PTH,VitD and Bone turnover..... finally a solution!! - USMLE Forums
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  #1  
Old 04-15-2014
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Correct Answer PTH,VitD and Bone turnover..... finally a solution!!

PTH
-main effect on bone is resorption.
-constant high levels of PTH would cause continuous bone resorption (catabolic).
-BUT......low dose intermittent pulses would cause bone formation (anabolic).
-I'll tell you how.......
*A pulse of PTH would cause a brief Resorption of bone.

* It would also cause an increase in VitD3, which would, along with PTH cause bone resorption.

* Now the brief period of resorption of bone would come to a halt when the PTH levels fall (because VitD requires the concurrent presence of PTH to exert its action on bone resorption).

* So in the absence of PTH, there would be no bone resorption going on ie VitD would exert only its bone forming action).

* The brief period of resorption would cause a breakdown of "Old Bone"
and thereafter osteoblasts would lay down "New Bone" under influence of VitD.............
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so vitD causes bone resorption in presence of PTH and bone formation in absence of PTH ?
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VitD causes

1. Bone resorption (dissolving "Old Bone") in the presence of PTH.

2. Bone formation......this is accomplished by increasing the absorption of calcium and phosphate from intestines (because we need both for bone formation) and by increased osteoblastic activity. (please note that this action DOES NOT require the presence of PTH)

And remember the above processes occur simultaneously in our body (Old bone is continuously replace by New bone). Its a DYNAMIC process.

BUT......in the absence of PTH (or low levels).......VitD exerts only its bone forming action only and not its bone resorping action.

In the presence of PTH its exerts BOTH.

Hopes this helps........
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thanks a lot..it was really helpful.. i always used to get confused with PTH , vitD and bone physio....
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Old 04-15-2014
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Yep....me too.....

I just spent the last 2 hours trying to figure it out. This was the only logical explanation I could come up with. So I decide to share it here, cause I knew EVERYONE would get confused over this.

The human body is not as easy as we think.........
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Old 04-17-2014
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I just read this topic again in Kaplan notes it says at normal levels of vitD3 it causes bone formation as it increases absorption from gut nd kidney. But it also says at abornarmally high activity levels vitD3 can also cause bone resorption and in that case even if PTH isn't present we get bone resorption. So in 2 cases we get bone resorption by the action of vit D3 when PTH is present and when vit D3 is abnormally high even in absence of PTH...
Quote:
Originally Posted by medstud. View Post
PTH
-main effect on bone is resorption.
-constant high levels of PTH would cause continuous bone resorption (catabolic).
-BUT......low dose intermittent pulses would cause bone formation (anabolic).
-I'll tell you how.......
*A pulse of PTH would cause a brief Resorption of bone.

* It would also cause an increase in VitD3, which would, along with PTH cause bone resorption.

* Now the brief period of resorption of bone would come to a halt when the PTH levels fall (because VitD requires the concurrent presence of PTH to exert its action on bone resorption).

* So in the absence of PTH, there would be no bone resorption going on ie VitD would exert only its bone forming action).

* The brief period of resorption would cause a breakdown of "Old Bone"
and thereafter osteoblasts would lay down "New Bone" under influence of VitD.............
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Old 04-17-2014
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I might be wrong... Even in presence of abnormally Hugh levels of Vit D3 we need PTH for bone resorption.. without PTH we won't get bone resorption... Not sure again..
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Cleared my doubt... In case if excess vit D3 it increases plasma calcium which causes decrease in PTH. ( calcium level is the main drive for PTH ). So in excess level of vit D3 it is characterized by decreased PTH , increased plasma calcium , increased plasma phosphate but normal or decreased phosphate excretion. So excess vit D and secondary hypoparathyroidism causes bone resorption nd bone mass decreases.
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Quote:
Originally Posted by adrenaline106 View Post
I might be wrong... Even in presence of abnormally Hugh levels of Vit D3 we need PTH for bone resorption.. without PTH we won't get bone resorption... Not sure again..
Ok.
*Vitamin D DOES cause bone resorption even without the presence of PTH.

BUT...... This action of Vitamin D is very insignificant at physiologically normal levels.

* In Hypervitaminosis D---- This bone resorbing capacity of Vitamin D increases significantly so as to cause osteopenia (even in the absence of PTH).

* Vitamin D has the same mechanism of action as PTH.... RankL mediated activation of osteoclasts.....

Pretty confusing topic, I must say.............
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Old 04-17-2014
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Refer this post

How come Vitamin D increases resorption and yet cause osteomalacia?
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yea.. really... just finished it...
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Originally Posted by medstud. View Post
Ok.
*Vitamin D DOES cause bone resorption even without the presence of PTH.

BUT...... This action of Vitamin D is very insignificant at physiologically normal levels.

* In Hypervitaminosis D---- This bone resorbing capacity of Vitamin D increases significantly so as to cause osteopenia (even in the absence of PTH).

* Vitamin D has the same mechanism of action as PTH.... RankL mediated activation of osteoclasts.....

Pretty confusing topic, I must say.............
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thanks a lot.. it finally cleared my doubt...
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