A TSH blood test measures the amount of thyroid-stimulating hormone (abbreviated as TSH). This hormone is secreted by the pituitary gland. It stimulates the thyroid gland to release thyroid hormone (thyroxine, also called T4). Thyroxine controls the rate of basal metabolism, that is, the energy needed to keep the body functioning at rest.
A TSH test is ordered to help diagnose disorders of the thyroid gland, hypothalamus, and pituitary gland.
A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle and is collected in a syringe or vial for testing in the laboratory. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
Generally, there is no preparation needed for a TSH test.
The normal concentrations of TSH in blood range from 0.2 to 4.7 mcU/mL. Abnormally high levels of TSH may indicate the following: congenital hypothyroidism, a genetic condition marked by arrested physical and mental developmentprimary hypothyroidism, that is, decreased function of the thyroid caused by problems within the thyroidsecondary hypothyroidism, that is, decreased functioning of the thyroid caused by problems in other organsthyroiditis, that is, inflammation of the thyroid
Abnormally low levels of TSH may indicate hyperthyroidism, which is an overactive thyroid gland. A low level is also seen with hypopituitarism, which is an underactive pituitary gland.