T3 Uptake
T3 uptake is an older test that indirectly estimates how many thyroid hormone binding sites are available on proteins in your blood. It is used to help interpret total thyroid hormone results.
T3 uptake does not measure hormone directly; instead it reflects protein binding capacity. Combined with total T4, it helps calculate the free thyroxine index, correcting for changes in binding proteins from pregnancy, estrogen, or other factors. It is less common now that free hormone tests are widely available.
A high T3 uptake typically indicates fewer available binding sites, which can accompany an overactive thyroid or lower binding-protein states. Because it is an indirect measure, it is meaningful mainly when paired with total T4. Your clinician interprets the combination.
A low T3 uptake generally indicates more available binding sites, which can occur with an underactive thyroid or higher binding-protein states such as pregnancy.
Reference ranges are lab-specific and this test is most useful within the free thyroxine index calculation rather than alone. Your clinician can interpret it alongside your total T4.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.