Disruption of thyroid hormone homeostasis in Ugt1a-deficient Gunn rats by microsomal enzyme inducers is not due to enhanced thyroxine glucuronidation

Microsomal enzyme inducers (MEI) that increase UDP-glucuronosyltransferases (UGTs) are thought to increase glucuronidation of thyroxine (T4), thus reducing serum T4, and subsequently increasing thyroid stimulating hormone (TSH). Ugt1a1 and Ugt1a6 mediate T4 glucuronidation. Therefore, this experiment determined the involvement of Ugt1a enzymes in increased T4 glucuronidation, decreased serum T4, and increased TSH after MEI treatment. Male Wistar and Ugt1a-deficient Wistar (Gunn) rats were fed a control diet or diet containing pregnenolone-16α-carbonitrile (PCN; 800 ppm), 3-methylcholanthrene (3-MC; 200 ppm), or Aroclor 1254 (PCB; 100 ppm) for 7 days. Serum T4, triiodothyronine (T3), and TSH concentrations, hepatic T4/T3 glucuronidation, and thyroid histology and follicular cell proliferation were investigated. PCN, 3-MC, and PCB treatments decreased serum T4, whereas serum T3 was maintained in both Gunn and Wistar rats (except for PCB treatment). TSH was increased in Wistar and Gunn rats after PCN (130 and 277%) or PCB treatment (72 and 60%). T4 glucuronidation in Wistar rats was increased after PCN (298%), 3-MC (85%), and PCB (450%), but was extremely low in Gunn rats, and unchanged after MEI. T3 glucuronidation was increased after PCN (121%) or PCB (58%) in Wistar rats, but only PCN increased T3 glucuronidation in Gunn rats (43%). PCN treatment induced thyroid morphological changes and increased follicular cell proliferation in both strains. These data demonstrate that T4 glucuronidation cannot be increased in Ugt1a-deficient Gunn rats. Thus, the decrease in serum T4, increase in TSH, and increase in thyroid cell proliferation after MEI are not dependent on increased T4 glucuronidation, and cannot be attributed to Ugt1a enzymes.