T3 (Free and Total) Test (Free and Total Triiodothyronine)

A free or total triiodothyronine (free T3 or total T3) test is used to assess thyroid function. It is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor treatment of a person with a known thyroid disorder.

A free T3 or total T3 test may be ordered when someone has an abnormal TSH test result. It may be ordered as part of the investigative workup when a person has symptoms suggesting hyperthyroidism, especially if the free T4 level is not elevated.

Signs and symptoms may include:

  • Increased heart rate
  • Anxiety
  • Weight loss
  • Difficulty sleeping
  • Tremors in the hands
  • Weakness
  • Diarrhea (sometimes)
  • Light sensitivity, visual disturbances
  • The eyes may be affected: puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes.

Free or total T3 may sometimes be ordered at intervals to monitor a known thyroid condition and to help monitor the effectiveness of treatment for hyperthyroidism.

Results Interpretations

Total T3 or free T3 results are typically evaluated with other thyroid testing results (i.e., TSH and T4).

In general, high total or free T3 results may indicate an overactive thyroid gland (hyperthyroidism) and low total or free T3 results may indicate an underactive thyroid gland (hypothyroidism). The test results alone are not diagnostic but will prompt the healthcare practitioner to perform additional testing to investigate the cause of the excess or deficiency.

When used for monitoring treatment for thyroid or pituitary disorders, results of thyroid tests will inform the healthcare practitioner whether treatment is effective and/or whether an adjustment to dose is necessary. For example, 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.

The following table summarizes some examples of typical test results and their potential meaning.

Keyword: T3 test; free and total T3; free and total triiodothyronine.

* The Content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.