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Thyroid Function Tests (TFTs)

This information describes thyroid function tests.

Thyroid function tests are blood tests. They let your doctor see if you have the right amount of thyroid hormone in your blood. The doctor uses the test results to make sure you are getting the right dose of thyroid hormone replacement.

Thyroid Stimulating Hormone (TSH)

Thyroid stimulating hormone (TSH) test is the most important blood test used. That is because it is more sensitive than other tests. TSH is secreted by the pituitary gland in the brain. It stimulates your thyroid to work. The normal range differs slightly from one lab to another. At MSKCC the normal range is 0.35-5.5 mIU/L.

  • If your TSH is low, you are in a state of hyperthyroidism. That means your thyroid function is too active.
  • If your TSH is high, you are in a state of hypothyroidism. That means your thyroid function is not active enough.
  • In patients without thyroid cancer, the goal is to keep TSH within the normal range.
  • In most patients with thyroid cancer, the goal is to keep TSH below the normal range. This decreases the risk of the cancer coming back. Your doctor will tell you what level is right for you.

Free Thyroxine (FT4)

Free Thyroxine (FT4), is another blood test we use. It tells us how much “free” or available thyroid hormone you have in your blood.

  • In patients without thyroid cancer, the goal is to keep the FT4 within the normal range.
  • In patients with thyroid cancer, the goal is to keep FT4 near the upper limits of normal. At MSKCC the normal range is 0.9ng/dl - 1.8 ng/dl.

Thyroglobulin

Thyroglobulin is a protein in the blood. It is produced only by thyroid cells. We measure the level of the protein if you had thyroid cancer. This helps us monitor your cancer after treatment. Rising levels could mean that you need more tests and follow up to be sure your cancer has not returned.

If you have medullary thyroid cancer, other tests are also used to monitor your cancer.