From the beginning of the series Tuvok was meant to be a Lieutenant, and is referred to as such in the series, but in the pilot episode "Caretaker" was given the rank pips of Lieutenant Commander-two silver, one black.
From the beginning of the series Tuvok was meant to be a Lieutenant, and is referred to as such in the series, both in the opening titles and dialogue. However in the pilot episode "Caretaker" Tuvok can clearly be seen to be wearing the rank pips of Lieutenant Commander-two silver, one black.