• Test Code - PL0494
  • Test Name - Free Thyroxine (T4)
  • Views - 834 Views
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Test Details & Preparation


  • Free Thyroxine (T4) testing is done to help evaluate thyroid gland function to help diagnose thyroid disease to screen for hypothyroidism in newborns to monitor effectiveness of treatment.

  • Free Thyroxine (T4) testing is performed when you have signs and symptoms of thyroid disease, usually after an abnormal result on a TSH test commonly performed as a screening test on newborns soon after birth when you are being treated for a thyroid disorder.

  • A blood sample drawn from a vein in your arm or from pricking the heel of an infant for Free Thyroxine (T4) testing.

  • No test preparation is needed however, certain medications can interfere with the free T4 test, so tell your doctor about any drugs that you are taking. If you take thyroid hormone as treatment for thyroid disease, it is recommended that your blood sample be drawn before you take your dose for that day.

  • In general, high free T4 results may indicate an overactive thyroid gland (hyperthyroidism), and low free T4 results may indicate an underactive thyroid gland (hypothyroidism). However, Free Thyroxine (T4) testing alone is no diagnostic and will tempt the doctor to perform additional testing to investigate the cause of the excess or deficiency.

  • Both decreased and increased free T4 results are associated with a variety of temporary and chronic thyroid conditions. Low free T4 results in conjunction with a low TSH level or high free T4 results along with a high TSH may indicate a pituitary gland condition.

  • In people with hyperthyroidism, free T4, free T3, and TSH are regularly checked while they are on anti-thyroid drugs to assure that the drugs are working and to decrease doses if thyroid hormone levels get too low.

  • In people with hypothyroidism, TSH and free T4 are regularly checked to assure that the right dose of thyroid hormone is being given to make TSH normal.


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