Tirpitz threatened Arctic convoys without leaving port

Why did the Royal Navy treat Tirpitz as such a serious threat to Arctic convoys?

An inadequately covered convoy had no safe answer to a battleship attack, so the possibility of Tirpitz sailing shaped British decisions.

Royal Navy caution about Tirpitz makes sense from a convoy commander's perspective. A merchant convoy without heavy covering ships would be dangerously exposed if the battleship appeared. Earlier experience of a carrier being caught by enemy battleships reinforced the fear of an unsupported formation meeting a capital ship. Tirpitz could therefore disrupt routes and escort decisions through the credible possibility of attack even when she remained in harbour.