T3 uptake (T3U) is used with measurement of thyroxine (T4) to calculate the free T4 index. The calculated free T4 is useful in the assessment of thyroid diseases. Elevations are associated with hyperthyroidism or thyroid hormone resistance whereas low concentrations are associated with hypothyroidism.
Decreased: Pregnancy, estrogens, hyperproteinemia, acute intermittent porphyria.
Increased: Androgens, hyperproteinemia, stress, acute liver disease.