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Physiology Supplemental Cards > Thyroid and Gastrointestinal Hormone Physiology > Flashcards

Thyroid and Gastrointestinal Hormone Physiology Flashcards

1
Q

draw out the hypothalamic-pituitary-thyroid (HPT) negative feedback loop

A
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2
Q

the human thyroid gland is the ___ gland in the body

A

largest

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3
Q

is the thyroid gland well-vascularized?

A

yes

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4
Q

describe inactive thyroid glands

A
  • colloid (thyroglobulin) is abundant
  • follicles are large
  • cells lining the follicle are flat
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5
Q

describe active thyroid glands

A
  • follicles are small
  • cells lining the follicle are cuboid
  • edges are scalloped with many small resorption gaps resulting from the uptake of colloid
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6
Q

what are the two primary functions of the thyroid?

A
  1. secretion of the two biologically active thyroid hormones by follicular cells:
    1. thyroxine (T4)
    2. triiodothyronine (T3)
  2. secretion of calcitonin by parafollicular cells (aka C cells)
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7
Q

thyroid hormones stimulate enzymes involved with glucose oxidation, thus increasing ___ and ___

A
  • basal metabolic rates
  • heat production (calorigenic)
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8
Q

describe parafollicular c-cells

A
  • dispersed throughout the thyroid
  • parathyroids and thymus release calcitonin in response to high blood calcium levels
    • calcitonin lowers blood calcium levels by inhibiting the resorption of bone by osteoclasts
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9
Q

which 3 key hormones regulate calcium homeostasis?

A
  • 1,25-dihydroxycholecalciferol
    • formed from vit D in the skin via the sun and successive hydroxylations in the liver and kidneys
  • parathyroid hormone (PTH)
    • secreted by chief cells of the parathyroid glands in response to low blood Ca2+
  • calcitonin (CT)
    • secreted by parafollicular cells in the thyroid glands in response to high blood Ca2+
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10
Q

thyroid hormones (T3 and T4) are synthesized from ___ and ___

A

iodine and tyrosine

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11
Q

describe the production of T3 and T4

A
  • T3 is primarily produced extrathyroidally (mostly in the liver) from its precursor, T4
  • T4 is produced exclusively in the thyroid
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12
Q

describe the potency of T3 relative to T4

A

T3 is 4x more potent than T4

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13
Q

although the vast majority of the thyroid hormones produced by the thyroid is ___, almost all of the ___ produced gets quickly converted to ___ peripherally once the hormones are released from thyroglobulin stores

A
  • T4
  • T4
  • T3
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14
Q

what is rT3?

A
  • “reverse T3”
  • inactive form
  • made mostly in the liver
  • 20% of T3 formed from T4 is converted to rT3
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15
Q

what enzyme is responsible for converting T4 to T3 and rT3?

A
  • 5’ de-iodinase enzyme converts T4 to T3
  • 5 de-iodinase enzyme converts T4 to rT3
  • important to realize that the # (T4, T3) is referring to the number of iodines bound
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16
Q

HPT axis:

hypothalamus stimulates the anterior pituitary to produce ___, (aka thyrotropin), which stimulates the thyroid gland to produce ___, which can then be converted to ___

A
  • thyroid stimulating hormone (TSH)
  • T4
  • T3
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17
Q

what 3 things can increase T4 and T3 production?

A
  • low T4
  • low T3
  • cold exposure
  • increasing T4 and T3 is calorigenic
    • increase basal metabolic rate (BMR) and body temperature
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18
Q

T3 is mainly produced extrathyroidally. what does this mean in relation to HPT axis regulation?

A

T3 can act independently of HPT axis regulation in peripheral tissues

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19
Q

describe how thyroid hormones target nuclear receptors and modulate gene expression

A
  • stress, dietary iodine, illness, fasting, and cortisol (steroids) can alter activity of deiodinase enzymes
  • under stress/fasting, body convertes less T4 to T3, and more T4 to rT3
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20
Q

without sufficient ___, thyroid hormones are not released and TSH builds up.

A
  • iodine
  • can cause goiter
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21
Q

describe how thyroid hormones are calorigenic

A

cause cells to consume more energy and produce heat as a byproduct

22
Q

the amount of T4 produced and releasd by the thyroid gland is controlled directly by TSH released by the ___

A

anterior pituitary

23
Q

what is the best known stimuli for increasing the rate of TSH secretion by the anterior pituitary?

A

exposure to cold

24
Q

what amino acid is thyroid hormone derived from?

A

tyrosine

25
Q

what receptors do thyroid hormones bind to?

