The TSH is essentially telling your thyroid to "release more hormone." This is why your TSH rises when your thyroid is underactive. A high TSH means that the pituitary gland is releasing its hormone to try to get the thyroid to respond and produce more thyroid hormone.
On the opposite end, when the pituitary senses that there is too much thyroid hormone circulating, then it slows down or even stops releasing TSH. The lowering of TSH means that the thyroid is no longer getting the message to release hormone, and thyroid hormone production will slow down.