Thank you sugarcatmom for reply!
All 3 times we collected urine from his litter box and brought it to vet for urine test. I cleaned the box with rinsing water.
His last T4 test was 1 month ago and T4 level dropped from 8 ( high) to 0.2 ( very low), so he now get methimazole 1/4 pill/day instead 1/2 pill/day. Cat still drinks and pees more than usual.