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Physiology 1 > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

Thyroid gland functions

A

Extremely important
1. Regulates metabolism bodywide (not just energy metabolism)
2. Crucial determinant of normal growth and development
3. Optimizes sensitivity of particular tissues to other hormones
4. Systemic effects of thyroid hormone
(Mess with the thyroid hormone and you’re messing with the whole body)

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2
Q

Thyroid hormone function in growth and development

A
  • Crucial determinant of normal growth and development of many systems including CNS, bone and respiratory
  • Fetal Thyroid tissue particularly important-(maternal thyroid can’t cross placenta)
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3
Q

Systemic Effects of Thyroid Hormone

A
  1. Increases basal metabolic rate
  2. Promotes growth and maturation
  3. Central nervous system effects
  4. Autonomic Nervous System
  5. Temperature regulation
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4
Q

Increased basal metabolic rate effects

A 
Systemic effect of thyroid
Increases cardiac output
Blood oxygenation
Glucose uptake
Glycogen formation
Lipid mobilization
Protein synthesis
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5
Q

Promotion of growth and maturation effects

A

Systemic effect of thyroid
Required for normal skeletal growth
Promotes bone and cartilage growth
Stimulates both GH secretion and somatomedin production

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6
Q

Central Nervous System effects

A

Systemic effect of thyroid
Necessary for perinatal maturation (pre and post natal development)
Promotes neural branching and myelinization of nerves

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7
Q

Autonomic nervous system effects

A

Systemic effects of thyroid hormone

Increases sympathetic activity

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8
Q

Temperature Regulation by Thyroid

A

Systemic effect of thyroid hormone

Increases thermogenesis

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9
Q

Gross thyroid structure

A

Species variations but doesn’t matter

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10
Q

Arterial supply of Thyroid Gland

A

Copius arterial supply!!

Second best perfused gland in the body

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11
Q

Functional Components of Thyroid Gland

A
  1. Folicular cells
  2. Colloid
  3. Parafollicular cells
  4. Capillaries
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12
Q

Folicular cells

A

Collection of cells surrounding a cavity (the follicular lumen) that stores material secreted by these cells

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13
Q

Colloid

A

The material inside the follicular lumen

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14
Q

Parafollicular Cells

A

Single or small groups of cells tucked among the bases of the follicular cells
Participate in Ca (calcitonin) regulation

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15
Q

Capillaries

A

Abundant

Rich supply to follicular cells

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16
Q

Follicle Balloon example

A

Whole balloon- follicle
Area Inside balloon- follicular lumen
Helium/Air Inside- colloid
Rubber- made up of lots of follicular cells

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17
Q

Thyroid Hormones

A

T3/T4
Amine Hormones
Derived from tyosine-amino acid skeleton/foundation of thyroid hormone molecules

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18
Q

Active/Inactive Forms of Thyroid Hormones

A

Active- T4 (Thyroxine)
Active- T3 (Thyronine)
Inactive- ReverseT3

19
Q

Reverse T3

A

Inactive form of thyroid hormone
Triple iodonated tyrosine, but 4th iodine was removed at the wrong site on the ring
Essentially a mistake with little if any biological activity (essentially zero potency)

20
Q

Thyroxine

A

T4, active form of thyroid hormone
Amine hormone
MAIN secretory component (gland makes more of it) but less potent

21
Q

Thyronine

A

T3, active form of thyroid hormone
Amine hormone
mostly formed from conversion of of T4 to T3 in tissues
MOST biologically potent (binds more readily to receptors and carriers bind less tightly)

22
Q

Thyroid Hormone Terminology

A

Refers collectively to T3 AND T4 together

Always considered together

23
Q

Thyronines (Definition)

A

Talking about T3 OR T4

Considered separately

24
Q

Iodine Regulation

A
  • Integral part of thyroid hormones (equal to that of tyrosine) just “harder to find”
  • Therefore, it’s reserves are strictly regulated (very little eliminated and several systems work together for reabsorption and recycling)
25
Q

Sources of Iodine

A

Diet- absorbed into the blood stream from the small intestine
Therefore, formation of adequate amounts of thyroid hormone depends on adequate dietary supply
Source examples- tap water, iodized salt

26
Q

Iodide Pump

A

Follicular cells have an iodide pump (Active transport) which takes iodide from blood and pumps it into follicular cells

27
Q

Iodine Deficiency

A

Thyroid gland adapts to iodine supply so animals living in areas with iodine poor soils up regulate the iodine active transport system
Iodine trapping is facilitated
Chronic deficiency can lead to goiter
Sudden deprivation-upregulation takes time so sudden partial deprivation increases risk of hypothyroidism and complete deprivation is eventually fatal

