Saxappeal1

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Hey all,
I hope I don't sound stupid asking this question, but I just came across this bullet point in FA 2016 (pg 314) that states that a modest decrease in Mg will cause an increase in PTH secretion, yet a severe decrease in Mg will cause a decrease of PTH. Can someone please explain the relationship between Mg and PTH and how it fits into this bullet point? Thanks!
 

Taco Time

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Hey all,
I hope I don't sound stupid asking this question, but I just came across this bullet point in FA 2016 (pg 314) that states that a modest decrease in Mg will cause an increase in PTH secretion, yet a severe decrease in Mg will cause a decrease of PTH. Can someone please explain the relationship between Mg and PTH and how it fits into this bullet point? Thanks!
Hey Saxappeal1,

Low Mg induces low Ca levels. Magnesium is involved in hundreds of pathways in the body so understanding how it functions in every aspect is probably not worth while. PTH would initially be increased in an attempt to correct the low Ca. However, once the Mg deficiency continues/worsens to markedly low concentrations, there is a paradoxical block on PTH.

The only way that PTH secretions would be decreased with ongoing low levels of Ca (due to the low levels of Mg) would be if the body thought that there was enough Ca. This occurs when markedly low Mg concentrations leads to activation of G-proteins that mimic activation of calcium sensing receptors. "Ample" Ca is sensed --> no longer need to secrete PTH.

Here's a paper that goes into more detail if you're interested:
http://www.ncbi.nlm.nih.gov/m/pubmed/12105390/
 
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Saxappeal1

Saxappeal1

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Why does this paradoxical block happen? Thats where I am confused lol
 

Taco Time

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Northerncardinal

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Severe hypomagnesemia causes low PTH secretion cuz Mg is a cofactor for adenylyl cyclase to form cAMP, which in turn is needed for PTH secretion.

what i don't get is how mild hypomagnesemia causes hypocalcemia?
 
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Saxappeal1

Saxappeal1

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Well according to that relationship, wouldn't that mean that low magnesium levels result in low cAMP, low PTH and thus hypocalcemia at low levels? Or am I missing something?