FREE T4 IS THE METABOLICALLY ACTIVE FRACTION OF THYROXINE. FT4 ALONG WITH TSH GIVES AN ACCURATE PICTURE OF THYROID STATUS IN PATIENTS WITH ABNORMAL THYROID BINDING GLOBULIN (TBG) LIKE IN PREGNANCY AND INDIVIDUALS ON TREATMENT WITH ESTROGENS, ANDROGENS, PHENYTOIN OR SALICYLATES.THIS ASSAY IS USEFUL FOR DIAGNOSING BOTH HYPO / HYPERTHYROIDISM.

PATIENTS WITH AUTOIMMUNE THYROID DISEASE CAN DEVELOP ANTIBODIES TO TSH WHICH INTERFERE WITH TSH MEASUREMENT. THIS TEST AIDS IN UNDERSTANDING DISPARATE LABORATORY AND CLINICAL FINDINGS.