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About the test
What is this test done for?
T4 or thyroxine test measures the amount of this important thyroid hormone in the blood. T4, also the total T4 blood test, measures both free and bound thyroxine.
Why is it Done?
The test is done
  • to help in diagnosing thyroid diseases like hypothyroidism and hyperthyroidism.
  • along with TSH and T3 tests to aid in diagnosing cause of infertility in women.
  • in monitoring the doses in the thyroid treatment in a patient being treated for thyroid disorders.
When should it be performed?

The following are the indications to which prompt a T4 test:

  • An underactive thyroid gland (hypothyroidism) causing weight gain, tired feeling, morning stiffness, dry skin, constipation, females having hormonal problems like absence of menses or frequent menstrual cycles, cold intolerance.
  • An overactive thyroid gland (hyperthyroidism) which produces symptoms like weight loss, increased heart rate, diarrheas, nervousness, anxiety, irregular periods, heat intolerance.
  • It is also done for the patients who are already taking thyroid replacement medications to adjust the dosages according to the value of T4 (with corresponding value of TSH) and monitor efficacy of treatment.
  • The test is done, along with TSH, to screen the newborns for underactive thyroid.
  • In case of infertility, this test is done as hypothyroidism can be one of the reasons for infertility.
  • To differentiate between two similar thyroid disorders.

In a pregnant woman with known thyroid disease

How is it done
A blood sample will be collected from a vein in your arm. Fasting is not needed prior to sample collection.
  • Normal T4 (but corresponding TSH is high) - Mild hypothyroidism (subclinical)
  • Low T4 (but corresponding TSH is high) - Hypothyroidism
  • NormalT4 (but corresponding TSH is low) - Mild hyperthyroidism (subclinical)
  • High or normalT4 (but corresponding TSH is low) - Hyperthyroidism
  • Low or normalT4 (and corresponding TSH is low) - Pituitary disease problems
  • HighT4 (but corresponding TSH is normal) - Thyroid hormone resistance syndrome
Normal values of T4: 4.5 to 11.2 ng/dL
  • Above normal results could mean— Graves’ disease, taking high doses of thyroid supplements, thyroiditis, tumor of ovaries or testes which is rare. It could also mean that the person is eating lot of iodine rich food.
  • Below normal results could mean— hypothyroidism, malnutrition or fasting.
Other Tests
  • Every T4 test is done with its corresponding TSH value, for accurate diagnosis. Hence, TSH and T3 must always be done with T4.
  • Free T4 (the active form) in certain cases to assess levels of unbound and active T4.
  • Ultrasound of thyroid gland to detect goiter or thyroid nodules can be performed.
  • Auto antibodies of thyroid gland like ANA and TPO can be done to assess the autoimmune origin of the condition or whether it is hereditary.
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