Hypothalamic-Pituitary-Thyroid and GI Hormone Physiology

Question 1

what stimulates the thyroid gland?

Answer 1

cold temps of low concentration of hormones

Question 2

what hormone does the hypothalamus release to stimulate the anterior pituitary gland?

Answer 2

thyroid releasing hormone

Question 3

what hormone does the AP release to stimulate the thyroid gland?

Answer 3

Thyroid stimulating hormone (TSH)

aka thyrotropin

Question 4

what are the two major thyroid hormones?

Answer 4

T4 and T3

Question 5

what gland is known as the master gland of metabolism?

Answer 5

thyroid gland,

largest gland in the body

Question 6

what things increase the size of the thyroid gland?

Answer 6

bigger in men
as you age it gets bigger
increase in body weight

Question 7

what deceases the size of the thyroid gland?

Answer 7

iodine intake

Question 8

what are the 3 characteristics of an inactive thyroid gland?

Answer 8

colloid (thyroglobulin)
follicles are large
cells lining follicle are flat

Question 9

what are the 3 characteristics of an active thyroid gland?

Answer 9

• small follicles
• cuboidal follicle lining cells
• edges are scalloped with many small resorption gaps resulting from the uptake of colloid

Question 10

what are the two primary functions of the thyroid?

Answer 10

• secretion of the two biologically active thyroid hormones T4 and T3
• secretion of calcitonin by parafollicular cells (C cells)

Question 11

what two things increase when thyroid hormones are released?

Answer 11

basal metabolic rate

heat production

Question 12

what happens when blood calcium levels are high?

Answer 12

parafollicular cells release calcitonin, which inhibit the resorption of bone by osteoclasts

Question 13

what are the three key hormones that regulate calcium levels?

Answer 13

• 1,25 dihydroxycholecalciferol (vitamin D steroid hormone)
• Parathyroid hormone (chief cells in parathyroid glands in response to low blood calcium)**stimulates osteoclasts
• calcitonin (parafollicular cells in response to high calcium) **inhibits osteoclasts

Question 14

T4 and T3 are synthesized from what?

Answer 14

iodine and tyrosine

Question 15

where is T4 produced?

Answer 15

exclusively in the thyroid gland

Question 16

where is T3 produced?

Answer 16

extrathyroidally from T4
60% liver
20% intestines
20% converted to inactive form of T3 (rT3)

Question 17

which is more potent T3 or T4

Answer 17

T3 is 4x more potent than T4, because the vast majority of T4 produced gets quickly converted to T3 peripherally once hormones are released from thyroglobulin stores

Question 18

what enzyme makes the active form of T3 from T4?

Answer 18

5’ de-iodinase enzyme
**note the prime
made mostly in the liver

Question 19

what enzyme makes the inactive form of T3 (rT3) from T4?

Answer 19

5-de-iodinase enzyme
**note there is no prime
made mostly in the liver

Question 20

feedback inhibition at the level of the anterior pituitary most efficiently inhibits what?

Answer 20

thyroid releasing hormone

Question 21

which hormone can act independently of HPT Axis regulation?

Answer 21

T3, because it is made extrathyroidally

Question 22

without sufficient iodine, thyroid hormones are not released. What builds up and what happens to the thyroid?

Answer 22

TSH builds up leading to oxidative stress leading to hyperplasia and dysplasia of epithelial cells lining the follicles (goiter)

Question 23

what is the thyroid hormone mechanism of action?

Answer 23

binds to nuclear receptors that act as hormone activated transcription factors
-thyroid hormones act by modulating gene expression to direct protein synthesis

Question 24

what is the name of the protein in which T3 and T4 are stored?

Answer 24

thyroglobulin (TGB)

Question 25

what are the only cells in the body that can absorb iodine?

Answer 25

thyroid cells

Question 26

what is goiter?

Answer 26

the result of excess growth when thyroid cells are exposed chronically to too much TSH or HCG -may also occur from iodine deficiency, pregnancy or Hashimoto's disease

Question 27

common goitrogens?

Answer 27

- medications that may promote goiter often by interfering with iodine uptake in the thyroid gland eg: anti-thyroid meds, sulfonamide antimicrobials, amiodarone, lithium, salofalk - Environmental compounds eg: mercury, arsenic, nitrates

Question 28

Iodine deficiency disorder (IDD)

Answer 28

patients commonly present with goiter or diffuse thyroid enlargement -if goiter gets large enough, patients may complain of compressive symptoms, such as: hoarseness, shortness of breath, couch, dysphagia

Question 29

Congenital hypothyroidism (Cretinism)

Answer 29

most extreme IDD due to deficiency of iodine in the mother's diet during pregnancy resulting in fetal and/or neonatal hypothyroidism from birth

Question 30

signs and symptoms of cretinism

Answer 30

``` growth retardation reduced BMR large tongue coarseness of skin dry brittle hair premature tooth loss infertility ```

Question 31

how can both too much and too little thyroid hormone lead to goiter?

Answer 31

- Hashimoto's disease: autoimmune hypothyroidism which results in damage and under production of thyroid hormone production. Causes AP to release too much TSH - Grave's disease a autoimmune hyperthyroidism with exopthalmos(bulging eyes). Abs attack thyroid, thyroid defensively overproduces thyroid hormones leading to goiter

Question 32

the regulation of GI function involves what types of communications?

Answer 32

endocrine paracrine neurocrine

Question 33

what hormone can act in endo, para, and neurocrine function?

Answer 33

cholecystokin (CCK)

Question 34

what essential to normal GI functions?

Answer 34

balance of + and - hormonal regulation

Question 35

the brain in your gut is known as what?

Answer 35

enteric nervous system

Question 36

where are there no nerves of the enteric nervous system?

Answer 36

lumen of the gut

Question 37

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

Answer 37

- source: G cells of stomach - stimuli for release: stomach distention, peptides in gastric lumen - action: stimulates parietal cells to release acid and proliferation of gastric mucosa

Question 38

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

Answer 38

- source: I cells in mucosa of duodenum and jejunum - stimuli for release: digestive products of fats, fatty acids, monoglycerides - action: contracts gall bladder to expel bile to sm. intes.

Question 39

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

Answer 39

- source: S cells of duodenum mucosa - stimuli for release: acidic pH in lumen of sm. intes. - action: stimulates pancreas to release bicarb

Question 40

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

Answer 40

- source: mucosa of upper sm. intes - stimuli for release: fatty acids>AAs>carbs - action: inhibits gastric secretions and motility. increases insulin release when duodenum is overloaded with food and when BG is elevated

Question 41

what are the three major pancreatic hormones?

Answer 41

glucagon insulin somatostatin

Question 42

what does somatostatin do?

Answer 42

- slows production of insulin, glucagon, gastrin and other hormones - basically puts the brakes on digestion - excess leads to diabetes