Physiology-Thyroid & Parathyroid Flashcards
Who are the major players in maintaining bone health?
Osteoblasts: deposit bone, osteoclasts: degrade bone, osteocytes: maintain bone
What are modulators of bone remodeling?
Blood Ca, sex steroids, mechanical usage
Where are the C-cells (parafollicular cells) derived from?
Neural crest and ultimobranchial body
Why can the thymus have parathyroid glands?
They are both derived from the 3rd pharyngeal arch (inferior parathyroid gland)
What embryonic layer does the thyroid develop from?
Oral endoderm
What fills up the thyroid nodules?
Follicles of varying sizes
What do these cells do?
Thyrofollicular cells (thyrocytes) make thyroglobulin and store it in colloid. Iodide is transported into the cell and is oxidized to iodine. Iodine is released into the follicle and forms mono-iodo-tyrosine (MIT) and di-iodo-tyrosine (DIT) when complexed with thyroglobulin. MIT + DIT forms T3 (tri-iodo-thryimine) and DIT + DIT forms T4 (tetra-iodo-thyrimine)
Organification
Formation of MIT and DIT from iodine and thyroglobulin
What enzyme is involved in forming T3 and T4 from MIT and DIT?
Thyroid peroxidase + hydrogen peroxidase
How does the follicular cell respond to TSH?
Colloid is endocytosed and degraded by the lysosome. T3 & T4 are released basally into the blood. MIT and DIT are degraded for future use in thyroglobulin.
How does the hypothalamus pituitary axis function?
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Which follicle is more active?
2: Note how much larger the cells are in the smaller follicle.
How does a section of a thyroid gland look when a person with Grave’s has a goiter? With Hashimoto’s thyroiditis? With a nontoxic goiter?
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What do these cells do?
Parafollicular (C cells) secrete calcitonin, which reduces serum Ca levels and promotes bone formation by inhibiting the resorptive activity of osteoclasts.
What stimulates secretion of calcitonin?
10% increase in serum calcitonin
How is calcium carried in the blood?
50% free 50% protein-bound
Types of cells in the parathyroid
Chief cells (predominate cells and make PTH) and oxyphils
What type of people typically have more fat in their parathyroid glands?
Older
How is PTH stored and where does it act when released?
Stored in chief cell granules and released when Ca sensing receptors on chief cells sense low Ca levels. PTH increases bone resorption, vitamin D activation and works on gut and kidney to retain calcium.
Where does vitamin D act in calcium homeostasis?
Increases intestinal reabsorption of Ca
What is the feedback inhibition for PTH release?
Ca binds PTH promoter and inactivates it. Vitamin D binds to repress PTH gene.
Where is the PTH receptor located in bone?
Osteoblasts and osteoblast precursors. Osteoblasts then secrete RANKL that bind RANK on immature osteoclasts, causing them to mature, it also binds to mature osteoclasts and causes the to resorb bone.
How does the osteoblast prevent over-activation of osteoclasts when stimulated by PTH?
OPG is secreted simultaneously with RANKL and binds it up, preventing it from binding RANK.
Why are post-menopausal women at particular risk for osteoporosis?
Estrogen decreases levels of RANKL and increases levels of OPG, preserving bone and negating effects of PTH. When estrogen levels are decreased, PTH has a stronger effect and more bone resorption occurs.
What is the paradoxical effect of PTH used to treat osteoporosis?
If it is injected at intermittent bursts, it increases bone formation
How does calcitonin affect the kidney?
Stimulates secretion of calcium into the urine
4 things that stimulate bone formation
Increased serum Ca concentration, increased mechanical load, estrogen and testosterone.
How does PTH affect the kidney?
Decreased Ca excretion and increased phosphate excretion
Effect on the bones from hyperparathyroidism
Osteoporosis from increased bone resorption
