According to Mayo Clinic, the T3 uptake test measures the amount of thyroid-binding globulin that is bound to T4. In cases of hyperthyroidism, there is excess T4 present, and therefore, most of the TBG is bound to T4.
Mayo Clinic explains that to perform a T3 uptake assay, labeled T3 is mixed with the patient's blood sample. The T3 binds to any unbound TBG present in the sample. The excess labeled T3 is bound to a solid substrate. The amount of labeled T3 that binds to the substrate is measured and is called the T3 uptake. Therefore, the less free TBG there is in the sample, the more T3 uptake there is. There is less free TBG when there is more T4. Higher T4 levels produce higher T3 uptake values. Mayo Clinic says that normal T3 uptake values are 27 to 37 percent for males and 20 to 37 percent for females.