Thyroxine (T4) labeled with 125I. Thyroxine is a thyroid hormone that is responsible for regulation or metabolism. T4 detection by radioimmunoassay is a common test of thyroid disease. This product is offered at three specific activities. Specific activity indicates how much radioactivity there is per molecule of thyroxine. 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).