A TSH blood test includes the following:
TSH, Triiodothyronine (T3) and Tetraiodothyronine (T4).
While TSH is produced in the pituitary gland, T3 and T4 are produced in the thyroid gland.
TSH regulates the production of T3 and T4.
T3, in turn, regulates the production of TSH by signaling the pituitary gland to stop the production of TSH. That is called the negative feedback mechanism.
Malfunction in production in any of these hormones could lead to hypothyroidism or hyperthyroidism.