Routes of T₃s Metabolism

Abstract

Recent studies suggest that, as in the case of T₄, monodeiodination (to T₂s) is an important mode of metabolism of T₃s (rT₃ and T₃). Two T₂s, 3,3′-T₂ and 3′,5′-T₂, could derive from the metabolism of 3,3′,5′-T₃ (rT₃) (Fig. 8.1). Both of these have been demonstrated in human circulation. Serum levels of 3,3′-T₂ as well as 3′,5′-T₂ increase considerably after oral administration of rT₃ (Wu et al. 1976; Chopra et al. 1978d; Geola et al. 1979). Similarly, radioactive 3,3′-T₂ and 3′,5′-T₂ have been found in human plasma after intravenous administration of rT₃ labeled in the phenolic (outer) ring. Studies of thyroid glands hydrolyzed with pronase suggest that the 3,3′-T₂ and 3′,5′-T₂ content of human thyroglobulin is very low on a weight basis, approximating only about 0.05% and 0.06% that of T₄, respectively.