Thyroid Hormones and Protein-Bound Iodine

To the Editor:—  Dr. Paul Starr (205: 313, 1968) rightly stresses the importance of the triiodothyronine (T3) content of desiccated thyroid preparations. I am afraid, however, that the statement that T3 "stimulates metabolism without representation in the PBI value" may give rise to misunderstanding. Triiodothyronine is, in fact, included in the protein-bound iodine (PBI) value, but its concentration in the serum is very low, compared to that of thyroxine (T4).¹ Owing to the much shorter biologic half-life, much larger distribution space, and greater calorigenic potency of T3, however, a given serum concentration of T3 will be metabolically equivalent to about 25 times the same concentration of T4.²The statement in the last paragraph that T4 "binds completely to the thyroxine binding globulin (TBG), and the T3 does not," should be understood to mean that T3 binds less completely, rather than not at all.Finally, patients with high TBG