What does the T3 uptake measure in the body?

The T3 uptake test primarily measures the level of thyroid-binding globulin (TBG) in the bloodstream. TBG is a protein that transports thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), throughout the body.

When evaluating thyroid function, the T3 uptake test uses a specific methodology to assess how well these thyroid hormones are bound to proteins in the blood. A higher T3 uptake indicates a lower level of available TBG, suggesting more free (unbound) T3 is present, while a lower T3 uptake points to higher levels of TBG and less free hormone availability. This information is crucial in diagnosing and managing various thyroid disorders, helping to infer the functional reserve of the thyroid hormone.

Other choices relate to different aspects of thyroid function. For instance, measuring thyroid hormones directly would provide information about their actual levels in the bloodstream. Thyroid-stimulating hormone (TSH) is part of the feedback loop regulating thyroid hormones but does not directly correlate to T3 uptake. Similarly, thyroid peroxidase levels are more closely associated with autoimmune conditions affecting the thyroid rather than the protein binding capacity assessed by T3 uptake. Thus, the correct understanding of T3 uptake's purpose