Hypothalamic-Pituitary-Thyroid and GI Hormone Physiology Flashcards
what stimulates the thyroid gland?
cold temps of low concentration of hormones
what hormone does the hypothalamus release to stimulate the anterior pituitary gland?
thyroid releasing hormone
what hormone does the AP release to stimulate the thyroid gland?
Thyroid stimulating hormone (TSH)
aka thyrotropin
what are the two major thyroid hormones?
T4 and T3
what gland is known as the master gland of metabolism?
thyroid gland,
largest gland in the body
what things increase the size of the thyroid gland?
bigger in men
as you age it gets bigger
increase in body weight
what deceases the size of the thyroid gland?
iodine intake
what are the 3 characteristics of an inactive thyroid gland?
colloid (thyroglobulin)
follicles are large
cells lining follicle are flat
what are the 3 characteristics of an active thyroid gland?
- small follicles
- cuboidal follicle lining cells
- edges are scalloped with many small resorption gaps resulting from the uptake of colloid
what are the two primary functions of the thyroid?
- secretion of the two biologically active thyroid hormones T4 and T3
- secretion of calcitonin by parafollicular cells (C cells)
what two things increase when thyroid hormones are released?
basal metabolic rate
heat production
what happens when blood calcium levels are high?
parafollicular cells release calcitonin, which inhibit the resorption of bone by osteoclasts
what are the three key hormones that regulate calcium levels?
- 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
T4 and T3 are synthesized from what?
iodine and tyrosine
where is T4 produced?
exclusively in the thyroid gland
where is T3 produced?
extrathyroidally from T4
60% liver
20% intestines
20% converted to inactive form of T3 (rT3)
which is more potent T3 or T4
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
what enzyme makes the active form of T3 from T4?
5’ de-iodinase enzyme
**note the prime
made mostly in the liver
what enzyme makes the inactive form of T3 (rT3) from T4?
5-de-iodinase enzyme
**note there is no prime
made mostly in the liver
feedback inhibition at the level of the anterior pituitary most efficiently inhibits what?
thyroid releasing hormone
which hormone can act independently of HPT Axis regulation?
T3, because it is made extrathyroidally
without sufficient iodine, thyroid hormones are not released. What builds up and what happens to the thyroid?
TSH builds up leading to oxidative stress leading to hyperplasia and dysplasia of epithelial cells lining the follicles (goiter)
what is the thyroid hormone mechanism of action?
binds to nuclear receptors that act as hormone activated transcription factors
-thyroid hormones act by modulating gene expression to direct protein synthesis
what is the name of the protein in which T3 and T4 are stored?
thyroglobulin (TGB)
what are the only cells in the body that can absorb iodine?
thyroid cells
what is goiter?
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
common goitrogens?
- 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
Iodine deficiency disorder (IDD)
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
Congenital hypothyroidism (Cretinism)
most extreme IDD
due to deficiency of iodine in the mother’s diet during pregnancy resulting in fetal and/or neonatal hypothyroidism from birth
signs and symptoms of cretinism
growth retardation reduced BMR large tongue coarseness of skin dry brittle hair premature tooth loss infertility
how can both too much and too little thyroid hormone lead to goiter?
- 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
the regulation of GI function involves what types of communications?
endocrine
paracrine
neurocrine
what hormone can act in endo, para, and neurocrine function?
cholecystokin (CCK)
what essential to normal GI functions?
balance of + and - hormonal regulation
the brain in your gut is known as what?
enteric nervous system
where are there no nerves of the enteric nervous system?
lumen of the gut
what is the source, stimuli for release and action of gastrin hormone?
- 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
what is the source, stimuli for release and action of cholecystokinin (CCK)?
- 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.
what is the source, stimuli for release and action of secretin hormone?
- source: S cells of duodenum mucosa
- stimuli for release: acidic pH in lumen of sm. intes.
- action: stimulates pancreas to release bicarb
what is the source, stimuli for release and action of gastric inhibitory peptide?
- 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
what are the three major pancreatic hormones?
glucagon
insulin
somatostatin
what does somatostatin do?
- slows production of insulin, glucagon, gastrin and other hormones
- basically puts the brakes on digestion
- excess leads to diabetes
