Triiodothyronine (T3) labeled with 125I. Triiodothyronine is a thyroid hormone that affects growth and development, body temperature, and heart rate. T3 detection by radioimmunoassay is a common test of thyroid gland function. This product is offered at three specific activities. Specific activity indicates how much radioactivity there is per molecule of ligand. Several of our 125I-labeled ligands are offered at maximum specific activity (2200 Ci/mmol if one 125I labeling site is available, 4400 Ci/mmol if two labeling sites are available).