T3 (liothyronine) leaves the system pretty fast, i had total thyroidectomy, i stopped taking t3 (liothyronine) and started t4, am i in danger the first weeks or does t4 act fast?

Can take days: If without any T4 at all for an extended period of time, it can take several days for the blood level of T4 to build up. The reason for this is that T4 gets bound to storage/binding proteins, as well as traveling as free T4 through the bloodstream. Even without a thyroid gland, though, once your T4 level is at goal, the body is quite good at converting the T4 you take to T3 (liothyronine) as it needs to.