A

nuclear receptors (lipophilic)

26
Q

thyroid hormones form complexes that act as ____

A

hormone-activated transcription factors

27
Q

thyroid hormones act by modulating gene expression to ___

A

direct protein synthesis

28
Q

thyroid gland stores huge reservoirs of both T3 and T4 incorporated in ___, the protein in which the hormones are assembled and stored

A

thyroglobulin (TGB)

29
Q

___ is essential for normal thyroid function

A

iodine

30
Q

thyroid cells are the only cells in the body that can absorb ___

A

iodine

31
Q

how is iodine deficiency defined by WHO?

A
  • urine iodine excretion of >70% of dietary iodine intake
  • for adults, that is urine excretion of >/=105 mcg/day
32
Q

what is goiter?

A
  • the result of excessive tissue growth when the thyroid cells are exposed chronically to too much TSH or hCG
    • exogenous hCG shots for weight loss can also cause goiter
  • may also results from iodine deficiency, but more often results during pregnancy or chronic autoimmune hashimoto’s disease or thyroid cancer
33
Q

what can prevent most iodine deficiency in the USA?

A

iodine supplementation in salt

34
Q

name 5 medications that may promote goiter often by interfering with iodine uptake in the thyroid gland

A
  • anti-thyroid medications
  • sulfonamide antimicrobials
  • amiodarone
  • lithium
  • salofalk (mesalazine is active ingredient)
35
Q

what environmental compounds may promote goiter?

A
  • mercury, arsenic, and some other heavy metals
  • nitrates
  • pesticides
36
Q

describe congenital hypothyroidism (cretinism)

A
  • cretinism is the most extreme manifestation of iodine deficiency disorder
  • can be hereditary, but more often due to deficiency of iodine in the mother’s diet during pregnancy, resulting in fetal and/or neonatal hypothyroidism from birth
37
Q

how can too much or too little thyroid hormone lead to goiter?

A
  • hypothyroidism - hashimotos disease
    • reduced T4 and T3 causes overproduction of TSH by the anterior pituitary, causing goiter
  • hyperthyroidism - graves disease
    • auto-antibodies attach thyroid, thyroid defensively overproduces thyroid hormones, leading to goiter
38
Q

what are 4 functions of the GI?

A
  • secretion
  • digestion
  • absorption
  • gut motility
39
Q

the regulation of GI function involves what 3 forms of hormonal communication?

A
  • endocrine
  • paracrine
  • neurocrine
40
Q

describe endocrine regulation of the GI

A
  • hormones released in response to meal consumption travel in the bloodstream and have distant effects on target cells in the GI, pancreas, or both
41
Q

describe paracrine regulation of the GI

A
  • provide local effects on neighboring cells
42
Q

describe neurocrine regulation of the GI

A
  • connections from both the CNS and the enteric nervous system integrate central regulation of the gut in response to changes in the luminal environment
43
Q

what is an example of a hormone that can act in neurocrine, endocrine, and paracrine fashion in the regulation of the GI?

A

cholecystokinin (CCK)

44
Q

how are normal GI functions regulated?

A

balance of positive and negative hormonal events

45
Q

what is the source, stimuli for release, and action of gastrin?

A
  • source: G cells in stomach
  • stimuli: consumption and peptides in gastric lumen
  • action: stimulates gastric acid secretion and proliferation of gastric mucosa
46
Q

what is the source, stimuli for release, and action of cholecystokinin (CCK)?

A
  • source: I cells in duodenum and jejunum
  • stimuli: digestive products of fats, fatty acids, and monoglycerides
  • action: contracts gallbladder, expelling bile into small intestine
47
Q

what is the source, stimuli for release, and action of secretin?

A
  • source: S cells in duodenum
  • stimuli: acidic pH in lumen of small intestine
  • action: stimulates pancreas to release bicarbonate to neutralize acids
48
Q

what is the source, stimuli for release, and action of gastric inhibitory protein?

A
  • source: mucosa of upper small intestine
  • stimuli: fatty acids > amino acids > CHOs
  • action: inhibits gastric secretions and motility, and increases insulin release when duodenum is overloaded and blood glucose is elevated
49
Q

name the pancreatic hormone with the following characteristics:

  • purpose: counter-regulatory assistance for insulin
  • action: release of glucose from cells
  • released due to: low blood sugar
  • inhibited by: high blood sugar
  • deficiency: hypoglycemia
  • excess: hyperglycemia
A

glucagon

50
Q

name the pancreatic hormone with the following characteristics:

  • purpose: regulate blood glucose
  • action: adsorption of glucose from cells
  • released due to: high blood sugar
  • inhibited by: low blood sugar
  • deficiency: diabetes
  • excess: hypoglycemia
A

insulin

51
Q

name the pancreatic hormone with the following characteristics:

  • purpose: regulates GI hormones in pancreas and GI tract
  • action: slows production of insulin, glucagon, gastrin, and other hormones
  • released due to: high hormone levels
  • inhibited by: low hormone levels
  • excess: diabetes
A

somatostatin

Physiology Supplemental Cards (3 decks)

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