28
Q

Thyroglobulin

A

Large protein molecule (NOT a thyroid hormone) synthesized by thyroid follicular cells that contains TYROSINE in its amino acid sequence
Essential for thyroid hormone production

29
Q

Thyroglobulin synthesis

A

Made inside the follicular cells and transported into follicular lumen where it is stored pending iodide arrival (makes up the colloid)

30
Q

Processing of Iodine

A
  1. Enterocytes pick up iodine and transport it through systemic blood to thyroid follicular cells
  2. Thyroid follicular cells actively pump iodide from plasma into their cytosol
  3. Iodide is oxidized intracellularly and then transported into follicular lumen
  4. The iodine is highly reactive and quickly binds to a tyrosyl residue on thyroglobulin in the lumen
31
Q

MIT

A

Monoiodotyrosine

Results when one carbon on tyrosine ring is carbonated

32
Q

DIT

A

Di-iodotyrosine

Results when two carbons on tyrosine ring are iodinated

33
Q

Coupling of Iodotyrosines

A

MITs and/or DITs bind with each other on the thyroglobulin molecule in follicular lumen (MUST take place here)
MIT+DIT= T3 (triiodothyroNINE)
DIT+DIT= T4 (tetraiodothyroNINE)
(Thyroid gland is capable of making T3 directly but doesn’t usually happen)

34
Q

Storage of Thyroglobulin

A

Iodonated thyroglobulin molecule is stored in the follicular lumen NOT follicular cell (extracellularly) until needed (allows body to also store iodine)
Thyroglobulin has multiple substances on it (MIT, DIT, T3, T4)

35
Q

Release of thyroid hormone to blood

A

Hypothalamus releases TRH-> anterior pituitary releases TSH-> “cue” for thyroglobulin

  1. Upon cue- thyroglobulin is endocytosed from the follicular lumen and back into the follicular cell (intracellular)
  2. Proteolysis of thyroglobulin causes release of T3/T4 (thyronines) and MIT/DIT are deiodonated (both the iodines and tyrosine’s are reused)
  3. The LIPID SOLUBLE thyronines freely diffuse through basal cell membrane into intersitium and into the systemic blood
  4. Thyronines picked up by binding proteins and transported throughout body
36
Q

T3 Formation

A

Some takes place in follicular lumen
Most T3 formed OUTSIDE the thyroid gland in the tissue (from T4)
-Liver and Kidney (makes lots of T3 because has lots of enzyme)
-Skeletal muscle (makes lots of T3 due to mass)

37
Q

Plasma transport of Thyroid Hormones

A

Carrier proteins required to move through plasma (they are lipid soluble/plasma insoluble) but only unbound hormone is active

38
Q

Relative Binding Affinities

A

Only unbound hormone is active
T4’s carrier binds very tightly so less T4 can diffuse free from carrier
T3’s carrier binds less tightly (partly why T3 has greater activity than T4)

39
Q

Regulation of Thyroid hormones

A

Short loop Negative feedback (thyroid hormones very important so want to adjust levels quickly)
Hypothalamic-Pituitary-Thyroid Axis: main feedback is the T3/T4 inhibition of TSH (at pituitary level); smaller effect on thyroid releasing hormone (at the hypothalamic level)

40
Q

Stimulation of Thyroid Hormone Synthesis/Release

A

Short periods of stress/high metabolic demand (ex. cold)

  1. Hypothalamus responds by secreting thyroid releasing hormone
  2. TRH travels to anterior pituitary gland
  3. TRH there supersedes the “day to day” control of T3/T4 and causes increased release of thyroid stimulating hormone
  4. Thyroid gland responds by making/releasing more thyronines
41
Q

Day to Day Hormone Synthesis/Release

A

Plasma levels of T3/T4 act directly on anterior pituitary to influence thyroid stimulating hormone release
-Lower T3/T4 levels cause TSH release and vice versa

42
Q

Inhibition of Thyroid Hormone Stimulation/Release

A

Day to Day: Adequate plasma T3/T4 levels feed back to the anterior pituitary and inhibit release of further TSH
Prolonged high stress: extra high levels of T3/T4 cause TRH shutdown

43
Q

Thyroid Function Disorders

A

Hypothyroidism: Usually dogs
Hyperthyroidism: Typically cats (usually due to benign tumor)

Physiology 1 (7 decks)

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