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About the test
What is this test done for?
T3, is done to assess the functioning of the thyroid gland. TSH is produced by a small gland, pituitary, located at the base of the brain and T3 (triiodothyronine) and T4 (thyroxine) are produced by the thyroid gland. Most of the T3 in the blood is bound to a protein and less than 1% of the T3 is in unbound condition, called free T3. T3 test will measure total of both bound and unbound forms.
Why is it Done?
It is performed to help diagnose Grave’s disease, an autoimmune disorder which is the common cause of hyperthyroidism. It is done to rule out other forms of hyperthyroidism as well.
When should it be performed?

The following are the indications which prompt a T3 test, i.e. case of suspected hypo or hyperthyroidism:

  • Anxiety
  • Weight loss or weight gain rapidly
  • Difficulty sleeping
  • Increased heart rate
  • Tremors in the hand
  • Weakness
  • Diarrhea
  • Visual disturbances
  • Eyes: dryness, irritation, bulging of the eyes.
  • To monitor treatment for hypothyroidism, hyperthyroidism
  • As part of screening in infertility
  • As part of routine health screening in women above 35 years
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 T3 - Mild hypothyroidism (clinical)
  • High TSH & Low or Normal T3 - Hypothyroidism
  • Low TSH & Normal T3 - Mild hyperthyroidism (clinical)
  • Low TSH & High or normal T3 - Hyperthyroidism
  • Low TSH & Low or normal T3 - Pituitary problems
  • Normal TSH & High T3 - Thyroid hormone resistance syndrome
Normal value for T3: 100 to 200 ng/dL
  • Elevated levels could mean— Overactive thyroid gland (for example: Graves’ disease), T3 thyrotoxicosis. It could also mean that the person is taking thyroid medicines or certain supplements or has liver disease.
  • Below normal levels could mean— Severe short-term or some long-term illnesses, thyroiditis i.e., swelling or inflammation of the thyroid gland (of which Hashimoto disease is the most common type), starvation or underactive thyroid gland.
Other Tests
A separate Free T3 test should be done. Ultrasound of thyroid gland to detect goiter can be performed. Auto antibodies of thyroid gland like ANA and TPO can be done to assess the autoimmune origin of the condition and if it is hereditary.
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