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Human phys II, Exam 2 > Lect 16, thyroid > Flashcards

Lect 16, thyroid Flashcards

1
Q

thyroid hormones are derived from what amino acid? What element is required?

A
  • tyrosine
  • iodine is required
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2
Q

what cells in the thyroid gland are the site of synthesis?

A

follicular cell

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3
Q

Where is thyroid gland located

A
  • below larynx, over the trachea
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4
Q

Where, in the thyroid gland, are hormones stored

A
  • thyroid gland comprised numerous follicles which are filled with colloid and lined by follicular cells
  • hormones are stored in the colloid of the lumen
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5
Q

what is the major protein of the thyroid colloid

A

thyroglobulin

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6
Q

what is the first step in thyroid hormone synthesis

A
  • uptake (trapping) of iodide
    • iodide trap: Na+/I- co-transporter
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7
Q

activity of the iodide trap is modulated by

A

diet, low iodide diet -> increased trap activity to compensate for deficiency

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8
Q

What is second step of thyroid hormone synthesis, after iodide trapping

A
  • once iodide is inside cell, iodide is transported to the follicular lumen by pendrin
  • iodide is then oxidized to iodine by thyroid peroxidase
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9
Q

what happens to iodine in the follicular lumen (colloid): step 3

A
  • iodine attaches to a tyrosine on thyroglobulin
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10
Q

in the colloid, two coupling reactions occur, name them

A
  • 2 DITs yield thyroxine (T4)
  • 1 MIT and 1 DIT yields triiodothyronine (T3)
  • reaction catalyzed by thyroid peroxidase
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11
Q

which thyroid hormone is produced more

A

more T4 is produced because the DIT + DIT coupling reaction is faster

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12
Q

do all the MITs and DITs couple?

A

no, portions of MIT and DIT do not couple but remain attached to thyroglobulin

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13
Q

how are thyroid hormones secreted from the colloid

A
  1. follicular cell engulf part of colloid containing thyroglobulin
    • facilitated by megalin
  2. lysosomes attack colloid and proteases split iodinated products from thyroglobulin
  3. T3 and T4 diffuse into blood
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14
Q

in the follicular cell, once T3 and T4 are spit from MIT and DIT, what happens to MIT and DIT

A

they are deioninated by thyroid deiodinase

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15
Q

Name three reasons why T3 is more biologically active

A
  • T4 binds more tightly to plasma proteins
  • peripheral conceversion of T4
  • thyroid hormone receptors in the nuclear have a greater affinity to T3
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16
Q

What happens to T4 when it enters a target cell

A

most of it is converted to T3, so cytoplasmic levels of T4 and T3 are about equal

17
Q

binding of T3 or T4 to thyroid hormone receptors has what effect

A

bound thyroid hormone receptors bind to nuclear DNA, thus T3, T4 regulates transcription of genes

18
Q

regulation of thyroid hormone secretion

A
  • hypothalamus releases TRH : thyrotropin releasing hormone which acts on
  • Thyrotoph cell isn anterior pituitary to release TSH: thyroid stimulating hormone
19
Q

what two roles does thyroid stimulating hormone have

A
  • acts on thyroid to stimulate secretion of thyroid hormones
  • growth of thyroid gland
20
Q

describe role of T3 in negative feedback loop regulation of thyroid hormones

A
  • inhibits release of TSH by down regulating TRH receptor on thyrotrophs and decreasing TSH synthesis
  • inhibit release of TRH in the hypothalamus
21
Q

what is the key hormone in negative feedback loop regulation of thyroid hormones

A

T3

22
Q

role of somatostatin and dopamine in thyroid hormone regulation

A
  • somatostatin and dopamine, released by hypothalamus, inhibits TSH release
23
Q

TRH from hypothalamus utilizes what protein receptor mechanism on thyrotrophs

A
  • TRH binds to G protein coupled receptor
    • raising Ca2+
  • cause thyrotrophs to synthesize and release TSH
24
Q

describe effects of thyroid hormones on metabolism

A
  • increase metabolic rate
  • increase oxygen consumption
  • increase body temperature
25
Q

describe how hyperthyroidism promotoes futile cycles in BMR, Carb metabolism, protein metabolism, and lipid metabolism

A
  • causes increased synthesis and degredation of carbs, protein, and lipids
  • causes net degradation of tissue protein -> can lead to a loss of muscle mass
26
Q

physiological effects of thyroid hormones on CV system

A
  • increase CO and ventilation
  • T3: chronotropic and inotropic, and reduces vascular resistance
27
Q

physiological effects of thyroid hormones on sympathetic nervous system

A
  • increases synthesis of B adrenergic receptors in cardiac, skeletal muscles and adipocytes
    • no change in catecholamines
  • contributes to thermogenic response
28
Q

why is normal/high thyroid levels important in pregnancy

A
  • thyroid hormone is essential for normal maturation of CNS
  • low thyroid status during development is detrimental
29
Q

physiological effects of thyroid hormones on growth

A
  • required for normal growth and development
  • promotes bone formation
30
Q

cretinism

A
  • condition of severely stunted physical and mental growth due to untreated congenital deficiency of thyroid hormones (congential hypothyroidism)
  • due to maternal nutritional deficiency of iodine
31
Q

how is a goiter formed

A
  • iodine deficiency
  • thyroid cant produce hormone to maintain feedback mechanism
  • pituitary responds by secreting more TSH which stimulates growth of thyroid gland
  • -> hypothyroidism and goiter (large thyroid)
32
Q

what is hashimotos thyroiditis

A
  • antithyroid antibodies destroy thyroid gland function
  • early stages: elevated TSH but normal T3 and T4
  • if untreated: see gradual decline of T3, T4
33
Q

what is Graves disease

A
  • hyperthyroidism
  • antibodies mimic TSH and bind to TSH receptors on the thyroid
  • stimulates thyroid hormone production
  • get loss of feedback control
34
Q

exophthalmos is a common sign of

A

graves disease: hyperthyroidism

35
Q

function of 5’/3’ monodeiodinase

A

involved in peripheral conversion: removes iodine from T4 -> T3

36
Q

differentiate between type I and II peripheral conversion

A
  • type I: liver, kidney, thyroid
  • type II: pituitary, CNS, placenta
37
Q

when is type I peripheral conversion reduced

A

caloric restriction and severe stress

  • *conversion of T4->T3 at liver, kidneys, thyroid is reduced
  • these tissues become unresponsive to thyroid hormone = decreased metabolic rate

Human phys II, Exam 2 (11 decks)

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