What it is: A T3 (triiodothyronine) test measures the level of triiodothyronine, the more biologically active thyroid hormone, in the blood — as either total T3 or free T3.
Why it's ordered: Typically ordered when TSH is suppressed but T4 is normal, to check for T3 toxicosis (a form of hyperthyroidism driven by excess T3), or to further evaluate suspected hyperthyroidism.
What it detects: Most T3 is converted from T4 in peripheral tissue. T3 is generally less reliable than T4 for diagnosing hypothyroidism, since levels can drop during illness or hospitalization unrelated to thyroid disease, but it's useful for confirming certain hyperthyroid patterns.
Collection & prep: A standard blood draw from a vein in the arm, usually as a follow-up test after TSH and T4. No special preparation is typically required.
Results:
High – supports a diagnosis of hyperthyroidism, particularly T3 toxicosis
Low – not a reliable standalone indicator of hypothyroidism, since illness can lower T3 independent of thyroid function
Possible complications of thyroid dysfunction: T3 results are always interpreted alongside TSH and T4 rather than in isolation, since T3 alone can be misleading, especially in people who are acutely ill or hospitalized.
Statements made in the Clinical Manual have not been evaluated by the FDA. The products mentioned are not intended to diagnose, treat, cure or prevent any disease.
