Early temporal effects of three thyroid hormone synthesis inhibitors in Xenopus laevis

Thyroid axis disruption is an important consideration when evaluating risks associated with chemicals. Bioassay methods that include thyroid-related endpoints have been developed in a variety of species, including amphibians, whose metamorphic development is thyroid hormone (TH)-dependent. Inhibition of TH synthesis in these species leads to developmental delay, and assays designed to capture these effects take several weeks to complete. In an effort to develop a shorter term approach, the early responses of various endpoints were evaluated in Xenopus laevis throughout 8d of exposure to three TH synthesis inhibitors: methimazole (100mg/L), 6-propylthiouracil (6-PTU) (20mg/L), and perchlorate (4 mg/L). Endpoints included thyroid gland histology and cell numbers, circulating TH concentrations, and thyroidal TH and associated iodo-compounds. Thyroidal 3,5-diodo-L-tyrosine (DIT) and thyroxine (T4) were significantly reduced from day 2 onward by all three chemicals, while 3-monoiodo-L-tyrosine (MIT) was significantly reduced by methimazole and perchlorate, but not by 6-PTU. These reductions were the earliest indicators of TH synthesis inhibition. Histological effects were apparent on day 4 and became more exaggerated through day 8. However, reductions in circulating T4 and increases in thyroid gland cell numbers were not apparent until day 6. Reductions of thyroidal MIT, DIT, and T4 and circulating T4 are indicative of inhibitory effects of the chemicals on TH synthesis. Changes in thyroid histology and cell number represent compensatory effects modulated by circulating TSH. These observations establish a basis for the development of short term amphibian-based methods to evaluate thyroid axis effects using a suite of diagnostic endpoints.