Thyrotropin(TSH) CLIA kit (Chemiluminescence Immunoassay) is an in vitro diagnostic quantitative test kit. TSH is a hormone that acts as a messenger to the thyroid gland. When released by the pituitary gland—a small gland located at the base of the brain—TSH stimulates the thyroid to produce more thyroid hormone. TSH levels seem counterintuitive. A high TSH suggests an underactive thyroid gland, while low TSH levels indicate an overactive thyroid.