Thyrotropin-mediated induction of thyroidal iodothyronine monodeiodinases in the dog.

Abstract

To investigate the effect of TSH administration on thyroidal iodothyronine-deiodinating activity, in vitro conversion of T4 to T3, T3 to 3,3′-diiodothyronine (T2), and rT3 to T2 by canine thyroid, liver, and kidney homogenate was examined, comparing tissues from TSH-treated dogs (3 IU bovine TSH daily for 3 days) with tissues from sex- and weight-matched saline-treated controls. The in vitro conversion activities were studied in the presence of 4 mM dithiothreitol at pH 6.5 for outer ring (T4 to T3 and rT3 to T2) and at pH 7.8 for inner ring (T3 to T2) deiodination. Iodothyronine-monodeiodinating activity in dog thyroid is heat labile (inactivated at 56 C × 5 min) and pH dependent, inhibited by propylthiouracil (0.2 mM) and ipodate (0.2 mM), unaffected by methimazole (up to 4 mM), ascorbate (up to 0.1 M), or KI (up to 4 mM), and is localized in the microsomal fraction. TSH markedly stimulated thyroidal inner and outer ring iodothyronine-monodeiodinating activities. Baseline rates of T4 to T3, T3 to T2, an...