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About the test
What is this test done for?
Free T4 and Thyroid Stimulating Hormone (TSH) are the tests done to assess the functioning of the thyroid gland. TSH is produced by a small gland calledpituitary, located at the base of the brain and hormones T3 (triiodothyronine) and T4 (thyroxine) are produced by the thyroid gland. T4 is an important thyroid hormone. Most of the T4 in the blood is bound to a protein and less than 1% of the T4 is in unbound condition, called free T4. Free T4 affects tissue function in the body, but bound T4 does not. A total T4 blood test measures both free and bound forms.
Why is it Done?
The test assesses if a person is suffering from thyroid disorders that lead to either hypothyroidism (under functioning) or hyperthyroidism (over functioning). TSH stimulates the thyroid gland to produce hormones like T3 and T4. Free T4 is not bound to protein in the blood, and hence, is not affected by protein levels in blood. It is the active form of thyroxine. The free T4 test gives therefore, a more accurate account of thyroid hormone function. This test has in most cases replaced the total T4 test.
When should it be performed?

The following are the indications to which prompt a free T4 and TSH 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 TSH and monitor efficacy of treatment.
  • The test is done to screen the newborns for underactive thyroid.
  • to determine causes for infertility
  • To differentiate between two similar thyroid disorders.

In a pregnant woman with known thyroid disease 4. How is the test performed?

A: A blood sample will be collected from a vein in your arm. Fasting is not needed prior to sample collection.

How is it done
A blood sample will be collected from a vein in your arm. Fasting is not needed prior to sample collection.
  • High TSH & Normal Free T4
    • Interpretation: Mild hypothyroidism (subclinical)
  • High TSH & Low Free T4
    • Interpretation: Hypothyroidism
  • Low TSH & Normal Free T4
    • Interpretation: Mild hyperthyroidism (subclinical)
  • Low TSH & High or normal Free T4
    • Interpretation: Hyperthyroidism
  • Low TSH & Low or normal Free T4
    • Interpretation: Pituitary disease problems
  • Normal TSH & High Free T4
    • Interpretation: Thyroid hormone resistance syndrome

NORMAL VALUES OF TSH: 0.4 to 5.5mU/mL. If you are on thyroid medications then the value should be between 0.4 to 3.0 mU/mL.

Above-normal results mean—

If the TSH value is above the normal range, then it suggests that the thyroid gland is underactive (hypothyroidism) and the pituitary gland is releasing more TSH when thyroid is not capable of producing enough hormones.

Below normal results mean—

If the TSH value is below the normal range, then it suggests that the thyroid gland is overactive (hyperthyroidism) which suggests release of less TSH from pituitary as thyroid gland is over producing hormones.

Normal values of free T4: 0.7-1.9 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
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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