What is a TSH Blood Test?

A TSH blood test measures the thyroid-stimulating hormone (TSH) in the body. The test evaluates the possibility of a thyroid problem in the patient. It is performed through a simple blood draw, and is used for both an initial diagnosis and continuous monitoring of thyroid function. The TSH blood test is generally performed in a lab, or in a physician's office, and takes only a few minutes.

The TSH blood test is often ordered by a physician when signs of either hypothyroidism or hyperthyroidism are present. It can also help pinpoint the specific cause of hypothyroidism. The thyroid blood test is also ordered periodically for patients who are on medication for either thyroid condition, to determine if the current medication doses are correct. Newborn babies sometimes have a TSH blood test to check for low thyroid function.

Abnormal levels of thyroid-stimulating hormone might indicate a thyroid problem. High TSH levels typically indicate hypothyroidism, which means the thyroid gland is underactive. Low levels of TSH could mean the thyroid is overactive, a condition which is called hyperthyroidism. If the TSH blood test comes back indicating either high or low TSH levels, additional thyroid blood tests might be ordered.

High or low levels of TSH in patients already diagnosed with a thyroid condition can indicate improper medication levels. Low levels in a patient with hypothyroidism might indicate the dose is too high, while high TSH levels could mean the medication dose is too low. The test results help a physician adjust the medication dosage to keep the TSH levels within the normal range.

The T3, T4, and thyroid antibodies tests are other possibilities. Thyroid-stimulating hormone regulates the production of T3, or triiodothyronine, and T4, or thyroxine. The T3 and T4 tests can give a more accurate picture of the thyroid problems occurring in the patient. Low T3 and T4 levels usually accompany high TSH levels in those with hypothyroidism. High T3 and T4 levels are associated with low TSH levels in hyperthyroidism patients.

Certain factors affect the accuracy of a TSH blood test, including stress or chronic illnesses. Certain medications can also affect the results, including dopamine, lithium, heparin, levodopa, corticosteroids, methimazole, and propylthiouracil. X-rays or tests using radioactive materials within the last four to six weeks sometimes cause inaccurate test results as well. Thyroid-stimulating hormone levels might also be low during the first trimester of pregnancy. The physician should know about any of these factors before the TSH blood test is performed.