Parathyroid Basics
Inferior parathyroid
*Think 3 is less than 4, 3 = inferior, 4 = superior
PTH secretion is primarily controlled by _______
Free (ionized) calcium, decreased free calcium --> increased PTH secretion
What enzyme does PTH upregulate to increase serum calcium?
1-alpha-hydroxylase
What is the active form of vitamin D?
Calcitriol (1,25-dihydroxycholecalciferol)
Name the cell type that releases PTH
Chief cells
Mild hypomagnesemia causes what change in PTH secretion?
Increased PTH secretion!
↓ extracellular magnesium → ↓ activation of the calcium-sensing receptor (CaSR) on parathyroid cells → ↑ PTH secretion (CaSR normally suppresses PTH secretion)
Where in the renal tubule does PTH increase Ca2+ reabsorption?
Distal convoluted tubule
Where is vitamin D converted to its storage form and active form?
Storage form: liver by 25-hydroxylase
Active form: kidney by 1-alpha-hydroxylase
Name the cell type that contain calcium sensing receptors
Oxyphil cells
Marked hypomagnesemia causes what change in PTH secretion?
Decrease PTH secretion!
Magnesium is essential for the cellular machinery required to produce and release PTH.
Does PTH or calcitriol decrease phosphate reabsorption in the renal tubule?
PTH
What is the impact of calcitriol in the GI tract?
Increase intestinal absorption of calcium and phosphate
What type of receptor is the calcium sensing receptors?
G-protein coupled receptor
An increase in albumin causes what change in PTH?
No change, PTH secretion is not directly affected by albumin levels. When albumin levels change, total calcium changes proportionally, but ionized calcium typically remains stable.
How does PTH increase bone resorption?
↑ expression of RANKL on osteoblasts
PTH ↑ expression of RANKL on osteoblasts → RANKL binds the osteoclastic RANK receptor → ↑ osteoclast activity → bone resorption → release of ionized Ca2+
How does calcitriol impact the bone at low vs high levels?
Low levels: Bone mineralization
High levels: Bone resorption
What is the primary blood supply to both superior and inferior parathyroid glands?
Inferior thyroid artery
Low pH causes what change in PTH secretion? Why?
↓ pH → ↑ H+ in serum binding to proteins → ↓ Ca2+ binding to proteins → ↑ ionized Ca2+ concentration → ↓ PTH
What is the main effect on serum calcium and phosphate for the following hormones: PTH, calcitriol, calcitonin?
PTH: increases calcium, decreases phosphate
Calcitriol: increases calcium, increases phosphate
Calcitonin: decreases calcium, decreases phosphate
Where is the hormone that is stimulated by high phosphate/calcitriol and inhibits 1-alpha-hydroxylase released form?
Osteocytes and osteoblasts in the bone
Hormone in question: FGF23