Alternative Names thyroid stimulating hormone, thyrotropin
Definition This test measures the amount of thyroid stimulating hormone, or TSH, in the blood.
TSH is a hormone secreted by the pituitary gland that causes the thyroid gland to release thyroid hormone, or thyroxin. Thyroxin levels are important because thyroxin controls the rate of basal metabolism, or the energy needed to keep the body functioning at rest.
Who is a candidate for the test? This test is ordered to help diagnose disorders of the thyroid gland, hypothalamus and pituitary gland.
How is the test performed? To measure the amount of TSH in the blood, 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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
What is involved in preparation for the test? A person should request specific instructions from his or her doctor.
What do the test results mean?
The normal concentrations of TSH in blood range from 0.2 to 4.0 mU/L (micro units per litre).
Abnormally high levels of TSH may indicate the following:
cretinism, or arrested physical and mental development with bone and soft tissue wasting due to an underactive thyroid
primary hypothyroidism, or decreased function of the thyroid caused by problems within the thyroid itself
secondary hypothyroidism, or decreased functioning of the thyroid caused by problems in other organs such as the pituitary or hypothalamus
thyroiditis, or inflammation of the thyroid
Abnormally low levels of TSH may indicate either hyperthyroidism, which is an overactive thyroid gland, or hypopituitarism, which is an underactive pituitary gland.
Author: David T. Moran, MD Reviewer: eknowhow Medical Review Panel Editor: Dr John Hearne Last Updated: 26/05/2005 Contributors Potential conflict of interest information for reviewers available on request
