The TSH (Thyroid Stimulating Hormone) Ultrasensitive test measures the levels of TSH hormone in the blood. TSH is produced by the pituitary gland located at the base of the brain. Its function is to stimulate and regulate the functioning of the thyroid gland. It signals the thyroid gland to increase or decrease the production of thyroid hormones T3 and T4 when their levels are low or high, respectively. Therefore, when the levels of T3 & T4 decrease, the pituitary gland is stimulated to release TSH. This high TSH level, in turn, stimulates the thyroid gland to release more thyroid hormones (T3 & T4). Vice versa happens when the levels of thyroid hormones increase.