free T3 test (FT3): indication and interpretation

When to order free T3 test ?

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.

What does the test result mean?

Increased or decreased thyroid hormone results indicate that there is an imbalance between the body's requirements and supply, but they do not tell the health practitioner specifically what is causing the excess or deficiency.

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

TSH	FREE T4	FREE OR TOTAL T3	PROBABLE INTERPRETATION
High	Normal	Normal	Mild (subclinical) hypothyroidism
High	Low	Low or normal	Hypothyroidism
Low	Normal	Normal	Mild (subclinical) hyperthyroidism
Low	High or normal	High or normal	Hyperthyroidism
Low	Low or normal	Low or normal	Non-thyroidal illness; rare pituitary (secondary) hypothyroidism
Normal	High	High	Thyroid hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid hormone function)

If someone is being treated with anti-thyroid medication for hyperthyroidism and the free or total T3 (or more frequently, the free T4 or TSH) is normal, then it is likely that the medication is effective in treating the condition. If the free or total T3 or free T4 is elevated, then the medication is not effective in treating the condition and the person may be experiencing symptoms associated with hyperthyroidism.

See more at: https://labtestsonline.org/understanding/analytes/t3/tab/test/

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