transport protein

25
Q

More than ___% of thyroid hormone is bound to transport proteins in circulation.

A

99

26
Q

In a radioactive iodide uptake scan, what is the normal uptake of iodide after 24 hours?

A

25%

27
Q

What is the organification defect?

A

Iodide cannot be incorporated into tyrosine residues. It can be tested for by blocking NIS with an inhibitor. If the level of iodide rapidly drops off, then you know the iodide that was previously administered is NOT actively being incorporated into tyrosine residues of TG.

28
Q

Which deiodinase works on the outer and inner ring of tyrosine?

A

type I (D1)

29
Q

Which deiodinase works only on the outer ring of tyrosine?

A

type II (D2)

30
Q

Which deiodinase works only on the inner ring of tyrosine?

A

type III (D3)

31
Q

What increases vs. decreases TBG? What do changes in TBG affect?

A
  • estrogen and hepatitis increase TBG
  • nephrotic syndrome and steroids decrease TBG
  • changes in TBG don’t usually affect bioavailable T3, but DO affect total T3/T4 levels
  • changes in TBG do not change “free” T3/T4
32
Q

Where is TBG made?

A

in the liver; it is a 394 aa glycoprotein

33
Q

What is the affinity of the thyroid hormone receptor for T3 vs. T4?

A
  • high affinity, low capacity for T3

- low affinity for T4

34
Q

What are some key facts about the thyroid hormone receptor?

A
  • nuclear receptor family
  • forms heterodimers with retinoic acid receptor (RXR)
  • expressed in nearly every cell type
  • high affinity for T3, low affinity for T4
35
Q

Which receptor initially transports T3 and T4 across the membrane of cells in peripheral tissues so that they can access the intracellular nuclear thyroid receptor?

A

MCT (monocarboxylate transporter)

36
Q

What is T3 critical for in the CNS?

A

normal brain development:

  • neuronal cell migration/differentiation
  • myelination
  • synaptic transmission
37
Q

How does T3 affect metabolism?

A

It increases cellular oxygen consumption and heat production by increasing mitochondrial activity (generates “futile cycles”).

38
Q

What are some characteristic hyperthyroid symptoms?

A

tachycardia, ophthalmopathy (bulging eyes), irritability, hyperactivity, heat intolerance, weight loss, nervousness, muscle wasting

39
Q

What are some characteristic hypothyroid symptoms?

A

lethargy, fatigue, hair loss, cold intolerance, brittle nails, decreased appetite, weight gain

40
Q

What is thyroid storm?

A

hyperthyroid (often underlying and undetected) coupled with/precipitated by severe acute illness; it is an emergency life-threatening situation

41
Q

What is the ONLY acute treatment for thyroid storm?

A

propylthiouracil (PUT)

FHB Exam 4: Endocrinology and Reproduction (14 decks)

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