Thyroxine I-125 is a radiolabeled thyroid hormone that has been used for diagnostics of thyroid disease. Thyroxine is the main hormone secreted into the bloodstream by the thyroid gland. Thyroxine plays vital roles in digestion, heart and muscle function, brain development and maintenance of bones. A thyroxine test measures the level of Thyroxine in the plasma and Thyroxine I-125 was used as a radioiodine-labeled tracer for this assay. Currently Thyroxine I-125 primary use as a tracer for binding assays and other radiometric biological studies.

A radioconjugate of synthetic active thyroid hormone, liothyronine (T3), labeled with Iodine 125. Liothyronine involves many important metabolic functions and is essential to the proper development and differentiation of all cells. I-125 liothyronine may be used in radiotherapy in thyroid cancers. It was used for the thyroxine T4 determination in serum by competitive protein-binding analysis.

Diotyrosine I 125 is the diagnostic aid.