Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.

To tie tubes is one of the methods to regulate pregnancy, it is not permanent. To untie tubes, you can do a procedure called: tubal legation reversal. In Islam the prophet (peace be upon him) approved people’s basics method of controlling birth by discharging outside intercourse intimate relationship.

The utilization of such medical methods in our days is allowed based on that concept. Now if we add the age factor, medical problems and the case of having already kids’ al-hamudullilah, it should be allowed for you to tie your tubes even if it means no more children are possible.

Almighty Allah knows best.