Developmental changes of thyroid hormones in the fathead minnow, Pimephales promelas

The fathead minnow is widely used in ecotoxicological studies and such investigations have begun to focus on potential disruption of the thyroid axis. However, normal levels of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) and their developmental patterns are unknown. To provide these baseline data, radioimmunoassays were developed and validated for analysis of T4 and T3 after extraction from plasma or whole fish. Female fish showed consistently higher plasma levels of T4 than male fish. Analysis of thyroid hormones during development showed a significant rise in both T4 and T3 during the pre-hatch period, indicating embryonic production of both thyroid hormones. After hatching, whole-body content of both T4 and T3 significantly increased in early development, peaking at 16 days post-hatch (dph) and 9 dph, respectively, when T4 reached 32.88 +/- 3.30 ng g(-1) body weight and T3 reached 24.17 +/- 3.15 ng g(-1) body weight. Thyroid hormones subsequently declined to a low plateau in later development with approximately 5 ng g(-1) body weight T4 and 2 ng g(-1) body weight T3. These data suggest a prominent role for thyroid hormones in early developmental processes when we predict that the ecotoxicological effects of thyroid disrupters will be most significant.