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Thyroid Function Tests (TFTs)

This information describes thyroid function tests (TFTs).

Thyroid function tests are blood tests that let your doctor see if you have the right amount of thyroid hormone in your blood. 

Thyroid Stimulating Hormone (TSH)

Thyroid stimulating hormone (TSH) is secreted by the pituitary gland in your brain. It stimulates your thyroid to work. The level of TSH in your blood tells us if your thyroid is too active or not active enough. If you do not have a thyroid due to surgery, the TSH tells your doctor if you are getting the right dose of thyroid hormone replacement medication.

The normal TSH range differs slightly from one lab to another. At Memorial Sloan Kettering Cancer Center (MSK), the normal range is 0.55 mIU/L to 4.78 mIU/L.

  • If your TSH level is low, you are in a state of hyperthyroidism. That means your thyroid function is too active.
  • If your TSH level is high, you are in a state of hypothyroidism. That means your thyroid function is not active enough.
  • In people without thyroid cancer, the goal is to keep the TSH level within the normal range.
  • In most people with thyroid cancer, the goal is to keep the TSH level 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 that measures your thyroid function. It tells us how much “free” or available thyroid hormone you have in your blood. At MSK, the normal range for FT4 is 0.9 ng/dL to 1.8 ng/dL.

  • In people without thyroid cancer, the goal is to keep the FT4 level within the normal range.
  • In people with thyroid cancer, the goal is to keep the FT4 level near the upper limits of the normal range.

Thyroglobulin

Thyroglobulin is a protein in the blood that is produced only by thyroid cells. We measure your thyroglobulin level to help us monitor your cancer after treatment. If your thyroglobulin level rises, it could mean that you need more tests and follow-up care to be sure your cancer has not returned.

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