FT3, FT4, RT3 and TSH Panel

This term was created for, but is not limited in use to, bioMerieux's VIDAS FT3 assay, which is intended for the quantitative measurement of free triiodothyronine (FT3) in human serum or plasma.

Free T4, or thyroxine, is the active thyroid hormone that regulates metabolism, energy, and body temperature. This test measures the unbound form of T4 circulating in your blood—the portion your body can actually use. Low free T4 suggests an underactive thyroid (hypothyroidism), causing fatigue and weight gain. High free T4 indicates an overactive thyroid (hyperthyroidism), causing rapid heartbeat and anxiety. Free T4 is often ordered alongside TSH to assess thyroid function comprehensively.

Thyroid stimulating hormone (TSH) is produced by the pituitary gland and regulates thyroid hormone production. A TSH test is the primary screening tool for thyroid disorders. Elevated TSH suggests an underactive thyroid (hypothyroidism), while low TSH may indicate an overactive thyroid (hyperthyroidism). Thyroid function affects metabolism, energy, weight, and mood. This is one of the most frequently ordered tests because thyroid dysfunction is common and affects overall health.

This test measures the ratio of active thyroid hormone (T3) to reverse T3 (rT3), reflecting thyroid hormone metabolism efficiency. T3 is the active form that powers metabolism, while reverse T3 is inactive and can block T3 action. An unfavorable ratio (high rT3 relative to T3) suggests poor conversion, seen in stress, illness, or metabolic dysfunction. This ratio helps assess thyroid function beyond standard TSH testing and guides treatment decisions.

Reverse T3 (rT3) is an inactive thyroid hormone produced alongside active T3. Your body converts thyroid hormones (T4 and T3) into either active T3 or inactive rT3, depending on your metabolic needs and stress. High reverse T3 can indicate that your thyroid hormones are being shunted away from active forms, potentially causing fatigue and metabolic slowdown despite normal TSH and T4 levels. This test is useful for investigating persistent thyroid symptoms and understanding how your body is utilizing thyroid hormones during stress, illness, or metabolic imbalance.

The free T4 index is a calculated measure of thyroid hormone activity that estimates the amount of thyroxine (T4) available to affect your metabolism. Unlike total T4, it accounts for proteins that bind thyroid hormones, providing a more accurate picture of active thyroid hormone. This test helps evaluate thyroid function and diagnose hyperthyroidism or hypothyroidism when TSH alone may be unclear.

This panel measures both thyroid stimulating hormone (TSH) from the pituitary gland and free T4 (thyroxine), the active thyroid hormone. Together, these tests provide a comprehensive assessment of thyroid function. TSH shows whether the pituitary is signaling the thyroid correctly, while free T4 measures the actual active hormone. This combination is more reliable than either test alone for diagnosing thyroid disorders.