T3 (Triiodothyronine), Free (LAB137)

TSH secretion is highly sensitive to thyroid hormone changes. Hence, TSH testing is the preferred initial test for the assessment of thyroid function.

Further investigations of abnormal thyroid function include free T4 (fT4) and free T3 (fT3) measurements. However, fT3 measurement is most useful when TSH is suppressed, but fT4 is normal or decreased, suggesting a diagnosis of T3 thyrotoxicosis.

For this reason, BJH offers a “Thyroid Function Cascade”. If TSH is < 0.30 or > 4.2 mcIU/mL, then free T4 is performed. If fT4 is <1.7 ng/dL (normal or decreased), then fT3 is performed.

• TSH, free T4, and free T3 are often ordered together out of convenience rather than necessity.

• The Thyroid Function Cascade is the preferred screen for the assessment of thyroid status in ambulatory patients.

1. Lin Y, Riek AE, Gronowski AM, Farnsworth CW. Limited Utility of Free Triiodothyronine (fT3) Testing. JALM 2023;8:847-55.