Effect of age on the conversion of thyroxine (T₄) to 3,5,3′ triiodothyronine (T₃) by liver homogenate from fed and fasted Sprague-Dawley rats

Aging is associated with numerous alterations in thyroid hormone homeostasis. Because T₃ accounts for much of the peripheral metabolic effect of thyroid hormone, and T₄, to T₃ conversion has been shown to be altered by fasting, at least in nonsenescent rats, the effect of age on this aspect of thyroid hormone metabolism was assessed. In the fed state, increasing age did not alter T₄, to T₃ conversion by rat liver homogenate (5.03 ± 0.7 vs. 4.44 ± 0.8%)(mean ± SD). Age did, however, alter the peripheral thyroid hormone response to fasting. Liver homogenates from fasted mature rats produced approximately 74% the amount of T₃ generated by tissues from similarly aged fed controls (p < 0.001). Fasting did not, however, decrease the amount of T₃ generated by liver homogenates from senescent rats. The in vitro addition of 5 mM dithiothreitol (DTT) increased T₃ generation 60.7 ± 9.0% (mean ± SD) (p < 0.001) by tissues from fed mature rats; DTT (5 mM) did not alter T₄, to T₃ conversion by tissues from fed aged rats. DTT (5 mM) added to tissues from fasted mature rats eliminated the decline in T₃ generation associated with fasting. DTT (5 mM) had no effect on tissues from fasted senescent rats. In conclusion, in the fed state, increasing age does not alter T₄, to T₃ conversion by rat liver homogenate. Age does alter the peripheral thyroid hormone response to fasting.