Alternatively, the T3 resin uptake test can be performed and provides an indirect measurement of the level of thyroid binding protein. The FT4 index is the total T4 multiplied by the T3 resin uptake and should be proportional to the true free T4 level. In pregnancy, the total T4 is elevated, the T3 resin decreased and the free T4 index is normal. The availability of the TSH screening has largely eliminated any confusion caused by changes in thyroid binding proteins as the TSH will remain normal in these circumstances. The thyroid gland takes up iodine and uses this to make thyroid hormone.
Radioactive iodine is taken up and metabolized by the thyroid in exactly the same way.
This is measured by counting the radioactivity over the thyroid gland. The test is safe since the radiation dose is very small, although it is usually not carried out in children or pregnant women. This is called a thyroid scan. The scan can be used as an alternative to the radioactive iodine uptake as described. In addition, the scan gives an idea of the shape and size of the thyroid gland and can be used for patients with thyroid nodules to determine whether the nodule is functioning.