What is the most prevalent thyroid hormone in circulation after administration of radioactive iodine?

The most prevalent thyroid hormone in circulation after the administration of radioactive iodine is thyroxine, which primarily exists bound to thyroxine-binding globulin (TBG). When radioactive iodine is administered, it helps to image or treat thyroid disorders by concentrating in the thyroid tissue. The thyroid gland then synthesizes and secretes thyroid hormones, mainly thyroxine (T4), which is produced in larger quantities compared to triiodothyronine (T3).

In the bloodstream, a significant proportion of thyroxine is bound to transport proteins, with TBG being the main carrier. This binding is crucial for regulating the availability of thyroid hormones because only the free (unbound) hormones are biologically active. However, the bound form of thyroxine accounts for the majority of the total thyroid hormone in circulation, making it the most prevalent after administration of radioactive iodine.

While free thyroxine and free triiodothyronine do circulate, they exist in much smaller concentrations compared to their bound counterparts. Triiodothyronine that is bound to TBG also occurs, but the total amount of bound thyroxine is higher given its predominant role in thyroid hormone physiology. Thus, the correct answer reflects the physiological and biochemical context of thyroid