Why TSH is so low on NDT or T3 containing medicine?

Why TSH is so low when a person takes medicine/supplement containing T3 thyroid hormone? Is a person hyperthyroid when TSH is “below the norm?”

“Among T3, rT3, and T4, T3 was the most potent and rT3 was the least potent. The relative potency of T3:T4:rT3 appeared to be approximately 100:12:1”