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 (T-4) and 3,5,3'-triiodothyronine (T-3) and their developmental patterns are unknown. To provide these baseline data, radioimmunoassays were developed and validated for analysis of T-4 and T-3 after extraction from plasma or whole fish. Female fish showed consistently higher plasma levels of T-4 than male fish. Analysis of thyroid hormones during development showed a significant rise in both T-4 and T-3 during the pre-hatch period, indicating embryonic production of both thyroid hormones. After hatching, whole-body content of both T-4 and T-3 significantly increased in early development, peaking at 16 days post-hatch (dph) and 9 dph, respectively, when T-4 reached 32.88 +/- 3.30 ng g(-1) body weight and T-3 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 T-4 and 2 ng g(-1) body weight T-3. 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. (C) 2004 Elsevier Inc. All rights reserved.