Thor's Uru hammer, the feared Mjolnir, is nowhere to be found during Thialfi's travels of the Lord world. Where is this mightiest of mighties and why does Lord Thor not hold it still?

Thor's Uru hammer, the feared Mjolnir, is nowhere to be found during Thialfi's travels of the Lord world. Where is this mightiest of mighties and why does Lord Thor not hold it still?