How can I retrieve the value if a server side control value whose value is updated on client side on page_load

I have a hidden variable and its value is being updated using javascript(client side) which I make a call from server side code. 
After making the call I am not able to retrieve the updated value from Server side variable.
I went through this forum http://forums.asp.net/p/1289684/3668484.aspx but not able find a way how to implement functionality with IFRAME.
Please provide me some sample code. I am trying to call the client side code and retrieve the updated value from server side in page_load event.
Thanks
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